Geo.Turf

An Elixir spatial analysis tool for the Geo library ported from TurfJS.

Warning

At the moment the library is in early development mode. The API could, and propably will, change on any moment.

Installation

Add geo_turf to the dependencies in mix.exs:

def deps do
  [
    {:geo_turf, "~> 0.5.0"}
  ]
end

WGS84 and SRID metadata

GeoTurf's geodesic calculations require WGS84 longitude/latitude coordinates (EPSG:4326). Public geodesic functions accept Geo structs whose srid is either 4326 or nil.

nil does not mean that GeoTurf detected WGS84. It means that the caller is asserting that the coordinates are WGS84, matching the default metadata on Geo structs. GeoTurf validates declared SRIDs but cannot infer a coordinate reference system from coordinate values.

Geometries with another declared SRID must normally be reprojected before use. If coordinates are already WGS84 and only their metadata is stale, correct the individual geometry explicitly:

geometry = %{geometry | srid: 4326}

Geo.Turf.Helpers.assert_wgs84!/1 applies this validation recursively to GeometryCollection children and lists. There is deliberately no global validation bypass, because disabling the guard would allow projected coordinates to produce plausible but incorrect geodesic results.

Usage

The library can perform functions on Geo objects as well as basic mathematic functions useful in spatial analysis. At the moment Geo.Turf expects WGS84 coordinates.

For example:

• Geo.Turf.Measure.along/3: Make a %Geo.Point{} at a definded distance from the start of a %Geo.LineString{} .
• Geo.Turf.Measure.along_midpoint/1: Make a %Geo.Point{} at a the middle of a %Geo.LineString{} .
• Geo.Turf.Measure.center/1: Makes a %Geo.Point{} at the center of a Feature.
• Geo.Turf.Measure.close_to/4: Check if 2 %Geo.Point{} items are close to each other.
• Geo.Turf.Measure.distance/3: Check the distance between 2 %Geo.Point{} items.
• Geo.Turf.Measure.length_of/2: Gives the length of a %Geo.LineString{} or the circumference of a %Geo.Polygon{}

Suggestions

If there are functions you need from TurfJS or just things you thought of please just make an issue for it. Including a test for it would be great.