[postgis-users] Newbie questions: SRIDs, function return values

[Andy Anderson]

I haven't done this, but it does appear that projections can be  
defined on-the-fly in PostGIS. So for area you could define, for  
example, an Albers Conic or Lambert Azimuthal equal-area projection  
centered on the geographic bounding box of the region of interest and  
secant half-way through its extent. The areas calculated with this  
projection should at least be comparable, though probably also of  
equal inaccuracy.

Distance is more prone to directional inaccuracy, especially at  
smaller scales. It might work best to again determine the geographic  
bounding box, and then choose a projection based on the larger  
dimension (transverse Mercator for north/south or Lambert conformal  
conic for east/west).

-- Andy

> On Thu, May 15, 2008 at 6:20 AM, Burgholzer,Robert
> <rwburgholzer at deq.virginia.gov> wrote:
>> Paul,
>> Nice example on that generic spatial ref system for calculating
>> distances and areas in the continental US.   This reminds me of a  
>> thread
>> about this a week or two ago.  It seems to me that the need exists  
>> (or
>> at lease it could be useful) for an area/distance functions that can
>> make some intelligent guesses about the best coordinate system to
>> reproject (transform) a given set of geometries into for the  
>> purposes of
>> accurate measurements.  Something like:
>>
>> St_AreaUnits(geom_col, units)
>> St_distanceUnits(geom._col, units)
>>
>> I am wondering if there is some logic out there that could be  
>> applied to
>> create a function that would evaluate the extent of the area and  
>> compare
>> it to the projection information described in the spatial_ref_sys  
>> table
>> to determine what srid (with the desired units) would have the  
>> greatest
>> chance of encompassing the area of interest and then do the transform
>> and measurement for the user.
>>
>> Any recommendations as to the functions to look at within the  
>> existing
>> postgis suite to find the gizmos that pull apart the spatial
>> descriptions to determine this?
>>
>> Or is this line of reasoning based on flawed assumptions?