The Human Subject Who Uses Gps is Positioned Continually at the Periphery of the World

Working with GPS Receivers

The Global Positioning System (GPS) provides a low cost and high accuracy means of finding the exact location and altitude of any place on Earth using inexpensive, hand-held GPS receivers. GPS receivers costing well under $200 can provide 15-meter accuracy and GPS receivers enhanced with WAAS capability that cost slightly more can provide better than 3-meter accuracy. Connecting a GPS receiver to a laptop running Manifold provides a high quality means of automatically acquiring position and other GPS data at a very low cost.

Manifold's GPS Console provides direct interaction with a GPS (Global Positioning System) device connected to the computer via a serial port or USB connection. The GPS Console allows setting moving map mode or automatically fetching position and other data from the GPS device.

The Manifold GPS console works with any GPS receiver or other device that can communicate with a PC using non-proprietary, standard NMEA version 2 protocol using the sentences enumerated at the end of this topic, or with Garmin devices using either Garmin protocol or NMEA protocol. See the notes below under the heading Choosing a GPS Device for more information.

To acquire data from a GPS receiver:

1. Attach an NMEA compatible or Garmin protocol GPS receiver to a serial port or USB port on the computer. If a serial port connection is used, note which serial port (COM1, COM2, etc) is used. If you are using an NMEA compatible device, make sure it is set to NMEA version 2.

2. Launch Manifold. Open the project into which the GPS Console will save objects. Create a drawing if one does not yet exist. We must have at least one drawing in the project to accept data from the GPS device to use Track commands.

3. Open the GPS pane with a SHIFT-ALT-G or with a View - Panes - GPS Console command from the main menu.

4. In the GPS Console, choose the Drawing into which objects will be saved. The drawing cannot be a read-only drawing (such as a read-only drawing on an Enterprise server in Enterprise Edition).

5. Press Properties in the GPS Console toolbar. If a serial port is used, choose the serial port and the serial port properties for the GPS device. Most GPS devices use the default baud rate of 4800 and other default settings. If a USB port is used, choose either the Garmin USB connection or the generic USB Human Interface connection if a non-Garmin USB GPS is in use. Press OK.

6. Press in the Connect / Disconnect button to connect to the GPS device.

7. Press in the Track option buttons for the GPS tracking actions desired, typically the Track Points or Track Line button as well as the Track Columns button. If Track Columns is used, choose the destination fields within the drawing's table for the various data fields acquired from the GPS receiver.

8. Press Track to manually acquire one reading of data from the GPS receiver as set forth by the Track options.

9. To automatically acquire repeated readings of data, choose the Refresh interval and press the Track Sequence button.

10. Manifold will acquire readings of data as set forth by the Track options until the Track Sequence button is pushed out again.

Note that the GPS Console can write to a drawing even if it is not opened. Once GPS tracking is started by pushing in Track Sequence, tracking will continue until the Track Sequence button is pushed out even if the GPS Console pane is hidden.

The Track action buttons work independently of each other, except that Track Columns requires Track Points to be enabled. If the Track Points button is engaged the system will create points whether or not the Track Center or Track Line buttons are engaged. Push in the Track Line button and the system will begin growing a line to follow changes in position as well as creating points. Push out the Track Line button and the system will stop growing the line but will continue to create points. Push the Track Center button in or out at any time to start or stop "moving map" mode where the drawing is centered to the current GPS position.

Most people working with the GPS Console will work with a map that shows some sort of base map drawing in one layer with a blank drawing into which GPS points or lines are written as a second layer. The base map layer provides orientation for the user while the new drawing layer keeps the data created by the GPS conveniently organized in a second layer.

Controls

	Connect / Disconnect - Press IN to connect to the GPS device using serial port parameters specified in the GPS properties dialog. The GPS dialog cannot fetch data from the GPS device until it is connected. Connecting to the GPS also serves as a test of the connection.
	Track Center - "Moving map" mode: with each reading of data from the GPS receiver move the drawing so that the acquired position of the GPS is always in the center of the currently active window. When Track Center is engaged and either a Track or Track Sequence command is issued the active window will be panned to center on the location reported by the GPS.
	Track Line - With each reading of data from the GPS receiver extend a line to the current position. This option creates a line object that shows the sequence of tracked positions.
	Track Points - With each reading of data from the GPS receiver create a point at the current position. This option creates a series of points that show the sequence of tracked positions.
	Track Columns - Used to choose which names will be used for destination fields of downloaded data. Otherwise, has no effect unless Track Points is selected. Raises a dialog allowing choice of the destination fields within the drawing's table for the various data fields acquired from the GPS receiver. Choose a blank destination in the Save To column to not import a given GPS field. When selected, Track Columns will write the selected data fields into the specified destination fields in the drawing's table with each reading of data from the GPS receiver.
	Track - Press to fetch data from the GPS. Each press of this button commands one acquisition of data and the creation of objects or moving map action as specified by the Track Center/Line/Points/Fields buttons.
	Track Sequence - Start / Stop acquiring data from the GPS receiver repeatedly using the given time interval between readings. When this button is pressed in, each specified time interval repeatedly commands data acquisition and the creation of objects or moving map action as specified by the Track Center/Line/Points/Fields buttons.
	Download Waypoints - Enabled only when connected to a Garmin device using Garmin protocol or when an NMEA device is detected as sending waypoints. Press to download waypoints. Press again to cancel downloading. When pressed, the system will begin downloading waypoints from the GPS receiver, arranging them in order and creating a point object for each waypoint. A line object will be created for a route. To cancel the download process without creating any objects, click the Download / Waypoints button again before the download is complete. See the Waypoints and Projections paragraph below for important notes on projections.
	Upload Waypoints - Enabled only when connected to a Garmin device using Garmin protocol. Press to upload waypoints. Press again to cancel uploading.
	Properties - Set up the serial port connection to the GPS receiver by specifying either a USB connection or the serial port used and the serial protocol options required by the GPS receiver.
Drawing	Choose the drawing into which objects and data acquired from the GPS will be placed.
Refresh	Specify the time interval and time units (minutes, milliseconds or seconds) between automatic data acquisition when tracking.
Datum	Datum used by the GPS receiver. The datum setting dialog is resizable.
Altitude	Value reported by the GPS receiver for the height of the GPS antenna. Although handheld GPS receivers integrate the antenna into the instrument case, remember when using an external antenna the GPS device reports the position of the antenna.
Bearing	True heading computed by the GPS receiver for its motion based on differences between recent positions measured. The true heading is the magnetic heading corrected for magnetic variation reported for the current position by the GPS device.
Mag bearing	Magnetic bearing.
Mag variation	Magnetic variation.
Latitude	Current position latitude.
Longitude	Current position longitude.
Quality	Circular error probability computed by the GPS receiver for the position it is reporting.
Speed	Speed in centimeters per hour computed by the GPS receiver for its motion based on differences between recent positions measured.
Satellites	The number of GPS satellites in view of the GPS receiver.
(status bar)

