[postgis-users] earth distance via projections

[David Blasby]

> proj +proj=tpeqd +lon_1=21d0 +lat_1=52d15 +lon_2=4d54 +lat_2=52d21
>
> This PROJ command converts all input points to x/y-coordinates from 
> which correct distances can be computed to either Warsaw or Amsterdam. 
> This method has been used by Bell telephone to calculate costs for 
> long distance calls.
>
> Can this be done with PostGIS? PostGIS can transform coordinates using 
> standard EPSG projection parameters, but can it also do 
> transformations with user defined parameters, like lat/lon above?


I was going to suggest this, but its a bit more complex - you have to 
use some lower-level projection routines:

 transform_geometry(geometry,text,text,int)
    -> tranform_geometry( <geom>, <proj4 string - input projection>, 
<proj 4 string - output projection>, <new SRID>)

Plus you have to build the proj4 string up from your input points (use 
the concatenate operator (||) and the X(point), Y(point) functions).

> PS: PostgreSQL's contrib directory has an "earthdistance" function, 
> which computes distances over a grand circle. Does anyone know how 
> exact this is, compared to a projected computation?

I it does distances on a sphere not an ellipse.  If its okay to use that 
approximation, the routines are probably pretty quick and easy to use.

dave