Community and Regional
Planning Program
The University of Texas at Austin School
of Architecture
Using the Global Positioning System Equipment in the School of Architecture
Our GPS capabilities consist of one GeoExplorer 3 GPS receiver (from
Trimble) and a single license of GPS Pathfinder Office software loaded
on the Tholos computer in Sutton Lab. The receiver plus a plastic
box containing cords, battery charger/support port, and antenna, are available
for check-out from the Audio-Visual Library on the 3rd floor of
Sutton Hall. A tiutorial for Pathfinder Office is also available for
check out, and can also be found with the other GIS materials on the GIS
bookshelf of Sutton Lab. The the GeoExplorer 3 (receiver) manual
(sole copy so far) is on the GIS bookshelf in Sutton, but we are making a
second copy for the AV Library. Please take very careful care of the receiver
and the manual!
Outline:
Step 1: Prepare in advance!
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Check satellite status and conditions for your field mapping day and
time.
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Use the QuickPlan utility under Start-Programs-GPS Pathfinder Office
2.70 to check to see when the best satellite availability is for the
day and time on which you want to do GPS mapping. Also check PDOP and SNR
levels.
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If you get a message that your almanac is more than 30 days out of date,
update the almanac file with current ephemeris information. This information
can be gathered by going outside with the GPS receiver and taking a reading,
then transferred to Pathfinder Office (see data transfer below), or can be
downloaded from the Trimble web site at
http://www.trimble.com/gpsdataresources.html.
The almanac file needs to be put in the following folder: C:\Program
Files\Common Files\Trimble\Almanacs\
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Think through your conceptual data design
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Are the type of feature(s) you will be mapping best represented as points,
lines, or polygons?
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What attributes do you want to collect in the field?
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Implement this design in Pathfinder Office
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Create a digital data dictionary using our Pathfinder Office software
(loaded on the Tholos computer in Sutton Lab). The data dictionary
will define what kind of feature(s) you are going to map (point, line, and
polygon), plus the attributes and possible attribute values each feature
will have.
Start GPS Pathfinder Office 2.70. You should see a dialog
box asking you to select a project. Choose New. Give the project a
name (e.g., waller_creek). The default folder will be e:\temp\pfdata\
(for "pathfinder data") which is fine. Press OK,
and then press OK again. A folder with three sub-folders will be created
to help you organize your project (backup, export and base). Remember that
this is where all your data should go (e.g.,
e:\temp\pfdata\waller_creek\)
To create a new data dictionary, choose Utilities - Data Dictionary
Editor. Look on the Help menu for more details. When you save
this file, make sure to save it under your project folder.
Note that Pathfinder Office has good Help available through the software.
There is also a tutorial for using this software, available in the Sutton
Lab on the GIS shelves, or available for check-out from the Audio-Visual
Library on the 3rd floor of Sutton.
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Optional: use the Configuration Manager under Start-Programs-GPS
Pathfinder Office 2.70 to configure GPS options like precision filter, coordinate
system, antenna height, etc. See online help for more information. You can
choose not to do any configuration - this means that you will use the default
configuration that comes with the GPS receiver. This is fine while you are
starting to learn.
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Note: the creation of a data dictionary can be done in the GPS receiver itself,
but it is much more difficult to do it there.
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Transfer the data dictionary (and optionally any new configuration file)
to the GeoExplorer 3 receiver
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First place the GPS receiver on its plastic support module port (this support
module also functions as the battery recharger).
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Connect this port to the computer
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Turn on the GPS receiver
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Follow the instructions in Pathfinder Office help for Data Transfer
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If Pathfinder Office shows that you are not making a connection, check
the cords, and try choosing the other data port
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Check the battery on the GPS receiver and recharge as necessary
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Turn on the GPS receiver and press the STATUS button to check battery levels
- if you do not see the screen with the battery gauge right away, keep
pressing the STATUS button until you do. A full battery should cover several
hours of data logging
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If it is low, place the GPS receiver on its plastic transfer/battery recharger
port and plug the port into an electrical outlet
Step 2: Collect data in the field
Note: The instructions for data collection in the field are not nearly so
good as for using Pathfinder Office. I have printed out most of manual for
the GeoExplorer3 and you should be able to find it in Sutton Lab on the GIS
shelves or available for check-out from the Audio-Visual Library on the
3rd floor of Sutton. I recommend practicing a lot with the
receiver to get the hang of it, so make sure to leave enough time to do that.
