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Frequently Asked Questions about GPS:

1. What GPS equipment do we have?
2. What is the accuracy of our GPS equipment?
3. What factors affect accuracy?
4. What is 'differential correction'?
5. Just for curiosity, how are the GPS satellites’ status being controlled?
6. What is the projection of our GPS surveyed data?
7. What are the general steps for a GPS survey project?
8. How should I choose the GPS field survey time?
9. What is the feature attribute data dictionary and how is it created?
10. What of the TDC1 data collector must be checked before going to the field?
11. What of the GPS Pathfinder receiver must be checked before going to the field?
12. How is the equipment assembled for use in the field?
13. How do I collect field data?
14. How do I transfer field data to a PC and how do I perform differential correction?
15. How do I display the results of my survey?
16. How do I export my final data to a GIS?
17. What are useful references and web sites for finding out more about GPS?



1. What GPS equipment do we have?

We have two Trimble GPS Pathfinder Pro XRS Systems. This is a high-performance geographic data-acquisition tool. The Pro XRS system consists of:

  • GPS Pathfinder Pro XRS receiver.
  • TDC1 field data collector with 1MB memory (enough to store 30,000 positions).
  • Asset Surveyor (v. 5.0) software that for use with TDC1. It communicates with the GPS receiver to set specific GPS parameters and collects field coordinates and attribute data.
  • Pathfinder Office software (running on GISLab laptop and desktop PCs), which allows comprehensive GPS-postprocessing and data-export functions.
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2. What is the accuracy of our GPS equipment?

The accuracy without differential correction is about 10 meters. After differential correction, the accuracy can reach submeter. Note: The horizontal coordinates derived from GPS are typically two to five times more accurate than the vertical coordinates (elevations) for any given GPS position.

3. What factors affect accuracy?

The following factors affect the GPS survey accuracy. They are also the main GPS configuration parameters you need to know.

Number of visible satellites (SV): If you require 3-dimensional coordinates (northerning, easting, and elevation), a minimum of four satellites is needed. If you require 2-dimensional coordinates, at least three satellites are needed, but the 2 dimensional mode can significantly reduce the final accuracy.
Position Dilution of Precision (PDOP): This is a measure of the current satellite geometry. It takes into account each satellite’s location relative to the other satellites in the constellation. A low DOP indicates a higher probability of accuracy. A high DOP indicates a lower probability of accuracy. A PDOP of 4 or below gives excellent positions. A PDOP between 5 and 8 is acceptable. A PDOP of 9 or more is poor. The default PDOP mask in Trimble GPS receiver is usually 8.
Satellite elevations: When a satellite is low on the horizon, the satellite signals must travel a greater distance through the atmosphere, resulting in a lower signal strength and delayed reception by the GPS receiver. Position data should be collected using only satellites that are at least 15° above the horizon.
Multipath: Satellite signals can reflect off larger nearby objects, such as buildings or cars, causing the GPS antenna to receive an erroneous signal (longer pathlength = false location). This phenomenon is known as multipath, which can induce errors of dozens of meters. Optimal accuracy can be obtained by collecting data in an environment devoid of large reflective surfaces and that has a clear view of the sky. Avoid standing next to a building.
Distance between base station and rover receivers: Accuracy degrades as the distance between base station and rover increases. To obtain submeter accuracy, you can generally only capture data within 300 mi of a base station. Using the satellite Differential GPS services (like Landstar, the subscription service for GISLab), this is not an issue--all of North America is within the baseline length limit for the network of basestations.
Signal-to-Noise Ratio (SNR): This is a measure of the information content of a signal relative to the signal’s noise. As this proportion decreases, information gets lost in the noise. A value above 20 is very good. The quality of a position is degraded if the signal strength of any satellite in the constellation is below 6. The default SNR mask is 6. The satellite signals do not penetrate metal surfaces, buildings, tree trunks, or similar objects. When recording tree location data, for example, consider placing the antenna a meter or two away from the tree.
Occupation time at a point: Accuracy at a spot can be improved by collecting over a longer period of time at the location, then averaging these positions. However, unless you are willing to occupy a point for half a day or so, the sample will not be big enough to be statistically better than collecting 3 positions (~3 seconds) and moving on to the next point.
Differential correction: This involves using a second GPS receiver at a known location (a base station) to correct for the drift of GPS signals. See Number 4 for more details.

 
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4. What is the "differential correction"?

Differential correction is a technique that greatly increases the accuracy of the collected GPS data. It involves using a receiver at a known location, the "base station," and collecting GPS positions at unknown locations with other, mobile receivers, "rovers." The data collected at the known site is used to determine what errors are contained in the satellite data. The information from the base station is then applied to the data collected by the rovers and the offset differences are used to remove errors from the rover positions. The base station position needs to be very accurate as the differential correction position accuracy depends on the accuracy of the coordinates of the base station.

