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PCCC GIS I Exercise 2 (AV9.1) - Locate your car with GPS, map it with GIS v2, Apr 2007 - page 6 / 31





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Press Add File (or Data). Press the Open button.

An Open Screen appears, with your data file on it.

The filename now appears in the Data Transfer window, with a directory path that is something

like C:pfdata\default\r100313A. This is setting up the path for the transfer of the file from the GPS unit to the computer, PAY ATTENTION to the directory path, as always.

  • Press Transfer All, and the data transfer process will begin.

After transfer is complete, you

may remove the GPS unit and power it down. It's all computer work from here on. Optional: If you want to view your points prior to differential correction, select View > Map A map screen will appears. Note that it has a scale, and indicates the distance from one side of the window to the other. You can select either positions or feature properties (icons at the top of the window) and then select each point. Note that the Attribute Name Field is identified as Comment, and the Value Field is identified as the car owner's name. Visualize how this might look in a GIS attribute table:









Comment is the name of the attribute field, while the car owner’s name is the attribute. Step 4 - Apply differential correction to the GPS data Discussion: Although you can use uncorrected GPS data if necessary (procedure noted below), maximum accuracy is achieved if “differential correction” is applied to GPS data prior to mapping. Differential correction employs the concept that the real position of a fixed base station is always known (even if its GPS position appears to “wander”), thus the base station can feed a “correction” signal to the data that you collected with your GPS rover unit.

Each second that a point is recorded in a GPS unit, it appears to have a slightly different latitude (Y), longitude (X), and vertical (Z) position, due to atmospheric and ionic disturbances that can disrupt travel paths of satellite signals and degrade GPS data precision and accuracy. Therefore, if you plotted the location of each of the 100 points collected by the GPS unit while you were standing over your car for 100 seconds, the GPS plot might look like a pile of spaghetti on a plate, because each of those 100 points plots in a slightly different location:

Uncorrected rover data:

A nearby GPS base station (up to 60 miles away) will be receiving the same signals from the same satellites that the rover unit receives. However, the base station, because it “knows” its fixed position, is able to determine the difference in position between the real, fixed position (red "x" below) and the base station’s apparent GPS position (red open circle):





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    50 feet


For the case above, at time “t”, the apparent GPS position of the base station is about 50 feet northwest of the real fixed position. PFO software uses this “difference in position” information from the base station to “correct” the rover unit’s point by moving it 50 feet back to the


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