SRTM

A small library that provides a simple interface to query locations on the Earth for elevation data from the NASA Shuttle Radar Topography Mission (SRTM).

Installation

Add srtm to your list of dependencies in mix.exs:

def deps do
  [
    {:srtm, "~> 0.9"},
  ]
end

Examples

iex> SRTM.get_elevation(36.455556, -116.866667)
{:ok, -51}

Elevations are in meters. The lookup reads the sample the coordinate falls into rather than interpolating between the surrounding ones, so the answer describes a grid cell roughly 30 m across. Coordinates the dataset holds no measurement for — voids, and open water in the raw tiles — come back as nil:

iex> SRTM.get_elevation(2.984654, 59.686144)
{:ok, nil}

Caching

Elevation data ships as HGT files, each covering one degree of latitude and longitude and taking up about 25 MB. The first lookup inside a cell downloads its file; later ones are served from the caches.

Downloaded files are written to ./srtm_cache by default. Point :disk_cache_path somewhere else, or turn the disk cache off to keep them out of the file system entirely:

SRTM.get_elevation(36.455556, -116.866667, disk_cache_path: "/var/cache/srtm")
SRTM.get_elevation(36.455556, -116.866667, disk_cache_enabled: false)

Parsed files are kept in memory with :persistent_term as well, trading memory for lookup speed. Read SRTM.Cache.PersistentTerm before caching a large number of cells; its purge/0 frees the memory again.

Sources

Files are downloaded from SRTM.Source.AWS and then SRTM.Source.ESA, in that order, until one succeeds. AWS serves the Terrain Tiles dataset, which covers the whole globe. ESA serves the raw SRTMGL1 tiles, which span 60°N to 56°S and omit open water, so it only works as a fallback behind a source with global coverage.

Sources and caches are both behaviours, so you can plug in your own — see SRTM.Source and SRTM.Cache.

See the documentation for further information on configuration.

License

MIT