Introduction
97
System Guide to Post-process GPS Surveying
Surveying with GPS
A GPS surveying system consists of at least two GPS receivers. The receivers collect
data simultaneously to determine the location of one receiver relative to the other(s).
The positional relationship between the receivers is presented in the form of a vector,
i.e. (delta X, delta Y, delta Z) or (delta N, delta E, delta H) between the receivers. This
is a 3-dimensional relationship. It is analogous to a conventional observation of
horizontal angle, vertical angle and slope distance. Vectors are produced between all
combinations of GPS receivers used during the survey.
It's important to remember that GPS observations result in vectors defining the
relationship between the points observed. In other words, it is the vector between the
points that is the result of a GPS observation, not the coordinates of the surveyed
points themselves. To determine coordinates of surveyed points, a set of control
coordinates must first be supplied for one of the points. Based on this set of
coordinates and the GPS vectors observed for all other points, coordinates for the
other points can be computed. In this manner, coordinates for the newly surveyed
points are determined. Similar to a conventional traverse, your initial set of
coordinates can originate from a known control point incorporated into your survey.
Alternatively, assumed coordinates can be used.
Post-process GPS Surveying
Post-process GPS surveying can be broken down into two primary tasks, data
collection and data processing. Let's examine each task in detail.
Post-process Data Collection
Data collection can be performed using two different methods; static and kinematic.
The following is a description of each method.
Static Data Collection
As the name implies, static data collection is stationary in nature. The GPS
systems simultaneously collect raw data from all available satellites while
remaining stationary on their respective points. Data collection continues at these
locations for an amount of time dependent upon several factors including the
distance between the receivers, the satellite geometry, and obstruction conditions
at the data collection locations (for example, trees or building blocking some of
the sky). When data collection is complete at these points, the GPS systems may
move to a new set of points to begin another static data collection session. In
most cases, one GPS system will remain on its current point in order to link the
previous set of points to the new set of points. After data collection is complete,
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