Print

We often speak about Mean Sea Level but may not realise the significance of this important benchmark position. Mean Sea Level changes depending where on earth it is being measured, and knowing where zero or Mean Sea Level is has important implications.

Recently GeoScience Australia published information about the new AUSGeoid09, that describes the average height datum (AHD) for the country. As that organisation states, "AUSGeoid09 is a three dimensional model used to convert ellipsoidal heights (as observed by GPS) to AHD heights to within ±0.050 metres accuracy across most of Australia. For example, if you use a GPS receiver to compute the height of your house, it will provide you with the ellipsoidal height. By subtracting the value of the AUSGeoid09 model at the latitude and longitude of your house, you can compute the AHD height."

Since Mean Sea Level varies from location to location, many countries have developed similar three dimensional models. Each of these models attempts to more closely represent the Mean Sea Level heights surrounding their nearest ocean shores. These average heights can vary considerable, as shown by 50 year average sea level collected for an area in Japan.

The measurement and calculation of Mean Sea Level follows specific procedures. Height Modernization describes the attempts to calculate heights through the use of GPS. Matthew Higgins of the Australia Department of Natural Resources and Mines describes Guidelines for GPS Surveying in Australia.

Where height measurement is being calculated or considered in applications such as flooding, mining and and other critical areas, knowing mean sea level is an important first step.

ASM Newsletter is distributed by e-mail every week. It reports on events in the spatial industry in Asia and events outside the region which will impact on Asia.