Geog 280: Basic Geographic Techniques

Exercise 4: Post Processing of GPS Data

You have now planned your GPS mission, and gone into the field to collect data.  The last part of your GPS mission is to make those data useful for mapping.  These activities can be called post processing. Post-processing activities consist of three types:

  1. Transferring ("downloading") the GPS data into a computer
  2. Correcting the GPS data for errors from S/A and other causes
  3. Translating the GPS data into an appropriate data format for your GIS software

Let's look at each step in at least a bit more detail.  Most of the processing has been done for you, but you will be observing the process for each step.  In this step you will use ArcView to view the GPS data, so you will need to do this page at the GIS Lab or elsewhere with ArcView.

Download Data to the Computer

To use your data in a GIS, the data must be transferred to a computer.  Receivers usually have a connection (a "port") for a cable.  The other end of the cable is connected to the computer (usually through a serial port, which is a connector similar to where the mouse is usually connected).

Then, special software that comes with the GPS receiver is able to contact the receiver and ask for the data to be transferred to the computer.  The software loads in the GPS data, often allowing you to view it right away in its raw state.  At this point you usually proceed to correct it for errors.

Before we correct it, let's take a few minutes to look at some "raw" GPS data similar to what you collected in your field work.  Follow the procedure below to start ArcView and load in the data:

  1. Resize and relocate this Netscape window so you'll be able to see both ArcView and Netscape.  I recommend making this Netscape window occupy the right 1/3 of the screen, so that it's tall and narrow, but any arrangement you find comfortable is fine.
  2. In the Start Menu-Programs, find the ArcView group, open it, and start ArcView. (If you're working with a demo copy of ArcView, start it and open an exercise, but then close any open views or other document windows within ArcView.)
  3. Resize the ArcView window so it doesn't overlap this Netscape window, for example, by putting it in the left 2/3 of the screen.
  4. In the project window (labeled "Untitled" in standard ArcView software), create a new view by making sure the Views icon  is highlighted and clicking once on the New button.  This opens a new, empty view window in ArcView.
  5. First add a digitized aerial photo of the campus to your view, which will help for comparison with the GPS data.  Do this by these steps:
    • choose View-Add Theme (or click on the equivalent button )
    • in the dialog box, navigate to the location of the data for this exercise.  In the GIS Lab, this is on the H: drive, in the directory \Class\280\Exer4.
    • change the lower left-hand drop-down box labeled Data Source Types to Image Data Source.  This should change the themes listed to just one:  SSU_Photo.  Click on this theme and on OK.  The SSU Photo item should appear in the view's legend.
    • Click on the theme's check-box to draw the photo.
  6. Add the GPS data by choosing View-Add Theme (or ) again.  Change the directory (double-click) to Uncorr.  Then change the Data Source Types box back to Feature Data Source.  You should see several themes in the left side of the box.  Hold the <Shift> key down and click successively on each one, then click OK. In the view, click the check-boxes to draw the themes.
  7. We want the grass theme to be drawn beneath the other point themes.  To do this, click on it in the legend and drag it so it's just above the SSU_Photo, then relase the mouse button.
  8. You may want to change theme colors to something more suitable, such as green for grass.  Or, if any of the themes don't show up well against the photo background because ArcView assigned them a dark color, you can change the theme's color by using the Legend Editor:
    • double-click on the theme in the legend to bring up the Legend Editor
    • in the Legend Editor, double-click on the colored box below Symbol to bring up the Palette (Symbol Window)
    • in the Palette, click on the paintbrush icon to get to the color palette
    • click an appropriate color in the palette.  The symbol in the Legend Editor should change to this color (move the palette if necessary to see Legend Editor controls)
    • in the Legend Editor, click on Apply to make the change in the view
    • you can now double-click on another theme in the view's legend if you want another change
    • when done changing colors, close or move the Legend Editor and Symbol Window
  9. Use the zoom tool  to zoom in on the area with GPS data.  If the grass area suddenly draws strangely, such as drawing outside the area rather than inside, you should zoom out a little ways so the whole theme is inside the view -- then it should draw properly.

All of the points and the grass area are supposed to be near the small grassy island in the quad area.  The grass itself is the island area.  Hopefully you can see with this display that the raw GPS data isn't very satisfactory -- the points and lines aren't placed in sensible locations, and lines zig-zag around.  You're seeing the effects of Selective Availability, and to a lesser extent of other errors.  The next step is to correct these errors.  Leave ArcView running with your display, as you'll use it again shortly.  You can enlarge this Netscape window temporarily to see more exercise at a time.

Correct the Data for Errors

The next step is to correct for errors due to Selective Availability and other problems such as atmospheric interference with the GPS signals.  The computer can correct for these errors with a technique called differential correction.  This technique uses the readings of a GPS receiver that is placed at one stationary location.  A receiver of this type is called a base station.  The base station is set up at a fixed location, and its position is precisely determined (often by using other GPS base stations).  Then the base station GPS receiver is turned on, and it records its position according to calculations from the satellite signals, just like an ordinary GPS receiver.  Because of S/A and other errors, the calculated positions will not be the same as the true, known position.  The base station appears to wander around, despite its fixed location!

We actually use these erroneous readings of the base station to correct our field receiver's readings.  It works like this: when we're in the field at a given moment, our receiver calculates its position.  The calculated position is off by some distance in some direction from the true position.  Let's say the calculated position is 34 meters northwest of our true position.  But of course we don't know what this error actually is at any given moment.

At the same time, the base station is also reading the same satellites, so the position it calculates for itself is also off by exactly the same distance and direction -- in our example, 34 meters northwest.

We know the true location of our base station, so we know exactly how much the calculated position is away from the true location -- 34 meters to the northwest.  To get the base station's true position, we would only need to move the calculated position 34 meters to the southeast.

We can get the true location of the field receiver for that time by doing the same correction -- moving it 34 meters to the southeast.

That's the procedure for differential correction.  In summary, the computer takes each reading of the field GPS receiver, finds out how much the base station's calculated position was off at the same moment, and shifts the field receiver's position exactly the same amount.

Let's now look at the data in ArcView as corrected by post-processing:

The differences between the raw, uncorrected data and the corrected data should be obvious.  The corrected data should have smooth lines, with reasonable locations for points.  Hopefully you've seen that GPS data must be processed before it can be used to map locations, especially at the local level.

Translate the Data to GIS Format

The final step in a GPS mission or project is to make the data usable to other software.  GPS receivers and software typically use their own specialized format for data, which GIS software cannot understand.  Fortunately, these days most GPS software is able to export data to commonly-used GIS formats, such as shapefile (ArcView) or DXF (AutoCAD format).  The data you've seen above has already been translated to shapefile format, so we won't do any more activities with ArcView at this point.  You should remember that just because you can get data doesn't mean your GIS can use it -- it must be in a format the GIS can use or import.

Export Your View to Submit with Assignment

As evidence that you've completed this part, you will export your View from ArcView to create a graphic file.  You will submit this with your assignment along with the answers to the questions.  You can use this procedure:

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Bryan Baker, Sonoma State University, bryan.baker@sonoma.edu
Updated 17 February 1999