[GRASS-user] Bug?: Projecting the same region from location to
location and back gives a different result!!!!
Glynn Clements
glynn at gclements.plus.com
Wed Nov 5 17:06:02 EST 2008
Corrado wrote:
> a very strange behaviour indeed:
>
> 1) location "A", one region defined TM, datum osgb36, region
> s=0,w=0,e=800000,n=1300000
>
> 2) location "B", one region defined lat/long, wgs84 180W,180E,60N,60S
>
> in location A, I use v.in.region to draw a vector called "outbox", I load it
> in location B with v.proj input=outbox ouput=outbox_fromA, I set the region
> in location B to the region of the box with g.region vect=outbox_fromA.
>
> The I go back: in location B, v.in.region to a vector called "back". In
> location A I use v.proj input=back ouput=back_fromB to project it and then
> set the region with g.region vect=back_fromB.
>
> The region I get in location A is compeltely different from the original
> location ....!!!!!!!!!!! But it should not be ..... it should actually be the
> same!
No it shouldn't.
The current region (and hence the vector map created by v.in.region)
will always be an axis-aligned rectangle.
The projection step will often introduce rotation and/or other
distortion, so the output of v.proj will be an arbitrary
quadrilateral.
Setting the current region to this map will produce the smallest
rectangle which bounds the quadrilateral.
Reverse-projecting the bounding region back to the original coordinate
system will produce another arbitrary quadrilateral, not the original
rectangle.
--
Glynn Clements <glynn at gclements.plus.com>
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