AUSPOS FAQ-4. Understanding the results

Online GPS Processing

• Question 4.1 - How do I read the processing report?
• Question 4.2 - What part of the report should I read?
• Question 4.3 - The ARGN GDA values differ from the 'official' values?
• Question 4.4 - The GDA94 'above Geoid height' differs from AHD?
• Question 4.5 - The system gives GDA and ITRF coordinates, what about WGS84?
• Further information

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Question 4.1 - How do I read the processing report?

The processing report is provided in ADOBE PDF format. To view the report you will need at least Acrobat Reader 4.0, which is free software from Adobe.

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Question 4.2 - What part of the report should I read?

The first time you should read it all. You should check every time that:

• the solution RMS is within a valid range;
• the height measurement is correct; and
• the a priori coordinate corrections are small and the coordinate precision is valid.

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Question 4.3 - The ARGN GDA values differ from the 'official' values?

For Australian users the online GPS processing service provides Geocentric Datum of Australia (GDA94) coordinates, as well as International Terrestrial Reference Frame (ITRF) coordinates, including any Australian Regional GPS Network (ARGN) GPS stations used in the processing.

The online service ARGN GDA94 coordinates will differ, at the mm level horizontally, from the 'official' GDA94 coordinates for the ARGN stations.

There is good reason for this. We consider the ITRF as highest accuracy reference frame available today, and while, when computed GDA94 was of high quality, the modelling and processing strategies used today are greatly improved. With this in mind, we process all our data in the ITRF and later transform all the stations in the processing job into GDA94, including the ARGN stations using transformation parameters [PDF_280k] derived from the National Mapping Division daily GPS solutions of the ARGN. So, any differences between the 'original' GDA and the online gps processing service GDA, are as a result of both the errors in the original GDA94 GPS solution, and modelling errors in our transformation process. Since these errors are only several mm in magnitude they can be ignored.

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Question 4.4 - The GDA94 'above Geoid height' differs from AHD?

In the GDA94 section of the AUSPOS report [PDF_107k], a component 'above geoid height' is given. This is computed by taking the ellipsoidal height resulting from the processing and subtracting the AUSGeoid98 value for that site. As such, the result is a reasonable approximation of Australian Height Datum (AHD), but, as there is a slope ranging from about +0.4 m in the north of Australia, to about -0.4 m in the south, between the geoid and the AHD, it is possible for the AHD value derived in this way to be perhaps half a metre different from the AHD value.

This AHD/geoid slope has become evident from a recent accurate GPS survey of the original defining AHD tide gauge bench marks, and is thought to be largely due to sea surface topography. Further investigation is currently underway, but there is a lot more data to be collected (particularly inland) before an exhaustive analysis will be available. In the meantime, it is best to use AUSGeoid differentially to accurately determine AHD values from GPS ellipsoidal heights.

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Question 4.5 - The system gives GDA and ITRF coordinates, what about WGS84?

Because it is very difficult to determine precise (<0.1m) WGS84 coordinates - ITRF and WGS84 are generally considered to be equivalent. To our knowledge, no transformation parameters exist between ITRF2000 and WGS84 because of their close alignment. Please note that within Australia GDA coordinates should be used, and within Australia, GDA should be considered equivalent to WGS84.

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Further information on this topic

• WGS84 and the Geocentric Datum of Australia 1994 [PDF_108k]
• ITRF and WGS84
• See "Malys, S. and J. Slater, 1994, Maintenance and Enhancement of the World Geodetic System 1984, Presented at The Institute of Navigation, ION GPS 94, Salt Lake City, Utah, September".

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