Properties Dialog

Device	Serial port or USB device to which the GPS receiver is connected. Most computers support COM1 and COM2 with connectors on the chassis. . Manifold will automatically search the first 1000 possible COM port configurations and allow use of any COM port that appears to be working. If a USB port is used choose either the Garmin USB connection or the generic USB Human Interface connection if a non-Garmin USB GPS is in use. USB choices will not be available if a USB GPS device is not detected.
Baud rate	Speed of the serial connection. Use 4800 for most GPS receivers.
Data bits	Number of data bits in the serial protocol. Use 8 for most GPS receivers.
Stop bits	Number of stop bits in the serial protocol. Use 1 for most GPS receivers.
Parity	Parity setting for the serial protocol. Use None for most GPS receivers.
Test	Test connection to the GPS receiver. Instructs Manifold to listen for valid NMEA sentences.

Consult your GPS receiver's user manual for information on what settings to use for the Properties dialog when using a serial port connection. The Properties dialog will fail to launch if there are no available COM ports or USB devices available. Unplugging a USB GPS device will automatically disconnect the GPS Console.

Pressing the Test button in the Properties dialog commands Manifold to try connecting to the GPS to see if it is alive and well and to report, if possible, what protocol the GPS recognizes. If a GPS is found, a message box will pop open to report whether it is using NMEA or GARMIN protocol.

If the GPS uses NMEA protocol, Manifold will report the NMEA tags recognized.

If the GPS uses GARMIN protocol Manifold will report a list of recognized GARMIN subprotocols recognized as well as the version of each subprotocol. If a GPS device switches from GARMIN to NMEA protocol, Manifold will automatically detect the change.

GPS Columns Dialog

The GPS Columns dialog allows specification of NMEA data that will be downloaded from the GPS, if available. Note that not all GPS devices will transmit all of the following fields. For example, only a depth sounder that repeats GPS location information while transmitting depth information as an NMEA repeater will provide GPS Depth information. The complete list of fields in the GPS Columns dialog is taken from all NMEA sentences that the Manifold GPS Console NMEA parser recognizes.

To capture a given field, double click into the Save To cell for that field and choose either the name of an existing column or to create a new column for that field choose [New Column] .

	Select All - Capture all fields.
	Select None - Capture no fields.
	Select Inverse - Uncapture all captured fields and capture all uncaptured fields.. A fast way to use all but one field: click Select None, capture the one field not desired and then click Select Inverse.
GPS Air Temperature	Air temperature.
GPS Altitude	Altitude.
GPS Bearing	True bearing.
GPS Depth	Depth from the sounding transducer to the sea floor. Note that transducers are normally mounted some distance below the water line of a vessel, so that to compute the depth of water we may need to add the distance between the vessel's water line and the transducer to the reported GPS Depth figure. Some depth sounders reporting depth in an NMEA stream have a configuration option which allows adding a fixed distance between the water line and the transducer so that the GPS Depth reported will be the distance from the water line and not from the transducer.
GPS Dew Point	Dew point.
GPS Humidity	Humidity.
GPS Humidity, Relative	Relative humidity.
GPS Latitude	Latitude of current position.
GPS Longitude	Longitude of current position.
GPS Magnetic Bearing	Magnetic bearing (computed from true bearing using variation).
GPS Magnetic Variation	Variation used by the GPS for this position and date.
GPS Quality	Estimated error of GPS position.
GPS Receiving Frequency	Receiving frequency.
GPS Satellites	Number of satellites used by the GPS to compute position.
GPS Speed	Speed of motion.
GPS Stream Bearing	Stream bearing.
GPS Stream Magnetic Bearing	Stream magnetic bearing.
GPS Stream Speed	Stream speed.
GPS Time	Time of measurement in Universal (Zulu) time.
GPS Time, Local	Local time
GPS Time Zone, Local	Local time zone.
GPS Transducer to Keel	Distance from the depth sounding transducer to the lowest point of the vessel. Transducers are normally mounted below the waterline but significantly above the keel. The distance from the transducer to the sea bottom must exceed this value to avoid grounding the vessel.
GPS Transducer to Water Line	Distance from the depth sounding transducer to the water line of the vessel. Transducers are normally mounted below the waterline but significantly above the keel. This distance should be added to the depth from the transducer to the sea bottom to get water depth.
GPS Transmitting Frequency	Transmitting frequency.
GPS Water Temperature	Water temperature.
GPS Waypoint Name	Used when downloading waypoint information.
GPS Wind Bearing	Absolute bearing of the wind.
GPS Wind Bearing, Relative	Apparent wind bearing considering the bearing of the vessel.
GPS Wind Speed	Absolute wind speed.

