Questions answered by Evan Bollard, GlobalPOS

Even though this may at times be considered more of a GIS question, it is very relevant to the operation of any GNSS receiver to ensure the correct datum is selected during use. If it is not set to collect data on the correct datum, or does not define what datum the data was collected on originally, then the data collected may not be as useful as first thought. It is very easy to collect very precise incorrect data.

In Australia the geodetic datum is currently GDA94/MGA94 (Geocentric Datum of Australia 1994/Map Grid of Australia 1994). Even though this is a geocentric datum it is not dynamic and may not be used by a particular GNSS receiver. Many receivers observe their positions on the ITRF (International Terrestrial Reference Frame) based on the current epoch, and then convert this value to a selected datum such as GDA94 (Lat, long coordinates) and MGA94 (UTM coordinates) based on a set of parameters (published or otherwise) at a specified epoch (ITRF 1992 for GDA94) If the drift of the Australian continent is not taken into account in these receivers by regularly updating these conversion parameters, or by using time derivatives in the setting of these parameters then incorrect results are the outcome.

One practical way of ensuring the output is as required is by observing existing published ground control marks to make a comparison between the observed coordinates and the published values. If operating in a ‘local’ coordinate system, these types of checks should also be performed for each job against known values.

Do you have a burning question about GNSS, need a practical answer to a problem you’ve encountered, or even just curious? Then, ask your expert! Send your questions to Evan care of [email protected]

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