A GPS (global
positioning system device) can be used both to collect georeferenced
data (GPS to GIS), and to locate or ground-truth existing GIS data (GIS
to GPS). Here's an overview:
- Basic features of the eTrex Vista Cx.
This is the GPS unit the ENVS Program owns. A quick guide is attached
to the bottom of this page; a picture of buttons is at right. Some
settings you should verify on your GPS prior to use include:
- Units:
Use decimal degrees (necessary for e.g. Moodle input) for position
format and WGS 84 (compliant with Google Earth and useful worldwide)
for map datum. You'll generally be using English units (e.g., feet) as
well.
- System: Enable WAAS (wide area augmentation system), and make sure battery type matches the batteries you are using.
- Time: Make sure time is correctly set.
- Collect georeferenced points (GPS to GIS).
Look at the Marking a Waypoint portion of the quick guide at bottom.
You can upload these points in the Dubach lab using a free application
called DNR Garmin.
- Note that it's
generally easier to annotate points separately, as the Garmin interface
for modifying the note field is clumsy. It's thus a good idea to bring
a logbook in which you can enter the point number along with any
descriptive notes, then later you can enter this information into an
attribute table in ArcGIS.
- When reception is poor, you
can select the Avg key at bottom for the GPS unit to average a set of
readings, which generally improves accuracy.
- DNR Garmin
is a free application developed by the Minnesota Department of Natural
Resources. DNR Garmin (only available for PC) can be downloaded here.
Connect your Garmin GPS via USB cable to a PC, then turn it on (you can
select Menu > Use with GPS Off) and launch DNR Garmin. It should see
and identify your Garmin GPS; if not, check under GPS > Set Port to
make sure it's set to USB. If you followed the GPS instructions, your
GPS readings will be in geographic (unprojected) format using WGS84
datum. Make sure that this is set accordingly on DNRGarmin by selecting
File > Set Projection, and clicking the EPSG button to select WGS 84
and No Projection. Also, check to make sure that the distance units in
your GPS (usually feet) are the same as in DNR Garmin (select GPS >
Units). To download waypoints, choose Waypoint > Download, and all
waypoints in your Garmin will be sent to the PC. You'll get a display
of all information sent, which can be edited as necessary. Since we are
keeping data unprojected, the lat/long fields should be equivalent to
the y_proj and the x_proj fields. This is an easy place to enter notes
or comments on waypoints. Now you will save your waypoints using File
> Save To > File..., and save to your user directory, remembering
that ArcGIS can have problems with spaces in folder/filenames. Save as
ArcView Shapefile (unprojected).
- Remember you can also download your GPS data to Google Earth for ready mapping; see the Google Earth help wiki for details.
- Keeping track of a route.
In addition to marking waypoints, the Garmin GPS units we're using have
a number of really nice features, including a good altimeter and
compass. One feature worth learning is Tracks, which stores breadcrumb
waypoints along a certain interval of a route you've taken. Just like
points, tracks can later be uploaded to GIS (via DNR Garmin; typically
you'll choose Line shapefile to connect the points of your track) or GoogleEarth (this is way cool). Tracks are accessed via the main menu. Here are some settings:
- Track
log and Clear: Whenever the GPS satellite receiver is on and track log
is set to On, it will be recording tracks, so prior to recording a
new track what I do is (a) make sure Track Log is turned off, then (b)
select Clear to clear the track log of any old tracks (make sure you've
saved existing tracks first! see below), then (c) turn Track Log on
again.
- Setup: For short treks you will generally use the
Distance method, recording a point at every 0.01 miles (the shortest
setting; about every 50 feet).
- Save: When you've
completed your track, make sure to select Save to record it, then (see
above) turn tracking off and then select Clear so that you're ready for
your next tracking adventure.
- Locate or ground-truth existing GIS data (GIS to GPS).
One of the common things you may want to do is find points of interest
identified from your GIS information and located in the field via GPS.
Here are two ways to do it:
- The manual way (good for just a few points):
- Go
to a point of interest in your GIS and write down the lat/long,
displayed at bottom right. (Note: the data frame must be set to display
decimal degrees, and you'll need to set units to 6 decimal places using
Tools > Options > Data View.)
- Fire up your GPS,
and mark a waypoint (hold down the joystick on the Vista Cx), scroll to
the lat/long and edit accordingly (a bit laborious), then click OK.
- The automatic way (good for more points with comments on each):
- Define
a new layer (shapefile) in ArcGIS to hold your points of interest.
You'll do this in ArcCatalog: navigate to the folder where you want to
store it, then select File > New > Shapefile, make sure
Feature Type says Point, then define a geographic (unprojected)
coordinate system: for the U.S., use North American 1983 HARN from
Geographic Coordinate Systems > North America. Once you have saved
it, you may wish to add fields to store comments on each point. To do
this, command-click/right-click in ArcCatalog, select Fields, and add a new field
called Comments with data type as Text.
- Add this
shapefile to your GIS map. Then click Editor > Start Editing (this
assumes you have the editor toolbar visible) and select the folder
where your shapefile is located, and make sure this shapefile is listed
in the Target window. Now click on the Sketch tool (the pencil), and
simply click on each place on the map where you want to add a point.
You can add information on each in the Comments field by opening the
attributes table and entering them into this field (assuming editing is
still enabled). When done, in the Editor menu select Save Edits and
Stop Editing.
- Now, you'll transfer this shapefile to
your GPS unit using DNR Garmin as above. Start up your GPS and connect
it via USB to your PC, then launch DNR Garmin. (You may need to select
GPS > Set Port to USB so you can see your GPS.) Then choose File
> Load From > Fileâ, enable shapefiles at the bottom, and
select the shapefile you made above. This should load all points, with
recognizable lat/long, into a table: you'll specify the point ID and
comment field when loading. (Note: your GPS will overwrite existing
points if the ID is the same! You can edit these point IDs [Ident
field] in DNR Garmin as needed.) When done, select Waypoint >
Upload, and you've transferred all points to your GPS!
- Once
you've done one of the two procedures above, you can find a waypoint on
your GPS, and click Go To to tell you how to get from where you are
to where the point is!
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