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mapping water drainage with SRTM elevation data in Python

September 18, 2009 Leave a comment

srtm enhanced relief map

With this mini-project, I wanted to try to model the effect of rainfall on the landscape, to see if I could map water drainage – finding watersheds, catchment areas and so on. After lots of tries, I stumbled across a potentially interesting mapping technique by accident, which does a good job of providing a relief map that emphasizes water courses.

The algorithm starts with a square grid of SRTM digital elevation data. (You can read more about this in a previous post)…

For each cell in the grid, a virtual raindrop falls. This drop then repeatedly moves to the lowest neighbouring cell, each time keeping a running total of how many meters it has descended by. Eventually, it falls into a local minima; a ‘well’, a cell with no neighbours lower than itself. This can be the coast, or it might be a lake or a low-lying piece of land in a valley. When it can’t fall any further, the map is coloured according to the total vertical distance travelled.

The height of the cell is relative to the nearest patch of flat land, rather than being relative to sea level. This makes it easier to follow the lie of the land.

The image here is a map of the degree square covering Edinburgh and the Scottish Borders (Dumfries is in the south, Edinburgh in the top-right corner).

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