Guidelines for the use of GNSS in surveying and mapping

Table I: Co-ordinate systems
Guideline Explanation Pt. 2 ref.
Use good current national control points for
surveys on the national co-ordinate system.
When performing a survey in a national co-ordinate system, it is essential to obtain at least two fixed points
with co-ordinates in the national system. Best practice is to obtain three or more points in the national
system which will allow a full check on the accuracy of the survey in the national system. A full discussion
on the use and concerns of national co-ordinate systems with GNSS surveys is explained in Part 2.
6.3.5
7.2.3
Use a good single point position (SPP) for the
reference station in local grid surveys.
When using GNSS for small control or detail surveys on an arbitrary local grid, the co-ordinate for the
reference station should be accurate to about 10m. This can be achieved by using a single point position of
a few minutes’ duration.
6.3.5
For small local grid surveys, use the
manufacturer’s recommended local grid
transformation.
As many real-time surveys are performed in small (less than 2km) areas, most GNSS manufacturers provide
a simple best-fit transformation from ITRS89 to a local grid. Use this procedure, but ensure the survey is
well within the area defined by the control points.
6.3.5
Use a rigorous transformation wherever
possible to compute national co-ordinates
directly from ITRS89 co-ordinates.
This will allow re-computation of data and co-ordinates from one system to another. This is good practice,
as any errors due to transformations can be eliminated and co-ordinates can be returned to the ITRS89
system. If co-ordinates are required in a national system, use the best possible transformation available at
the time of the survey.
6.2
6.3
Keep a back-up copy of all surveys in the
ITRS89 co-ordinate system.
Should it be found that an error in the transformation process has taken place, this will allow the recomputation
of the survey into any other co-ordinate system.
6.3.5
Follow the co-ordinate system guidelines for
large engineering projects.
For larger GNSS networks (greater than 15–20km or so), use a final co-ordinate system appropriate to the
project. North–south projects can use a customised Transverse Mercator projection. East–west projects can
use a customised Lambert Conformal Conic projection, and other orientations can use a customised
Oblique Mercator projection. If national co-ordinates are also required, compute these separately, directly
from the ITRS89 co-ordinates.
6.1.5
6.3.3
GUIDELINES FOR THE USE OF GNSS IN LAND SURVEYING AND MAPPING |21