Geog 280: Basic Geographic Techniques
Exercise 4: Post Processing of GPS
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
- Transferring ("downloading") the GPS data into a computer
- Correcting the GPS data for errors from S/A and other
- 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
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
- 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
- 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.)
- Resize the ArcView window so it doesn't overlap this Netscape
window, for example, by putting it in the left 2/3 of the
- In the project window (labeled "Untitled" in standard ArcView
software), create a new view by making sure the Views
is highlighted and clicking once on the New button.
This opens a new, empty view window in ArcView.
- First add a digitized aerial photo of the campus to your view,
which will help for comparison with the GPS data. Do this by
- choose View-Add Theme (or click on the equivalent
- 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
- Click on the theme's check-box to draw the photo.
- Add the GPS data by choosing View-Add Theme
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.
- 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
- 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
- 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
- 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
- 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
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
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
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
Let's now look at the data in ArcView as corrected by
- ArcView should still be running and visible, with the data
loaded as previously described in this page. Arrange this
window and ArcView to see both comfortably. Make sure the
view with the SSU photo and raw GPS data is still visible and
active (blue highlight in title bar of view1).
- Choose View-Add Theme (or its button). The
corrected data is in a different folder (in GIS Lab, it's
H:\Class\280\Exer4\Corr). You'll probably have to
double-click on the Exer4 folder in the dialog box to move
up a level, then move to the Corr folder.
- Hold <Shift> down and click on the themes in the folder,
than click OK.
- Click on the check-boxes of the themes to draw them.
- If the themes are difficult to distinguish from each other,
use the Legend Editor to change the symbol colors (see above for
instructions). You can also experiment with point-symbols
(in the Symbol Window, click on the "push-pin" icon to find it) --
assign different symbols to different types of points.
- Zoom in if necessary to see the details of the raw and
- You can also turn off all themes except a pair of corrected
and uncorrected themes. For example, you could turn on just
the lightposts for the corrected and uncorrected data.
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:
- In ArcView, make sure your View is active (blue in title bar
of View1). Keep the view zoomed in on the area around the
- Use the Text tool
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
- In the menu, choose File-Export. In the export
dialog box, change the drive to U: and the directory to your
personal folder (or if you're outside the GIS Lab, choose a
directory you'll remember). Then type
Ex4-xxxx, but substitute the first four letters of
your last name for the xxxx. Make sure the File
Type box in the lower left says Windows Metafile (WMF).
Then click OK.
- If you have a mail window already open to type answers, you
can switch to it now and attach the file you just created
(Ex4xxxx.wmf). Or, you can wait until you open a mail
message to enter the answers, then attach the file.
- After you export the file, you can quit ArcView. You
should save your ArcView project (in the GIS Lab, to your U:
folder; name it Exer4). You can then enlarge Netscape
to fill most or all your screen to finish this exercise.
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Bryan Baker, Sonoma
State University, email@example.com
Updated 17 February 1999