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Hello Samuel,

I've used the SRTM data that John Jacobs mentioned above, the newer release, with about 30m intervals. Overall, the data was great, but you need to beware of some substantial offsets (10-20m), which mean you need to adjust elevations if you do field surveys. This was in DRC, elevations around 800m above sea level, in a fairly hilly area.

As a first approach, it provides a fantastic "preview". You can get the data into QGIS, and design a basic network with the EPAnet plugin, all for free. It is only useful over fairly big distances, otherwise, the noise in the data and the resolution will make your calculations meaningless. Also, you need someone fairly comfortable with GIS systems, as the process is not super easy, so this might be a show stopper for you.

If you're going to invest substantial money in building a network, I'd advise you add some sort of field survey, at which point you're back to abney levels or smartphone apps.

Laurent