# [postgis-users] curve between geo points

François B osgeo at bonzon.com
Tue May 23 10:12:10 PDT 2017

```It depends on the type of your column. If it's standard "geometry", then
it's a straight line in the 2D space defined by your coordinate system. If
it's "geography", then it's a curve, aka geodesic or great circle.

>From the manual:

4.2. PostGIS Geography Type
https://postgis.net/docs/using_postgis_dbmanagement.html#PostGIS_Geography

The basis for the PostGIS geometry type is a plane. The shortest path
> between two points on the plane is a straight line. That means calculations
> on geometries (areas, distances, lengths, intersections, etc) can be
> calculated using cartesian mathematics and straight line vectors.
>

> The basis for the PostGIS geographic type is a sphere. The shortest path
> between two points on the sphere is a great circle arc. That means that
> calculations on geographies (areas, distances, lengths, intersections, etc)
> must be calculated on the sphere, using more complicated mathematics. For
> more accurate measurements, the calculations must take the actual
> spheroidal shape of the world into account, and the mathematics becomes
> very complicated indeed.

Note that many data formats (outside of PostGIS, e.g. Shapefile), do not
explicitly define this interpretation. See this Stack Exchange question:

Are long lines in shapefiles to be considered geodesics or straight lines
in the 2D latlong space?
https://gis.stackexchange.com/q/25670/7599

I'd like to know how PostGIS interprets the curve between two points on the
> earth, in the context of spatial analysis. Ex, the NW and NE corners of the
> State of Colorado - is it a 'straight line' in some projection, or a
> geodetic curve or the intersection of a normal plane with the surface (as
> in MS Sequel). Possibly, the function called implies this (2D VS. 3D).
>

> Redirecting to online documentation or discussion would be welcome.
>

> Best regards,
>

> Jim Walseth
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