Guidelines for the use of GNSS in surveying and mapping

6.3.5 Co-ordinate transformation issues in
GNSS
In GNSS land surveys, there are several key coordinate
system and transformation issues that
should be considered.
First, there is the use of national control points. For
any project a minimum of three but preferably four
points should be obtained to control the survey
network. An indication of the actual accuracy of
these points should also be obtained from the
national mapping agency. This needs to be
considered, as older control values from
triangulation surveys will not be as accurate as
GNSS values. If these older ‘fundamental coordinates’
are held fixed in any network adjustment,
the GNSS survey geometry will be distorted and
errors will result in the survey. Therefore, good
quality, recently surveyed national control points
should be obtained for any survey which requires a
tie to a national grid.
Secondly, if no national grid co-ordinates are
required and no national control is used for a
survey, it is important to use an accurate position in
ITRS89 as the basis for the survey. If this is not
done, scale errors will result in the baselines
observed from this base point. The approximate
magnitudes are shown below in Table M.
Thus, it is important to take a single point position
(SPP) using at least a low-precision static method,
preferably using dual-frequency data (see section
5.1). The results of this SPP should also be quoted
in any survey reporting, together with an estimate
of the accuracy of the final ITRS89 position. If the
survey is required to be later placed on the national
grid, errors will also result as the SPP ITRS89 point
used for the survey will only be accurate to a few
metres. It will also be difficult to ensure correct
transformations between co-ordinate systems if
future work is to be carried out.
Third is the use of manufacturers’ own single-step
transformations. These are often offered by GNSS
equipment manufacturers as a simple method of
obtaining true scale (flat earth) local grid coordinates
from ITRS89. In the worst case, an
arbitrary ITRS89 point is used with the
manufacturer’s own single-step transformation for
the whole survey. If future work is to be carried out
using another manufacturer’s equipment, many
potential problems could arise. It is far better to
conduct the control element of the survey
separately, based on precise ITRS89 points and
using rigorous transformations from ITRS89 to the
final local grid. It is best practice to use single-step
transformations generated from a set of four or
more points that have accurately surveyed coordinates
in ITRS89 and in the local grid. It is
critical that detail observations are completely
within the control framework when using singlestep
transformations.
If a manufacturer’s single-step or similar
transformation is used, all parameters and
transformation types should be quoted in any coordinate
listing or in the survey report with
equipment and software versions. A back-up copy
of the survey should also be kept in ITRS89 for any
other later transformations as required.
Table M: Scale error magnitudes
Error in true ITRS89 position (m) Baseline scale error (ppm) Approx. error at 5km
2m 0.3 2mm
10m 2.6 13mm
100m 25.5 130mm
48 | GUIDELINES FOR THE USE OF GNSS IN LAND SURVEYING AND MAPPING