[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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