Part of the satellite error is intentionally introduced into the GPS signals in the name of "national security" (although much of the problem of 'selective availability' of satellites was resolved in the late 90's by Presidential order, precisely because differential correction is such an effective work-around!). Differential correction works because most of the error in a satellite’s signal is the same in a wide geographic area. After differential correction, the accuracy of rover positions is improved to better than 5 meters even if the rover is 300 miles (500 km) away from the base station. If the rover is within 30 miles of the base station, the accuracy can reach submeter.

5. Just for curiosity, how are the GPS satellites’ status being controlled?

There are 24 operational NAVSTAR satellites orbiting the earth every twelve hours at an altitude of about 12,600 nautical miles (20,200km). The satellites are so high there is little atmospheric drag and their orbits are very stable. The satellites are constantly monitored by the Department of Defense (DoD). Each satellite contains several high-precision atomic clocks and constantly transmits radio signals using its own unique identifying code. The DoD has four ground-based monitor stations, three upload stations, and a master control station. The monitor stations track the satellites continuously and provide data to the master control station. The master control station calculates satellite paths and clock correction coefficients and forwards them to an upload station. The upload stations transmit the data to each satellite at least once a day to control its status.

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6. What is the projection of our GPS surveyed data?

The GPS can record coordinates in geographic projection (i.e. latitude, longitude, elevation) or UTM projection. The former projection is preferred because it is easier to transform to any other projections. The datum is World Geodetic System 1984 (WGS-84). Most software, such as ARC/INFO does not support WGS84 directly. But the Northern American Datum 1983 (NAD83) is, for all practical purpose, equivalent to WGS-84. In the next few years, maps published by the U.S. Geological Survey will use NAD83. NAD 83 is widely supported by most GIS software including ARC/INFO.

7. What are the general steps for a GPS survey project?

Before going to the field:

  • Plan: choose the best times to collect GPS data
  • Create feature attribute data dictionary and transfer it to the TDC1 data collector
  • Check the TDC1 data collector
  • Check the GPS Pathfinder receiver
  • Assemble the equipment

In the field:
  • Create rover files
  • Record features

Back in the lab:
  • Transfer data to the PC
  • Perform differential correction
  • Display data
  • Export data to a GIS


Please read the Trimble GPS manual: Pro XRS System, Chapters 3, 4 and 5.

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8. How should I choose the GPS field survey time?

Generally there will be enough GPS satellites available for field surveying. But planning allows you to choose the best time to collect GPS data based on the following information:

  • the number of visible satellites
  • where the satellites will travel
  • the PDOP (Position Dilution of Precision)
The more visible satellites, the higher possibility of high accuracy. Lower PDOP values indicate more accurate results. The Quick Plan software running on GISLab computers can help you predict satellite availability and determine the best observation periods for the field crew to work. The software is easy to use; you can directly play with it. If you need instruction on using Quick Plan, refer to the Quick Plan Software User’s Guide in the GISLab library.

Please make special note of the almanac. The almanac is a set of data which is used to predict satellite orbits over a moderately long period of time (about a month). An almanac file might becomes incomplete or invalid sooner if a new satellite is launched, if an existing satellite malfunctions, or if a satellite is moved to a new orbit. Quick Plan software needs an almanac no more than about a month old to produce reasonably accurate results. Therefore, it is important to keep your almanac up-to-date. The Almanac is transmitted from the satellites and automatically recorded by the receivers. A complete almanac message takes 12.5 minutes for the satellite to broadcast. Our Trimble Pathfinder receiver acquires the current almanac from satellites during regular operations, and maintains it in memory. You can download the almanac from the GPS Pathfinder receiver to an SSF file. Then load the SSF using the Options/Almanac menu item of the Quick Plan software.

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9. What is the feature attribute data dictionary and how is it created?

A feature attribute data dictionary is the description data associated with the geographic locations you collect. For example, if you are doing an inventory of trees, you may need to collect their ID and species. Then you need create an attribute table with items of Species and ID. In the field, you are prompted to enter the species and ID for each tree you are surveying. You may even want to choose one species name from several possible species names instead of wasting your precious field time to input the species name each time. All these can be archived by creating the feature attribute data dictionary.

To create a feature attribute data dictionary, you need use the Pathfinder Office software running on a GISLab computer. Choose Utilities à Data Dictionary Editor. It’s easy to create. If you need more information, see the Mapping Systems General Reference, Chapter 3.

After an attribute data dictionary has been constructed, it must be transferred to the TDC1 data collector. First connect the TDC1 data collector to a GISLab computer (see figure 1), then turn on the TDC1 data collector, and select File Transfer; On the Pathfinder Office side, select Utilities à Data Transfer, then change Data Type to data dictionary, highlight the file to be transferred, click add, and then click Transfer.

