[postgis-users] earth distance via projections

Paul Ramsey pramsey at refractions.net
Tue Apr 15 09:03:50 PDT 2003

Proj has some decent great-circle stuff too. May we should use the proj 
functions instead of our homebuilt functions in cases were proj has bee 
compiled into postgis.

Jan Hartmann wrote:
> Dave Blasby wrote:
>  > There are lots of ways of doing this.  There are routines for doing
>  > distance calculations on an ellipsoid, but I think its more general to
>  > re-project your data.
>  > ...
>  > 1. Find a cartesian projection thats accurate for where your data is
>  > (ie. UTM zone whatever)
> There is a more general way of measuring earth distances via 
> projections, (using standalone PROJ), but I don't know if this can be 
> done easily within PostGIS. Some projections give true distance from one 
> or two control points. An example is the "Two Points Equidistant" 
> projection, which gives correct distances from its two control points to 
> any other point on the map. The PROJ command (with Warsaw and Amsterdam 
> as control points) would be:
> 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?
> Jan
> 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?
> Pedro Salazar wrote:
>> On Tue, 2003-04-15 at 15:28, Pedro Salazar wrote:
>>> David,
>>> 2) What is the SRID for measuring the distance between geometries in
>>> meters? I'm using the SRID 4326 (WGS84). So, to what system should I
>>> transform my geometries to get the distance in meters?
>> I got it. I already understood that I have to project for a UTM
>> projection for example..
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      | Paul Ramsey
      | Refractions Research
      | Email: pramsey at refractions.net
      | Phone: (250) 885-0632

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