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Digital Elevation Models of the Earth's surface have a long history. Nevertheless, the first truly global DEM was create only ten years ago, and even the best of the modern varieties seem to have errors measured in tens or hundreds of metres. But there is hope that a global DEM with sub-metre errors is not far off.

This is significant because reliable DEMs are important for creating a number of products from other satellite sensors.

In a presentation at ISPRS, Jan-Peter Muller from University College London said that by 2013, a fully global 30-metre DEM based on US and Japanese data was highly likely. The real issue is whether or not such a global DEM will include bathymetry over the continental shelves of the continents, where the water depth is usually less than 100 metres.

As Muller recounts the history, the first global DEM was a NOAA DEM with 10 km postings. This was followed by GTOPO30 from the US Geological Survey, with postings at one kilometre. A study by Wil Featherstone and others at Curtin University in Perth showed that despite having errors of up to 100 metres, the dataset is still downloaded hundreds of times a day. It is obviously still fit for purpose, says Muller.

Since then, there have been a succession of attempts at a global DEM, most notably by the US-Italian Shuttle Radar Topography Mission. The global SRTM dataset has 90 metre postings, but it is full of holes, where the data is non-existent for a variety of reasons.

The USGS and the Earth Remote Sensing Data Acquisition Centre in Japan have agreed to co-operate on a fused ASTER/SRTM dataset to give 30 metre postings, using the Indian Cartosat optical satellite to fill in voids. Such a DEM has already been produced for India.

Another alternative that is also under consideration is the use of PRISM data from the Japanese Space Agency.

However, Muller said that none of these sensors would give coverage of the shape of the continental margins, where the water is less than 100 metres thick.

Speaking to ASM after his presentation, Muller said data did exist or was being created by most nations as part of UN agreements under which countries have the right to control the potential resources of the continental shelf. This requires that accurate bathymetry of the claimed region be submitted to the UN.

However, he noted that this data will likely not be released for 25 years.

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