Spectra Precision ProMark 800 Reference Manual - Manual (Page 80)

68
Precise Surveying - Field Applications & Concepts
Please refer to the specifications sheet of the model you are
using for more information.
Obviously, accuracy figures deteriorate when the system fails
to fix the position.
Accuracy
Measures Errors on coordinates determined with GNSS systems are not
constant (the solution varies statistically).
If you plot the horizontal coordinates of a reference point (X0,
Y0) computed by a GNSS system over a significant period of
time (static survey), you will obtain a scatter plot such as the
one below.
The origin of the (X0, Y0) axis system is the true position of
the reference point. Each dot represents a position solution
delivered by the GNSS System for this point.
How you analyze the scatter of solutions results in a different
accuracy figure characterizing the performance of the
system.
The main accuracy measures used by GNSS manufacturers
are the following:
1. rms (root mean square): accuracy is obtained by
computing the square root of the average of the squared
errors (a statistical method).
If error distribution along each axis is Gaussian (it is in
general), i.e. the mean error converges to zero, or close to
zero, then an error probability may be associated with the
rms accuracy. This probability is about 68%, which means
the computed position will be within the announced
Accuracy (rms) RTK Post-Processing
Horizontal 1 cm + 1 ppm 0.5 cm + 1 ppm
Vertical 2 cm + 1 ppm 1 cm + 2 ppm
X0
Y0