Figure 1. Connecting the TDC1 to the PC

(OSM-PF stands for the Pathfinder Office Support Module)


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10. What parts of the TDC1 data collector must be checked before going to the field?

Batteries! The TDC1 hand-held data collector requires two 9V batteries as the "main batteries" and another two 3V small lithium batteries as the "backup batteries". If you can not turn on the TDC1, it is likely that the main batteries are dead and you need to replace them. When the TDC1 detects that the main battery voltage is low, it will display the message "The main internal battery is low" on the screen. At such time, you may continue use the TDC1 for a short period, but the batteries should be replaced as soon as possible. The backup batteries are used to power the TDC1 memory when the main batteries are removed. These backup batteries contain enough power to preserve the memory for 400 hours when the main batteries are completely discharged or removed. When all batteries are dead or removed, all data in the TDC1 data collector may be lost! It is recommended that once you finish your survey, please download all data to a GISLab computer at your earliest convenience, then remove all the batteries if the GPS equipment is not going to be used for a period of time.

As for its configuration, the "Minimum posns" should generally be set to 1 to 30 (by default it is 180, which will slow down the speed of your survey). All the other default settings are generally good for most usage.


11. What parts of the GPS Pathfinder receiver must be checked before going to the field?

Batteries! Be sure to charge the receiver's batteries before leaving the office. Open the GPS Pathfinder receiver pouch, take out the four batteries (two active plus two spare), and charge them using the Pathfinder Office Support Module (OSM-PF).

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12. How is the equipment assembled for use in the field?

You need to assemble the GPS Pathfinder receiver, the TDC1 data collector, and the antenna all together in the backpack. See figures 2 and figure 3.

Figure 2. Assembling the GPS Pathfinder Pro XL system.

Figure 3. Assembled GPS Pathfinder Pro XL system.

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13. How do I collect field data?

After you finish all the necessary preparation before in the lab, you are ready to go to the field and collect field data. You need bring the TDC1 data collector and the GPS Pathfinder receiver to the field. You can collect both coordinate data and attribute data by operating the TDC1 data collector.

Before you start to capture data, a rover file must be created. The file will contain all of the features and positions that you will capture in the field. To create a rover file, Turn on the TDC1, select Data capture/Create rover file, select the data dictionary you want to use (usually the one you created using Pathfinder Office and transferred to TDC1, refer to Question 9: What is the feature attribute data dictionary and how is it corrected?). Before continuing, press "Pause" (F1). This will suspend the data collector's position-gathering until you are on your first point.

Now you are ready to record field features. Place the receiver antenna at/near the feature to be surveyed, choose Select feature to select the feature you need. Press "Resume" (F1), and the TDC1 automatically starts to collect the coordinate. At the same time, you can enter/select attribute values for the attributes associated with the feature, such as tree species, ID. When the TDC1 has acquired the minimum number of positions for the feature (~3), press "Pause" (F1). Move to the next feature, select its type, press "Resume" and then "Pause" as before. After you finish collecting field data, return to main menu and turn off the TDC1.

14. How do I transfer field data to a PC and how do I perform differential correction?

After your survey, you should transfer the field data stored in TDC1 to a GISLab computer as soon as possible (the same day as your survey or the following day). Do not leave your data in TDC1 for a long time. You may have to redo your survey if the data are lost.

To transfer TDC1 data to a GISLab computer: first connect the TDC1 with the computer (see figure 1). Then turn on the TDC1 data collector, and select File Transfer; On the Pathfinder Office side, select Utilities à Data Transfer, highlight the file (.ssf) to be transferred, click add, and then click Transfer.

To transfer the base file to a GISLab computer: download the Landstar file to your project /base directory.

To perform differential correction: Use the Pathfinder Office software, select Utilities à differential process, feed in necessary rover file, base file, output file, click OK.

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15. How do I display the results of my survey?

Use the Pathfinder Office software, choose View à map, select the file that you want to display, then click ‘OK’.

16. How do I export my final data to a GIS?

Use the Pathfinder Office software, choose Utilities à Export, select ARC/INFO UNIX generate file, or ArcView shape file, then click OK.

If you choose the ARC/INFO UNIX generate file, the output files will be an AML program, several coordinate files, and attribute files. Run the AML in ARC (either UNIX version or NT version), the surveyed coordinates and attributes will be converted into a coverage along with its attribute table. If you choose ArcView shape file, the output will be a shape file consisting of three files (.shp, .shx, .dbf). You can add it directly as a theme in ArcView.

17. What are useful references and web sites for finding out more about GPS?

You can find all the reference manual of our equipment on the shelf of the GISLab library. Besides, you can find great GPS resources in following links.

"How GPS Works": by Trimble Navigation
USDA Forest Service: GPS page
The Geographer's Craft Project, GPS resource
Iowa State University GPS page


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