Spectra Precision ProMark 800 Reference Manual - Manual (Page 60)

48
Precise Surveying - Field Applications & Concepts
Second Criterion:
Base Position
Known or
Unknown?
In addition to the good reception conditions required at the
base, you must also think about whether the base position
should be known with great precision or not. The explanations
below will help you understand what you need in terms of
base position accuracy.
1. If you want to obtain absolute, centimeter-accurate
positions attached to a particular coordinate system for all
your surveyed points, then the base position must be
known with the same centimeter accuracy in the same
coordinate system.
If the chosen position for the base is unknown whereas
you need centimeter accuracy for this point in the
coordinate system used, you can determine it through a
static post-processing survey. You will however need a
reference position to determine this point.
2. If you are only interested in performing relative
measurements (i.e. positions of points relatively to other
points), then the base can be installed on an unknown
point meeting the reception requirements. In this case,
the position to be entered in the base can be accurate only
to within a few meters.
Caution! In this case, keep in mind that you will not be
able to attach your points to a known coordinate system
unless later you accurately determine one of these points
in the desired coordinate system. With some field
software, such as FAST Survey, you can also use the
Localization function to attach your job to a local
coordinate system.
There are some disadvantages that you should be aware of
when installing a base on an unknown point. For every 15
meters of error between the estimated base coordinates
and the true base coordinates, one part-per-million (ppm)
of relative error will be introduced into the computed
vector between base and rover, plus the absolute
difference between the computed base position and the
real base position.
For example, assume that the coordinates assigned to the
base point are 30 meters off the true base position. This
30-meter offset from truth will produce 2 ppm (0.002 m
per kilometer or 0.010 ft per mile) of error in the vector
between base and rover.
If the rover is 5 kilometers (3 miles) from the base, this
will produce 0.010 m (0.030 ft) of error in the vector. In
most cases, the base receiver will estimate its position to