ftp://e0srp01u.ecs.nasa.gov/../srtm/version2/SRTM3/

You would have to change the “.gov/srtm” part in my download script to “”.gov/../srtm”:
http://github.com/Sjors/srtm2postgis/

I have had a quick go at putting together a simple server to return the height of a particular point using this data – I read all of the files into arrays in memory (only the uk is covered, so you need to use positions like (54,-1) – it is then a simple matter of checking which file contains the required data point, and grabbing it from an array (I do not do interpolation yet, so it is not precise.
It should be visible to the outside world at http://maps.webhop.net:1281/ if you are interested. The obscure port number is because it is a pure python server rather than running via apache, so I didn’t want to use port 80.
My real intention is to accept a GPX file describing a route, and return an elevation profile of it – a job for tomorrow evening….

Graham.

I have tried to explain it somewhat in the comments of the function posFromLatLon(lat,lon), but it could use a bit more clarification.

There are several things going on in that function. First it finds the correct tile. Then it figures out where your coordinates are within that tile. This will probably not be directly on top of a data point, so in stead it will give you the four closest points and the distance towards these points.

The application then fetches the altitude of these four points and performs bilinear interpolation.

At bit more background about the index:

As you have guessed correctly I use a bigint to create a unique index for every SRTM data point. The original data set consists of lots of tiles, each of which is exactly 1 degree longitude by 1 degree latitude.

Now here comes the more difficult part. Each tile has been given a name (e.g. “N45E001.hgt.zip”) that represents its bottom left (south west) corner. However the file itself reads like a book: it starts in the top left corner. So the first column of the last row is located at 45′ north and 1′ east and the first column of the first row is located at 46′ north and 1′ east.

I have created an index such that the tile “N00E00″ has index 0. Tiles are 1200 rows by 1200 columns, so the tile directly east of it (“N00E01″) starts at position 1200 * 1200 =1440000. The tile directly west of it starts at position -1440000. Since there 360 degrees of longitude, the next latitude starts at 1200 * 1200 * 360.

Once you know where a tile starts, you just need to know that the tile content starts at the top left corner and reads like a book.

Cheers

Graham.

Glad to hear that!

In retrospect, a big binary file (or several) might have been a better approach for this project. I was surprised to see the per record overhead that using a database creates [0].

The good news is that I recently abstracted the database part. There is a class Database that has only one method: fetchAltitude(position). So if you are in the mood, you could write a class that accesses the data directly from a file and it should work fine with the rest of my code.

Michael Kosowsky from http://www.heywhatsthat.com/
wrote C++ code to read the .hgt files directly rather than
through a database.

[0] http://code.google.com/p/route-altitude-profile/issues/detail?id=1

Cheers

Graham.