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.
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
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.
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
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
- 6 at most (this will prevent data collection when PDOP is greater
- 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.