SatRef - Knowledge Sharing Platform
Procedure for GPS Height Transformation
within a Small Local Area
JAMES WONG, LS/G(NT)
SatRef - Knowledge Sharing Platform
Purposes of this briefing
• To share the knowledge of how to create a Localized Height
Transformation Parameter File for the transformation of GPS
surveyed ellipsoidal height to HKPD Height in a small area.
• To release the height model data set for the creation of the
Localized Height Transformation Parameter File.
Relationship between Ellipsoidal Height (GPS Height) and
Orthometric Height (Local Height)
Current Problems
• The existing 7-Parameter Set published by the Geodetic Survey Section of
SMO is used for the datum transformation between ITRF96 and HK1980
Geodetic Datums.
• It is not used for transformation between the GPS surveyed ellipsoidal
height and orthometric height (local height).
• Hence, the problem is how to transform the GPS surveyed ellipsoidal height
to orthometric height for the determination of the local height for various
applications.
• Actually, there is no high accuracy Geoid Model of Hong Kong for the
determination of geoid-ellipsoid separation for the GPS height
transformation.
Height Model Data Set
Height Model Data
Which approach to use
• This question is almost impossible to answer since the approach used will depend
totally on local conditions and information.
• If you wish to keep the GPS measurements totally homogenous and the information
about the local map projection is available, the Classical 3D approach would be the
most suitable.
• If you are unsure of the local height information but the position information is
accurate and you wish to keep the GPS measurements homogenous in position, then
the Stepwise approach may be the most suitable.
• For cases where there is no information regarding the ellipsoid and/or map projection
and/or you wish to force the GPS measurements to tie in with local existing control
then the One-Step approach may be the most suitable. Alternatively if a large
number of common points are available and a more accurate approximation is
required the Interpolation approach can be used.
• The Two-Step approach also treats position and height information separately which
allows for position only control points to be used as well. Compared to the One-Step
approach, information regarding the ellipsoid and map projection has to be known.
The advantage is that this approach can be used for larger areas than the One-Step.
Two Step
• This transformation approach works by treating the height and
position transformation separately. For the position transformation the
WGS 84 coordinates are first transformed using a Classical 3D pretransformation to obtain preliminary local cartesian coordinates.
These are projected onto a preliminary grid using the specified
ellipsoid and map projection. Then the 2 shifts, the rotation and the
scale factor of a Classical 2D transformation are calculated to
transform the preliminary to the “real” local coordinates.
• The position transformation requires knowledge of the local map
projection and the local ellipsoid. However, as the distortions of the
map projection are taken into account, Two Step transformations can
be used for larger areas than One Step transformations.
• The height transformation is a single dimension height approximation.
Two Step
Advantages:
•
Errors in local heights do not affect the position transformation
•
The points used for determining the position and height
transformation do not necessarily have to be the same points.
•
The distortions of the map projection are taken into account which
enables you to use this kind of transformation for larger areas.
Disadvantage:
•
Knowledge of the local projection and local ellipsoid are required.
Establishing Transformation Relationship
using Leica LGO V3.0
Data Set (ITRF96 Geodetic Coordinates)
Data Set (HK80 Grid Coordinates + Height above HKPD)
Datum and Map Function
Datum and Map Function
Datum and Map Function
Datum and Map Function
Datum and Map Function
Datum and Map Function
Two Step – Transformation Report
Two Step – Transformation Report
Two Step – Transformation Report
Two Step – Transformation Report
Final Product :
A Localized Height Transformation Parameter File
• A localized height transformation parameter file called
“GPSTRF.DAT” or “TRFSET.DAT” is produced by Leica LGO
V3.0/ SKI-Pro V3.0.
• Users can import the finalized transformation parameter file
into the Leica LGO/ SKIPro software for coordinates
transformation in office.
• Users can also import the finalized transformation parameter
file into the Leica GPS receivers for carrying out RTK/ DGPS
surveys in real-time on site.
Example
Establishment of a Height Transformation Relationship
for Kowloon Area
Example: Height Model of Kowloon Area
Data Set (10 control points used)
Two Step – Transformation Report
Checking Point Distribution for Testing of the Finalized Height
Transformation Relationship (Total no. of points = 22)
Conclusions
• Average difference in Height above HKPD = 1 cm
• Maximum difference in Height above HKPD = 4 cm
• The height differences depends on the GPS measurement accuracy
and the height transformation accuracy.
• The height transformation accuracy is about 2 cm in this small area.
(i.e. 11 km x 12 km area)
Remarks
• This procedure is suitable for establishing GPS height transformation
relationship within the small local area, say 15 km x 15 km area.
• Checking points should be provided to test the transformation accuracy.
• The number of control points of the height model data set will be increased
if resources are permitted.
• The accuracy of the control points will be improved as far as possible.
• A Geoid Model should be eventually produced for providing the
territory-wide solution of the geoid-ellipsoid separation for GPS height
transformation.
Thank you !!