The availability of numerous additional columns, such as GPS Depth, opens the door to numerous applications if we have the equipment that can provide such NMEA information. For example, suppose our boat has a depth sounder that can read GPS latitude / longitude information from a GPS device (either external or built-in) and can then repeat it on an NMEA serial connection as an NMEA "talker" along with depth information generated by the sounder.

We can then automatically map depths for a water body by simply connecting the NMEA serial cable to a laptop running Manifold and then set the GPS console to record GPS Depth as well as GPS Latitude and GPS Longitude. We set Track Columns and Track Points on and then cruise around the body of water for all regions where we want to capture depths. We could use the resulting depths to create a surface that shows the bathymetry of the water body, or we could create labels from the depth values to create a chart with traditional depth sounding values in the chart. If we need to make any adjustments to the depth values captured, such as, for example, a correction for the distance between the water line and the transducer we could use the transform toolbar for tables to add a fixed value to all of the depths.

Protocol Requirements for Downloading and Uploading Waypoints

The Upload Waypoints command works only with Garmin devices connected using Garmin protocol that, further, support the relevant Garmin subprotocol. Some older Garmin devices do not support Garmin uploading protocol.

Although the Download Waypoints command works when either NMEA or Garmin protocols are used for connection, as a practical matter it is easiest to use with Garmin protocol. When used with Garmin devices in Garmin protocol, the Download Waypoints button is enabled upon connecting to the GPS device and one simply presses the button to download whatever waypoints are stored in the GPS device.

When using NMEA protocol, the Download Waypoints command will stay disabled until the GPS device actually transmits some waypoints. Many units using NMEA protocol do not transmit waypoints unless specifically instructed to do so via the devices user interface. For example, the device may only be able to send waypoints for download if they are part of a route and that route has been made "active."

It is such a hassle to decode the user documentation for most GPS devices to figure out what must be done to send waypoints down the pipe for download, and such an annoyance to fumble with thumb-based menus that most people who want to download waypoints from a GPS will simply buy an inexpensive Garmin device and use that with Garmin protocol.

Note that as in the case for uploading waypoints, when using a Garmin device it must be a reasonably modern device that supports the required Garmin download subprotocol.

Downloading Waypoints, Routes and Tracks

Most GPS devices can store waypoints, which are point locations noted by the GPS. Most GPS devices make it easy to mark a waypoint at a desired location. In addition, most GPS devices also allow designation of routes and acquisition of tracks. A route is usually a list of waypoints. A track is a series of positions recorded by a GPS device and is imported into Manifold as a line. While routes are formed from waypoints, tracks are entities independent of waypoints. Manifold can import waypoints, routes and tracks.

Download of waypoints, routes and tracks and upload of waypoints in Manifold works only with Garmin protocol, which means we must be using a GPS device manufactured by Garmin. Garmin units implement two different protocols for downloading routes and tracks, so Manifold provides two separate checkboxes, one for downloading routes and the other for downloading tracks. If a particular Garmin unit does not implement either the route or track download protocol, the relevant checkbox will be disabled.

To acquire waypoints from a GPS device:

1. Attach a Garmin protocol GPS receiver to a serial port or USB port on the computer. Note which serial port (COM1, COM2, etc) is used.

2. Launch Manifold. Open the project into which the GPS Console will save objects. Create a drawing and verify the projection of that drawing using Assign Projection if a drawing does not yet exist. Using Edit - Change Projection, change the projection of the drawing to Latitude / Longitude.

3. Open the GPS pane with a SHIFT-ALT-G or with a View - Panes - GPS Console command from the main menu.

4. In the GPS Console, choose the drawing into which objects will be saved. The drawing cannot be a read-only drawing (such as a read-only drawing on an Enterprise server in Enterprise Edition) and it must be in Latitude / Longitude projection.

5. Press Properties in the GPS Console toolbar. If a serial port is used, choose the serial port and the serial port properties for the GPS device. Most GPS devices use the default baud rate of 4800 and other default settings. If a USB port is used, choose either the Garmin USB connection or the generic USB Human Interface connection if a non-Garmin USB GPS is in use. Press OK.

6. Press the Download Waypoints button in the GPS console toolbar.

7. In the resultant Download Waypoints dialog, choose options desired such as whether to download waypoints, routes and tracks, which data fields should be downloaded and into which columns should they be imported. Press OK.

