[postgis-users] Orthographic projection problems

[Kevin Jardine]

I'm new to PostGIS and I'm trying to transform a latlong (SRID 4326) map 
  of the world into an earth-from-space orthographic projection.

I added this Proj 4 definition:

insert into spatial_ref_sys (srid, proj4text) values 
(200000,'+proj=ortho +lat_0=%lat +lon_0=%lon +x_0=0.0 +y_0=0.0 +units=m')

where %lat and %lon is the centre point of the orthographic projection.

My first attempts failed because adding something like

where distance(GeometryFromText('POINT(%lon %lat)',4326),the_geom) < 90

to my latlong geom select still selected lots of points that were not 
projectable (they are "behind the Earth").

I used the equations here:

http://mathforum.org/library/drmath/view/52049.html

to construct a polygon that I can use in an intersection to filter out 
the non-projectable points before the transform.

This works most of the time, but I still sometimes find that either the 
intersection or (usually) the transform fails, typically in a nasty way 
that crashes my PostgreSQL backend with a Signal 11 :(

I am thinking that surely there is an easier way to use PostGIS to do 
orthographic projections.

Any suggestions?

By the way, I am using proj 4.4.8, postgresql 7.4.5, and the latest geos 
2 and postgis cvs versions from a couple of days ago on a Linux box with 
2 Gb of RAM.

Cheers,
Kevin

-- 
Kevin Jardine
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Greenpeace International

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