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
<[EMAIL PROTECTED]> 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?
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