POSITION-FIXING 95
If several GPS points are laid over a site-plan, for exam-
ple, errors may be encountered over and above the limits
of accuracy. Again, this occurs when a plan surveyed using
a flat grid, based on the assumption of a flat earth, is com-
pared with GPS positions calculated on the spheroid.
This is a significant factor when using highly accurate
systems such as RTK GPS. If the distance between two
stations is measured using a tape-measure, and then an
RTK GPS unit is used to measure the coordinates and
calculate the distance between them, the result will be two
different values. This is because the RTK GPS unit is meas-
uring the distance and taking into account the curvature
of the spheroid. To obtain comparative measurement, the
scale factor (a value representing curvature) must be applied
to the direct distances measured by the tape-measure.In summary, GPS is an extremely useful tool for
position-fixing. It is relatively affordable and can result in
sub-metric accuracy given the right equipment and con-
ditions. It is essential, however, when using GPS, to be aware
and have a clear understanding of the issues discussed in
this chapter.Further Information
Ackroyd, N., and Lorimer, R., 1990, Global Navigation: A GPS
user’s guide. London.
Bettes, F., 1984, Surveying for Archaeologists. Durham.
Dana, P. H., The Geographer’s Craft Project. Dept of Geography,
The University of Colorado at Boulder. http://www.colorado.
edu/geography/gcraft/notes/datum/datum.html (updated
2003).
Judd, P. and Brown, S., 2006, Getting to Grips with GPS: Master-
ing the skills of GPS navigation and digital mapping. Leicester.
Schofield, W., 2001 (5th edn), Engineering Surveying. Oxford.