[postgis] distance(geometry,geometry) function

[Frank Warmerdam]

Dave Blasby wrote:

>> I assume you mean euclidean distance.  A lot of us keep our locations in
>> geodetic coordinates.  It would be nice if geometries could reference a
>> particular datum so these things would be handled transparently.
>>  (Postgis is nonetheless a very exciting development.)
> 
> 
> 
> Yes, these are euclidean distances.  The calculations in geodetic
> coordinates are complex.  
> 
> We have a function that, given a spheroid and 2 geodetic points, will
> calculate the distance between them.  In order to do distance() in
> generality, I need to be able to;
> 
> 1. Given a spheroid and the points A, B, and C.  A-B define a line in
> geodetic space (a great circle).  Find the minimum distance (also a
> great circle) between the line AB and C.
> 
> 2. Given a spheroid and the points A, B and U, V.  A-B and U-V define
> lines in geodetic space.  Tell me if they intersect or the minimum
> distance between then.
> 
> 3. Given a spherioid, the point A, and the polygon P tell me if point A
> is inside the polygon P.
> 
> Then I'd need to dust off my spherical geometry and make sure my
> euclidean assumptions will hold on an ellipsoid.
> 

Dave,

PROJ.4 does include a geodetic great circle distance calculation program. 

I wonder if it would be best putting off such advanced services untill 
you can build an entire
projections library into PostGIS.  I would certainly promote PROJ.4 as 
an option.  However,
I would assume this (building in full OGC projection support) would be a 
ways down the road
for PostGIS.

Best regards,

-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent


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