Wednesday, April 06, 2005

Using a GPS with GTViewer and Pocket GTViewer

GTViewer and Pocket GTViewer can all use a GPS to update their view position and reflect an actual location in real-time. The GPS functionality allows the use of any GPS receiver supporting the standard NMEA protocol or Trimble’s TSIP protocol. GPS receivers are generally connected to a COM port; however, modern GPS devices also use BlueTooth, PCMCIA slots, CF slots, and can be built into the laptop or handheld computer.

A GPS receiver is a nice addition to map viewing because it will keep the view centered on your location without having to manually pan, zoom, or query to get there. This “view synchronization” runs in the background and doesn’t prevent GTViewer from using any of the other commands (like Attribute Info, redlining, etc.). Even with inexpensive GPS receivers whose accuracy may not be precise enough to capture location points, it is still quite satisfactory for view synchronization.

GTViewer and Pocket GTViewer both provide several modes of operation. The GPS indicator can be set to obey one of the following 4 rules:

  • Keep In View Mode – the indicator will move around the view, but if it leaves the current view area, the view will be updated so that the indicator re-centered. This mode is ideal if you are moving slowly or covering a small area because the view will not continuously update because of the movement.
  • Keep Centered Mode – the indicator is always in the center of the view and the map moves under it. This mode is good for driving as the cursor is fixed and the map moves under it; however, this mode does require many more view updates and make it more difficult to perform other tasks when moving.
  • Free Mode – the indicator will be displayed, but the view will not update to keep it visible. This mode unhinges the GPS indicator from the view turning off the view synchronization, yet still showing the GPS cursor when it is in the view.
  • Centered/No Indicator Mode – identical to Keep Centered, except the indicator is not drawn. This mode is good if you are just interested in keeping the view synchronized with the current location, but do not want the indicator displayed in the view.

The GPS indicator not only shows the position on the map, it also has a small red arrow head showing the current bearing.

GTViewer and Pocket GTViewer also provide two different trail modes. A trail is a line drawn along the path the GPS indicator follows. The Redline Trail mode leave a redline along the path and it can be saved, exported, shared, etc. The Decaying Line mode draws a trail of a user specified length, but the line is temporary and will shrink over time. The line is draw in Red and “cools” to Blue.

GTViewer and Pocket GTViewer both have a GPS Info dialog that shows detailed information about the current GPS readings. One page shows the tabular data like Latitude/Longitude/Altitude, HDOP/PDOP/VDOP, Bearing and Speed, and Satellites in Use/Signal Quality/GPS Time. The second page is a skyplot showing the relative position of each satellite and its signal strength.

The GPS functionality also supports two simulation modes. One simulation will read a file of Latitude/Longitude pairs and “play” them back as if they where actually being received by a GPS receiver. The second simulation mode will read an NMEA stream from a file and “play” it back as if it were being received by a GPS receiver. The difference in the two modes is that mode uses a source file that can be easily manufactured with some known points. The other mode uses a richer set of information and runs all of the GPS Info settings including the Skyplot, the drawback is that the file is usually much larger and needs to be captured from an actual GPS receiver at some point.

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