Download Waypoints Dialog

The Download Waypoints dialog provides the following controls:

Download waypoints	Check to download waypoints.
Download routes	Check to download routes
Download tracks	Check to download tracks.
Reuse downloaded data	Recycle already downloaded data instead of fetching it again from the GPS device. Used to save already downloaded data into different drawings.
Create new drawing	Create a new drawing, called GPS Data by default.
	Choose all - Download all columns.
	Choose none - Download no columns.
	Choose inverse - Reverse values in Save To column so that those columns currently marked for download are now downloaded and vice versa.
Column	Columns available for download.
Save To	Columns into which downloaded data should be placed. Click into the Save To cells to choose from pull down menus of columns in the destination drawing's table. Choose [New Column] to create a new column in the drawing to accept the data.

Waypoints in a GPS normally have a latitude, longitude and altitude for each waypoint. In addition, each waypoint has a name and a symbol used to represent that point in the GPS. Some waypoints may also have a comment in the form of a short text string.

Once download occurs, the GPS Console will report the results of the download in the status bar at the bottom of the display.

Note that because GPS connections can be very slow the download of tracks can take a long time. Manifold will provide progress indications during the download process.

Due to the slow interface used by GPS devices the waypoint download process can be very slow at times, with only a few waypoints per second being downloaded. Tracks especially can take a long time. Manifold will provide progress indications during the download process.

When creating a new drawing using the Create new drawing option the new drawing will be created using Latitude / Longitude instead of the usual default Orthographic projection.

Reusing Downloaded Data

The GPS Console will cache downloaded data so that unnecessary downloads can be avoided if possible. The Reuse downloaded data option tells the GPS Console to recycle any data already downloaded instead of freshly downloading it. This option will be enabled only if there is some data available to reuse.

Use this option if you have downloaded data from a GPS device and there have not been any changes made to the data in the device, for example, no new waypoints added. If we have just downloaded data into a drawing and we would also like to download those points into a different drawing we can use this option to avoid the slow download process.

It is true that we could simply copy the downloaded points from the first drawing into the second, but it could be that the drawing into which the waypoints were first downloaded already had many points in it and that it is easier for us to simply download the waypoints again into a second drawing than it is to figure out which of the many points in the first drawing are the recently downloaded waypoints that should be copied.

To reuse downloaded data:

1. Connect to a GPS unit using Garmin protocol.

2. Select a target drawing in the Drawing box.

3. Download data using the Download Waypoints toolbar button.

4. Select another target drawing in the Drawing box.

5. Press the Download Waypoints button again.

6. Check the Reuse downloaded data option and click OK.

Waypoints and Projections

Downloaded waypoints will not be created if doing so would create an object outside the region that the projection in use can show. This prevents the addition of data that is impossible to show in the projection in use.

For example, the Orthographic projection used by default for new, blank drawings can only show one hemisphere of the Earth at a time. Waypoints that would appear in the hidden hemisphere of the default Orthographic projection will not be created. For this reasons, drawings created automatically when downloading waypoints will be created in Latitude / Longitude so waypoints will always be created no matter where they are located on Earth.

To avoid needing to think about projections the simplest approach to deal with the above is to create a new drawing that will accept the waypoints and then to immediately use Edit - Change Projection to change the projection of the drawing from the default Orthographic into Latitude / Longitude. There are no hidden hemispheres or disallowed locations in Latitude / Longitude so waypoints will always be created no matter where they are located on Earth.

Users who have confidence in their ability to use projections correctly can create a new drawing that uses a projection appropriate to their area of interest in which the waypoints will appear. The easiest way to do this is to create a map based upon a drawing that shows reasonable background features (such as roads or cities) for the area of interest and to then add a new drawing to the map by right clicking a layer tab in the map and choosing Add - New Drawing. The new drawing will automatically be created in the projection used by the map, which, if the map shows the area of interest correctly, will of course be a reasonable projection to use.

Adding a new drawing to an existing map to accept waypoints is also a good idea because seeing the waypoints appear against a background map that provides visual context is a good check against errors. For example, if we are working with a Garmin device and downloaded waypoints appear in the expected places we know we've chosen the correct route's waypoints (if there is more than one route full of waypoints stored on the GPS device) for download.

Uploading Waypoints

Many GPS device can receive uploaded points and store them as waypoints for later use. Such waypoints usually may be displayed in the GPS device's built-in maps (if any) and used for navigational purposes.

Manifold can upload points from a drawing into a GPS device as waypoints if the following conditions are observed:

• Uploads require a Garmin device connected using Garmin protocol.

• The drawing from which uploads occur can only contain points.

• The drawing must be in Latitude / Longitude projection.

• The drawing's table must contain a column that contains a unique, short name for each point.

• The GPS device must be capable of accepting uploads of waypoints. Note that some Garmin devices, such as the "hockey puck" simplified receivers like the GPS18 and later such devices, cannot store waypoints within the device nor accept uploads of waypoints.

To upload waypoints into a GPS device:

1. Attach a Garmin protocol GPS device capable of accepting waypoints to a serial port or USB port on the computer. Note which serial port (COM1, COM2, etc) is used.

2. Launch Manifold. Open the project into which the GPS Console will save objects. Make sure the drawing from which points will be uploaded is in Latitude / Longitude.

3. Open the GPS pane with a SHIFT-ALT-G or with a View - Panes - GPS Console command from the main menu.

4. In the GPS Console, choose the drawing from which points will be uploaded.

5. Press Properties in the GPS Console toolbar. If a serial port is used, choose the serial port and the serial port properties for the GPS device. Most GPS devices use the default baud rate of 4800 and other default settings. If a USB port is used, choose either the Garmin USB connection or the generic USB Human Interface connection if a non-Garmin USB GPS is in use. Press OK.

