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The altimetry atlas

When the World Wide Web was in its infancy i was working at Delft University of Technology on the processing of radar altimeter data. Using a radar altimeter one can measure the distance between a satellite and the sea surface very accurately. Because the sea is rather flat, especially compared to land, and salt water reflects radar signals very well radar works particularly well for this purpose. Back then this distance could be measured with an accuracy of some five centimeters using corrections for factors like wave height and air pressure. If you compute the orbit of the satellite accurately you can measure the sea level height. Combine all the measurements over the world's oceans and you get a global map of sea surface heights. Local sea surface height is affected by local gravity and ocean currents. The local gravity effects tend to be small scale and constant and ocean currents are dynamic and large scale. So one can separate the two.

With my colleagues i made maps of the gravity anomalies over the oceans, computed from the sea surface heights. On those maps you can see the influence of sea mounts and ridges, pulling the local sea surface towards them ever so slightly towards them.There were even unnamed features visible because the sea floor wasn't so well known then.

We eagerly showed the maps to visitors and at conferences. But that was all we could do with them. One can write a paper about the methodology used but one cannot include a full set of maps. Journals are not supposed to be picture books..

Around that time Tim Berners-Lee had made a very useful invention at CERN: the World Wide Web. Using a simple server and even simpler pages with hyperlinks you could publish whatever you wanted. It was obvious that my maps looked nice, but i wasn going to make a book aout of them. So i proposed to put them all on the new web. That way the could at least be shared with the world.

The maps had to be organised to make them browsable. The maps were divided into different map sheets. An overview map showing the whole ocean provided links to the separate map sheets using a clickable image map, an early feature of HTML which luckily still works. Zooming into a map was technically not feasable yet so you zoomed in by clicking on the overview map. I made a perl script to generate the map sheet pages, including hyperlinks to the adjacent map sheet pages. Using the hyperlinks the reader could jump from one map sheet page the next.

Amazingly, Delft University has kept the pages online to this day. At the moment of writing the altimetry atlas is still online at In the decades since i made the maps a lot of radar altimeter satellites have been launched with much better accuracy than the one these maps are based on. And the dataset has grown immensely over the years. But amazingly the web version of the altimetry atlas still works. Because we had to use such simple functionality it has not become obsolete, even in modern web browsers.

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en/kaarten/altimetry_atlas.txt · Last modified: 2020/04/04 20:41 by Edwin Wisse