Tips:
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Make sure your GPS receiver battery is fully charged
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In the Austin area during daylight savings time, plan to collect data either
before or after 1pm, as our correction data from TxDot comes in two files
for each day, one covering the time from 7am until 1pm and the other covering
the time from 1pm. During regular central time, these times are 6am and 12
noon respectively.
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Make sure you have at least 4 satellites in view (and above buildings) before
you start logging positions
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While you are logging or when you stop, note your PDOP values - low is good,
e.g., 4 or or lower is considered excellent, 4-6 is good, and above 8 is
poor.
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Buildings, trees, etc., can interfere with the satellite signals and cause
higher error. I have seen it recommended that near a building or under tree
canopy or other obstructions, that you log at least 30 and up to 50 positions
for a good point (the receiver will average these into a single point). In
the open air, I have read that 3 positions is adequate. But I recommend you
experiment with these and check your results later in the lab.
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If you are collecting a line or polygon, walk fairly slowly, or experiment
on the same feature at different paces. I have gotten much smoother data
walking slowly.
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Sometimes instead of walking a regular polygon, it is easier to collect a
series of points that you will then connect in the lab.
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If you are collecting data for which it is difficult to get the required
number of satellites or PDOP (e.g., the corner of a building), think about
other ways you could collect the data, e.g., at an offset, or at a couple
offsets.
Step 3 - Download the data from the GPS receiver to the
computer
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First place the GPS receiver on its plastic transfer/battery recharger port
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Connect this port to the computer
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Follow the instructions in Pathfinder Office help for Data
Transfer
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When you tranfer data from the GPS receiver to the computer, also make it
a habit to transfer the latest almanac data as well
Step 4 - check the data in Pathfinder Office
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Check your data by bringing it up in a map in Pathfinder Office - see the
Help for more information
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You can bring up other data as background including an orthophoto in .tif
format and also ArcView shape files and .dxf. files. Unless you set the
configurations differently, your GPS data is in latitude and longitude. If
your background data is in a different coordinate system, you need to first
set your GPS data to the matching coordinate system by choosing
Options-Coordinate System.
Step 5 - Differentially correct your data the next day
(or later)
Differential correction should increase the accuracy of your data. Many
conditions, including weather, can affect your GPS readings. Doing differential
correction means you are comparing your with data recorded at the same
time at a known base station in your area where weather conditions were likely
the same. The comparison and adjustment will correct those problems in your
data. Read carefully through the Differential Correction Help section
of the Pathfinder Office Help for detailed directions.
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In Texas, the Texas Department of Transportation makes correction data available
for several cities, including Austin, on the following day. You can acquire
this data from the following web site:
http://www.dot.state.tx.us/insdtdot/orgchart/isd/gps/gps.htm
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After getting to this site and clicking to go to the FTP site, look for the
document called FileFormat.txt - this explains the file naming system
and its relationship to date and time.
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Go to the folder containing files for the month in which you have collected
data
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Find the folder for your city
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Download the correct data file for your day and time.
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Note: the differential correction utility can work directly off the TxDot
web site as well, meaning you don't have to dowload the file.
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For the rest of the process follow the directions in the Differential
Correction Help section of the Pathfinder Office Help
Step 6 - Export your data to a GIS or CAD format
Once your data has been differentially corrected, you can export it to your
final format. Read carefully about through the Export Help section
of the Pathfinder Office Help.
Tips
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During the export process, you can choose the coordinate system to which
you want your data exported (e.g., from latitude/longitude to Texas State
Plane, Central Zone, NAD 83) so that it matches up with your existing data.
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If the export process says that no data was exported, check the Export
Properties, especially the Position Filter tab. You may have settings
that filter out data that you actually want. This sometimes happens when
you are trying to export uncorrected data to see how well it matches up prior
to correction. In this case, you need to set the Position Filter so
that it will export uncorrected data. In general, the ability to set a filter
is good protection against bad data.
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