6. Press the Upload Waypoints button in the GPS console toolbar.

7. In the resultant Upload Waypoints dialog, choose the column that will be used to provide the GPS Waypoint Name value for each point and, optionally, the columns that will provide the GPS Waypoint Symbol and GPS Waypoint Comment values if such are desired. Press OK.

Upload Waypoints Dialog

The Upload Waypoints dialog provides the following controls:

Column	Columns to which data may be uploaded.
Load From	Columns from which uploaded data should be taken. Click into the Load From cells to choose from pull down menus of columns in the source drawing's table.

Note that each point's name should be unique in whatever colum is used to provide the GPS Waypoint Name. Using the same name for a subsequent point will simply overwrite any previous waypoint by that name.

Examples

The following examples assume the GPS receiver has been correctly connected to the computer and the GPS Console has been set up.

To connect the GPS receiver and set up the GPS Console:

1. Attach an NMEA or Garmin compatible GPS receiver to a serial port or a USB port on the computer. Note which serial port (COM1, COM2, etc) is used.

2. Launch Manifold. Open the project into which the GPS Console will save objects.

3. Open the GPS pane with a SHIFT-ALT-G or with a View - Panes - GPS Console command from the main menu.

4. Press Properties in the GPS Console toolbar. Configure USB port or serial port properties for the GPS device. Press OK.

5. Press Connect / Disconnect in to connect to the GPS device.

When working with GPS devices we will often travel from position to position. We would like to use our current position as reported by the GPS to move a map so that our position is always at the center of the map.

Move a map to show GPS location at the center:

1. Open the map desired.

2. Choose a drawing in the map as the Drawing choice in the GPS Console. Connect to the GPS device by pushing in the Connect / Disconnect button.

3. Choose a reasonable refresh interval, such as every 5 or 10 seconds. What is reasonable depends on the travel speed and the scale of the map. There is no point in refreshing the display every second if one's travel speed is so slow or the scale of the map so big that it takes minutes for any change in position to be noticeable.

4. Choose (push in) Track Center in the GPS Console toolbar.

5. Press IN Track Sequence in the GPS Console toolbar.

We might want to show a reticule at the center of the map when in Track Center, "moving map," mode. Reticules are controlled from the World pane.

Show a reticule at the center of the map:

1. Open the subject map.

2. Open the World pane.

3. Push in Show Position Reticule.

A very common use of GPS Console is to create maps by driving the periphery of ranches or over roads in a vehicle equipped with a GPS receiver and a laptop running Manifold. The GPS Console is set up to record position by creating points over a regular time interval.

To create a series of points at a given time interval:

1. Open the subject map or drawing in which points will be created.

2. Choose the drawing as the Drawing choice in the GPS Console. Connect to the GPS device by pushing in the Connect / Disconnect button.

3. Choose a reasonable refresh interval, such as every 5 or 10 seconds. What is reasonable depends on the travel speed, the scale of the map and the desired accuracy.

4. [Optional] Choose (push in) Track Columns in the GPS Console toolbar.

5. [Optional] In the Track Columns dialog, choose [New Column] as the Save To method or all fields to be imported. Choose a blank value in the Save To method to not import a field. If mapping a parcel of land using higher accuracy GPS receivers many people would import GPS Altitude.

6. Choose (push in) Track Points in the GPS Console toolbar.

7. Press IN Track Sequence in the GPS Console toolbar. The system will begin creating points.

8. Press OUT Track Sequence in the GPS Console toolbar to stop creating points.

Note that the act of creating a point in a drawing from a GPS position automatically captures the latitude and longitude of that point. Even if we do not import the GPS Latitude and GPS Longitude fields from the GPS device at that instant using the Track Columns option, we always can see the latitude and longitude of that point by showing the Latitude (I) and Longitude (I) intrinsic fields in the drawing's table.

As an option we may wish to capture fields from the GPS that are not implied by the location of the points created. For example, we may wish to capture GPS Altitude so that each point has a data field giving the altitude of that point.

Example Scenario

We work for a rural township and would like to create a precise map of all addresses on rural routes. We dispatch a vehicle with a driver and a person to operate a laptop computer connected to a GPS device. They will capture a point for each address as follows:

1. Configure Manifold using a drawing that shows streets in the township. In the map, add a blank drawing called Points to receive GPS-generated points.

2. Click open the table associated with the Points drawing and create an Address field that is a variable length text field.

3. Configure the GPS console to save points to the Points drawing. Press in Track Center and Track Points and configure Track Columns to save Latitude and Longitude. Connect to the GPS device by pushing in the Connect / Disconnect button.

4. Enable Edit - Instant Data .

5. Drive to each address, pausing at the address.

6. At each address, press the Track Once button. This will pan the map to that location, save a point at that location and will raise the Instant Data dialog.

7. In the Instant Data dialog, select Address as the field and enter the street address.

8. Continue in this fashion for all addresses, driving to each and pressing the Track Once button. With each future address Instant Data will automatically position the cursor for data entry for the Address field.

When adding points it is helpful to see the results of a data entry as points appear. We can add a labels component to the map so that each point is labeled with the Address field as it is created. To do so, copy the drawing and paste it as a labels component using the Address field. Drag and drop the new labels component into the map. As each point is created the address for the label will appear.

