Global Positioning System - GPS Planning
 

Planning is a most important part of data collection. Without good planning, the GPS data may be of poorer quality. This will also effect any end products such as GIS data layers.

Almanac

Most GPS software packages come with a module that enables the data collector to assess the satellite conditions before going into the field.  First, the user will create an Almanac of satellites by putting the rover unit in an open area and collecting satellite transmissions for about four or five minutes.  Most units will display some type of message once the Almanac has been completely loaded into the receiver.  The Almanac is then downloaded into the planning module on a personal computer. 

After downloading the Almanac, the user enters critical information such as current latitude, longitude, and elevation for the proposed field work site.  The planning module can then create charts that give the user information (based on the Almanac and the critical information discussed above) on the best times of any given day to do the data collection in the field. 

Figure 2: Typical PDOP Chart

(SIC 2002)

The most useful charts available for planning are those that show the number of available satellites and PDOP (Position Dilution of Precision)[See Figure 2].  For high accuracy data, a minimum of six available satellites is required.  However, a minimum of four is needed at any given time to calculate a geographic position unless elevation is known. In such situations, only three satellites are necessary to obtain a geographic position.  In general, better data will result from more satellites in good positions in the sky.

 


Data Dictionaries

A way to organize field data is to use a data dictionary within the rover unit.  A data dictionary is user-defined and stored in the rover unit prior to going into the field for data collection.  Because most of the data collected in demining will be incorporated into a GIS or translated into a hardcopy field map, most rover/field units use the traditional  “point, line, and area” methodology, because it fits well into a GIS.

In simple terms, a data dictionary gives the structure for data in a particular project.  The user defines a data dictionary by feature classes (points, lines, or areas), feature names, attribute names, and attribute values.  By doing this and collecting data in this way, data will already be in the desired “layers” ready for use in a GIS. Differential correction is often used to reduce the error, but this is not a requirement.


Rover Unit Settings

These are a few rover unit configuration setting recommendations that may help:

  • SNR - (Signal to Noise Ratio):  The higher the SNR, the more satellites received.  However, a high SNR can lead to faulty data
  • PDOP - 6 at most (this will prevent data collection when PDOP is greater than 6)
  • Coordinates - International Standards call for Degrees, Minutes, Seconds  (Use UTM only on a local basis)
  • Datum - WGS84


Base Station Note

The base station functionality is discussed in the "GPS Processing" section. When collecting GPS data for the purposes of mapping or ground truth verification, the data collector must be absolutely sure there is a GPS base station within 500 km and that it is operational, and that the data is accessible.

 

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