[Proj] Projecting a rectangle to another coord. system?
Mikael Rittri
Mikael.Rittri at carmenta.se
Mon Sep 13 07:01:10 EDT 2004
Thanks for all replies.
Ed wrote:
>Glynn's response is right on the mark. But are you asking a
>really general question, or do you have specific values for A
>and B? If A and B come from a relatively limited list (rather
>than needing to support ANY arbitrary projection) the job, and
>Glynn says, can be made much simpler or even trivial.
I meant to be really general (hoping to find a published
algorithm for the most general case.)
The company where I work sells a toolkit (www.SpatialAce.com)
for building GIS systems. Currently, the toolkit supports about
nine (families of) projections, so we have special methods to
project a rectangle from projection A to a rectangle in
longitude/latitude, and then to projection B. As you say,
for most of the projections (but not all), projecting to
or from a long/lat rectangle is almost trivial.
But if, in the future, we would like to provide many more
projections, it would probably be worthwhile to implement
a general solution that works for any pair of projections.
And it must handle discontinuities well, since you never
know what an end user might do...
By the way, I disagree with Glynn's final sentence:
>There are cases where this approach can fail, i.e. where the
>projection of an interior point lies far outside of the
>projected border (e.g. projecting from a polar projection
>to a cylindrical one). However, these cases tend not to occur
>in practice, as the level of distortion tends to make the resulting
>data too inaccurate to be of any practical use.
Although I agree that data stored in a polar projection should not
be viewed in a cylindrical one, I think that one may have geodata
stored in longitude/latitude, which should be viewed in a polar
projection.
Again, thanks for all replies,
Mikael Rittri (Mikael.Rittri at carmenta.se, www.carmenta.se)
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