The result of the above scenario will be a drawing with a point for each address. Each point will have a latitude, longitude and address field. This same process can be used to locate all bridges, fire hydrants, manhole access points, transformers or any other facility. For locations in urban areas, use an WAAS-enabled GPS receiver to achieve accuracy of 3 meters or better.

Accuracy

Typical GPS accuracy is approximately 15 meters worldwide. For many years, the U.S. military deliberately damaged the accuracy of signals sent out by satellites to civilian GPS devices so that their accuracy averaged only 100 meters. The signal sabotage program was called "Selective Availability" (SA). SA was turned off by Presidential order in the year 2000 to allow all GPS devices whether military or civilian to operate at full GPS accuracy of 15 meters. Because most GPS devices use chip sets produced by a limited number of vendors, even the cheapest handheld GPS device delivers about the same default accuracy as the most expensive.

An easy way to see the accuracy of a GPS device is to connect it to a laptop computer and then start recording points without moving the GPS device. As points are collected the actual location for each point will vary slightly due to transient changes in what the GPS computes to be its location.

The above plot shows approximately 750 points collected with a Garmin GPS18 USB device at a stationary location. After collecting the points using Track Points we added an Order field to the drawing and then used the transform toolbar to add an arithmetic series to the Order column beginning with 1. This is just a simple way of numbering each point in order from 1 to 750. We then used thematic formatting to color the background color of the points with a spectrum of colors from purple to red so that there is a visual indication of the order in which points were acquired.

The width of the scattered points is about 6 meters (18 feet) and the height about 4 meters (12 feet) . The points were acquired with six satellites in view, but given that the Garmin GPS18 uses WAAS it is not spectacularly good accuracy. Presumably, if more satellites were in view (the GPS18 has 12 channels to handle up to 12 satellites at once) or if WAAS reception were better at the test location then accuracy would be improved.

Enhanced Accuracy

Two enhancement systems provide GPS accuracy better the default 15-meter GPS accuracy:

• Differential GPS (DGPS) - An older technology that requires additional receiving circuitry usually implemented as an external black box. Provides 3-meter to 5-meter accuracy.

• Wide Area Augmentation System (WAAS) - A newer system functioning throughout North America that can be implemented using almost the same receiving circuitry already designed for regular GPS function. Provides better than 3-meter accuracy 95% of the time. Europe and Japan are developing similar systems.

Many low cost handheld devices now include WAAS functionality built in for just slightly increased prices over non-WAAS products. At this writing it is possible to buy a WAAS-enabled GPS device for just over $100 that can connect to a laptop computer.

When mapping land parcels with GPS, in North America one should use a WAAS enabled GPS receiver if possible to achieve accuracy better than 3 meters over 95% of the time. WAAS also provides this same accuracy in altitude measurements. An accuracy of better than 3 meters is roughly equivalent to the accuracy of TIGER/Line and USGS 1:100,000-scale SDTS/DLG drawings, so it is a good match for much GIS work.

WAAS-enhanced GPS is also a good match for mapping civic infrastructure such as the locations of fire hydrants, 911 addressing, rural geocoding and many other applications.

Bearings, Magnetic Bearings and Variation

Bearings are radial measurements describing directional movement on the Earth where travel directly towards the North Pole is assigned a value of 0, directly East is 90 degrees, South is 180 degrees and due West is 270 degrees. True bearings are given based on this system that uses the bearing toward the North Pole as the zero value. The North Pole, in turn, is defined by the spin axis of the Earth. Bearings are also referred to as "headings".

Magnetic compasses do not point towards the North Pole of the Earth. Instead, they point approximately at the North Pole of the Earth's magnetic field, which is usually located hundreds of miles away from the North Pole defined by the spin axis, approximately 700 miles (1140 kilometers) away. In addition, due to fluctuations in the Earth's magnetic field the magnetic North Pole will move over a period of years relative to the true North Pole.

Because the magnetic North Pole is located hundreds of miles from the true North Pole the bearing shown by a compass will not the same as the true bearing. Instead, the compass shows magnetic bearing (also known as "compass" bearing). The difference between the magnetic bearing shown by a correctly compensated compass and the true bearing is called variation. Variation is different at different locations on Earth because different locations have a different geometric relationship between the location and the placement of the true North Pole and the magnetic North Pole. Other factors, such as magnetic anomalies in a particular region, can greatly influence variation as well.

Variation can be substantial. For example, the variation in the San Francisco Bay region is approximately 15.6 degrees. That means to head directly North one must follow a magnetic bearing of approximately 344 degrees as indicated by a compass or as indicated by a GPS device set to display magnetic bearings. Variation can change surprisingly fast in some regions of the Earth. In far north regions or in locations near magnetic anomalies positions that are only a mile (1.6 km) apart can have an entire degree difference in variation.

Variation also changes over time as the Earth's magnetic field drifts to slightly different locations relative to the Earth's spin axis. The changes are quite rapid compared to most geophysical processes: there are locations on Earth where significant (greater than 1%) changes in variation will occur within a few years.

Consumer GPS devices are intended to assist navigation in settings such as boating or hiking where magnetic compasses will also be used. Such GPS devices are often setup by default to display magnetic bearings even though the GPS computation within the unit provides a true bearing. Most GPS devices calculate the magnetic bearing they display by consulting a look-up table within the unit that contains an approximate variation for various locations on Earth. Since the unit knows its location and the current date, it can find the current variation for that location in the table (interpolating if need be) and adjust the true bearing to the magnetic bearing that is displayed.

