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:
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.
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:
is highlighted and clicking once on the New button.
This opens a new, empty view window in ArcView.
))
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.
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.
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.
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.
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:
to add a title to the view -- make up something
appropriate. Use the Pointer tool 
to select and enlarge the title so it's prominent (drag outward on
the black handles around the title). You could also draw a
white box behind the text so it shows up better agains the
photo.
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Bryan Baker, Sonoma
State University, bryan.baker@sonoma.edu
Updated 17 February 1999