Garmin, for example, at the present writing uses an internal lookup table based on the IGRF 90 international standard Earth magnetic field model to estimate the variation at any given location. The model as used by Garmin provides +/- 2% accuracy for more than 95% of the Earth's surface through the year 2010. Because variation changes more rapidly in some locations (such as northern Canada) where there is a high variation to begin with and a high annual rate of change in variation, in some locations the internal lookup table will result in accuracy worse than 2% by the year 2010. To deal with such cases Garmin provides the ability to manually specify the variation to be used.

Most units will normally report true bearings via their NMEA interface. They will also report the variation used for that location at the time of the measurement. When capturing data from a GPS it is best to capture the true bearings. The true bearing is an invariant quantity that is linked to the spin axis of the Earth. For tactical navigation purposes it is convenient to use magnetic bearings with GPS devices so that the bearing reported (and captured within Manifold) is the same as that seen on a compass. However, for archival purposes it is risky to use magnetic bearings because the actual direction indicated by a magnetic bearing depends upon variation, which in turn depends upon the location, the geophysical model employed and the date of the measurement. To maintain archival integrity over the years it is best to capture variation together with magnetic bearings if magnetic bearings will be used.

Note: A more precise term for variation is magnetic declination. "Variation" is a less technical term employed in marine and air navigation. We use the term variation because it is widely understood by lay audiences to be the difference between true bearings and magnetic bearings. A further desire is to avoid confusion with the meaning of "declination" in celestial navigation and astronomy where together with right ascension it is used to describe the position of a star.

Datums

The default datum used by almost all GPS devices is WGS84. Manifold assumes data coming in from the GPS receiver is in latitude / longitude coordinates using the WGS84 datum. When GPS information is added to a drawing, the GPS coordinates will be converted on the fly from latitude / longitude coordinates and WGS84 into whatever coordinate system (projection) and datum are used by the drawing.

If your GPS receiver is set to use some other coordinate system (projection) or some other datum, then Manifold's interpretation of the data it receives from the GPS receiver will not be correct. The easiest way to avoid such problems is to make sure your GPS receiver is set to use WGS84 and latitude / longitude coordinates. If your GPS receiver is set to use other coordinates or a different datum, the easiest path is to use the setup menu in your GPS receiver (see the documentation for your GPS receiver) to change it to use latitude / longitude and WGS84.

Troubleshooting

If your system is unable to read data from the GPS receiver check the following (using a log file if necessary):

• Is your GPS turned on and transmitting location data? Some GPS devices do not transmit location data until they have acquired a signal.

• Is your GPS a serial port interface device or a USB interface device?

• Is the GPS device plugged into your computer's serial port, or, if it is a USB device is it plugged into the USB port?

• Is your GPS interface set to NMEA version 2? Most modern GPS receivers when set to "NMEA" for their interface will automatically use version 2. Some GPS devices, such as Garmin, may come with their own proprietary interface protocol selected by default. Although Manifold can communicate with Garmin devices in Garmin protocol, for other proprietary formats you will have to change the interface protocol to NMEA version 2 in the GPS receiver's setup menu. If you experience any difficulty acquiring fields when working with Garmin protocol, set the device to use NMEA protocol.

• Is your serial port or USB port on your computer functioning correctly?

• Have you chosen the correct serial port in the Properties dialog? If using a USB port have you specified a USB connection?

• If you are using a manufacturer's cable to connect a serial port GPS device to a USB connection and that manufacturer provided custom USB drivers, did you install the USB drivers successfully?

• When using a serial port connection, do the baud rate and other serial port settings in use for the computer match the settings used by the GPS device?

• Some GPS devices will not begin transmitting until an active route is specified or some other setup procedure is followed. If this is the case with your device have you correctly set up the GPS?

• Use HyperTerminal to manually check the GPS interface as described below.

• When downloading waypoints from some Garmin devices, the route containing the waypoints must be set as the active route. See your GPS device user manual for information on how to designate a particular route as the active route.

If waypoints do not appear when you download them, make sure the drawing you are using is in Latitude / Longitude and not some other projection.

Details on GPS receiver interfaces and setup, by long tradition in the GPS hardware industry, are buried in a very obscure portion of the GPS receiver's user manual. It may well be a frustrating exercise to try and find out how to set a route active, how to configure your device to use NMEA 2 and so forth. For example, the manual for a recent Garmin device talks about waypoints and "active" routes but there is no command or documentation for designating an active route; instead, the user must infer that telling the unit to "navigate" a particular route makes it the active route.

Do an Internet search for pages with advice, tips and tricks on using GPS receivers.

Track Sequence Timing

The Track Sequence command might not operate continuously if CPU usage is too high. The Track Sequence command operates when the system has nothing else to do (to avoid crowding the drawing with too much data) so it may get overridden in extremely heavily loaded systems by higher priority processes. The solution is to avoid running too many demanding programs that place heavy loads on the system processor at the same time while running Manifold with the GPS console and Track Sequence.

Garmin Protocol

Garmin GPS devices can communicate with using either industry-standard NMEA protocol or using Garmin's own proprietary interface protocol. Manifold can connect to a Garmin GPS device using either NMEA protocol or Garmin protocol.

Technical Note on NMEA Parsing

The GPS Console handles NMEA sentences in a generic manner. For example, the handler for the $GPGGA sentence can handle arbitrary $xxGGA sentences. Manifold can handle more than 20 NMEA sentences, including DBT, DPT, FSI, GDA, GDF, GDP, GGA, GLA, GLF, GLL, GLP, GOA, GOF, GOP, GSA, GXF, GXP, HCC, HDG, HSC, MHU, MTA, MTW, MWV, Rnn (where nn is a decimal number), RMA, RMC, RMZ, VDR, VHW, VTG, WPL, ZDA, ZLZ, ZZU. Not all are literally exposed as available fields in the GPS Columns dialog, but all will be used as necessary to extract the fields available in the dialog.

Some devices attempt to extend NMEA through the use of proprietary sentences that are unique to that manufacturer. Manifold does not support proprietary NMEA sentences.

Choosing a GPS Device

There are hundreds of different brands of GPS receivers that work perfectly well using standard, non-proprietary NMEA format; however, on occasion you may encounter some devices that claim to be "NMEA compatible" that fail to communication via standard NMEA sentences.

Some such devices simply have bugs in their firmware while others inject proprietary sentences into what is otherwise a standard NMEA data stream. Devices that mix proprietary sentences with standard NMEA sentences can be perversely confusing to new users because they may work for some features, such as reporting latitude and longitude, but not work for other features, such as downloading waypoints. Even well-known brands, such as Magellan, may incorporate such communications incompatibilities into their NMEA interfaces, so users should not take it for granted that an "NMEA compatible" device really is NMEA compatible.

Another source of incompatibility may occur when using devices that connect via USB. This especially may be the case when working with USB devices using vendor-supplied USB drivers, in which case not only might GPS communications standards not be observed but a variety of non-standard approaches to providing Windows drivers might also be a factor.

Manifold.net recommends Garmin GPS devices because they are high quality, inexpensive, very reliable and do an excellent job of supporting standard industry protocols. In addition, Garmin has posted for free download all information necessary for software vendors to do a good job of supporting Garmin devices both in NMEA and in proprietary Garmin mode. Garmin also normally provides even many of its least expensive devices with full capability to connect with and interact with PCs. This makes it possible for users to acquire a Garmin device and connect it to their PC at a very low cost. Garmin devices make it easy to download waypoints, routes and tracks and are the only brand of GPS devices currently supported by Manifold for uploading waypoints.

Because of the high quality of Garmin firmware and support for standard protocols, manifold.net uses Garmin devices as the "gold standard" for GPS compatibility in NMEA and Garmin interfaces. If a Manifold feature does not work with a Garmin device, it is immediately investigated as a possible bug in Manifold. In contrast, if a Manifold feature works with the Garmin devices in our testing laboratories but is reported not to work with some other GPS manufacturer's device, the debugging process will begin with the assumption that the problem is in that other GPS manufacturer's firmware or communications protocol.

If you have a problem using a GPS device other than a Garmin device, Manifold technical support (at the cost of one or more support incidents) is always willing to investigate the matter for you to help verify that all settings (such as baud rate, etc.) are correct. However, beyond the initial stages of investigation good documentation from the GPS manufacturer may be required for resolution of the problem. Absent such documentation, if tests with a Garmin device do not duplicate the problem the fault will be assumed to be a problem in the GPS device you are using and not in Manifold.

Support for Proprietary GPS Features

manifold.net is able to provide the best support for those GPS vendors, such as Garmin, who do a good job of providing technical details for their devices for free download on their websites. We are always willing to listen to requests for support of proprietary GPS protocols and features, but only if the GPS manufacturer publishes complete details required for such support for free download and free usage.

When vendors publish on their websites details of formats for communicating with and controlling their devices, for uploading and downloading waypoints, for uploading cartography and so on, it makes it possible for software developers such as manifold.net to provide crisp and reliable support for those devices. We believe this minimizes the tech support load on both manifold.net and the GPS vendor while it increases customer satisfaction and makes it more likely that the GPS vendor will sell more devices.

There are now hundreds of GPS manufacturers, some of which are not really GPS developers but are more accurately understood to be integrators, in that they take an OEM version of a GPS chipset or module made by some other company and simply package it, relying upon the manufacturer of the module to provide software support. Some such integrators do not provide good support for their products because they do not understand the internal functioning of the modules and software they buy from the original developer.

If your GPS vendor does not provide good documentation online, consider switching to a vendor who does. Given the very low cost of GPS devices and the great competition in this market there is little reason to do business with vendors who do not provide full and precise documentation online.

See the Contacting manifold.net topic if you would like to recommend support for proprietary GPS features.

GPS Manufacturers

If you are a bona fide GPS manufacturer, manifold.net technical support will provide free Manifold System licenses to you so you can test your GPS devices with Manifold and better support your customers. This offer is open only to manufacturers and not to dealers, distributors, integrators or other non-manufacturing entities.

Technical Note on Datums

If your GPS receiver uses some coordinate system and datum other than latitude / longitude and WGS84 and cannot be conveniently reset, it is still possible to use data from that GPS receiver. To do so, first create a drawing in latitude / longitude and WGS84 and acquire the GPS data into that drawing. Next, use the Edit - Assign Projection dialog to specify the coordinate system and datum used by the GPS receiver for that drawing.

See the Technical Note for Experts at the end of the Projections topic for variations in WGS84 and NAD83 datums as used by certain GPS devices.

See Also

View - Panes

View - Panes - GPS Console

thomaswitither.blogspot.com

Source: https://manifold.net/doc/mfd8/working_with_gps_receivers.htm

Share this post