[GRASS5] [bug #870] (grass) r.proj problem: boxes appearing

Eric G. Miller egm2 at jps.net
Wed Dec 12 11:21:29 EST 2001

On Wed, 12 Dec 2001 16:22:07 +0100 (CET), Request Tracker <grass-bugs at intevation.de> wrote:

> this bug's URL: http://intevation.de/rt/webrt?serial_num=870
> -------------------------------------------------------------------------
> Hi Morten
> (dear RT bugtracker)
> I am sending this to the bug tracker as the information shall
> be recorded.
> To repeat the bug description: the maps generated by r.proj
>  - might be shifted by 0.5 to 1 pixel (not sure!)

   Projection introduces error.

>  - the resulting maps contain a sort of "box structure" where 
>    the information differs from the neighbors. This only applies to
>    the border of the boxes, not the box contents. In a Gauss-Boaga
>    (transverse mercator) projection the boxes are 500m x 500m
>    everywhere like a chess-board.
>    Note that this is only visible when generating an aspect map
>    in case you re-project an elevation map.

   Are you certain the original data doesn't contain these artifacts?
   Perhaps the projection exacerbated the effect?

> > And I have really never looked closely into what is happening 
> > inside nearest.c, bipolar.c and cubic,c. Could be some obscure bug there, 
> > but I will have to get my Grass burning again before I can do any further 
> > programming. Right now not possible. Next year when I'm out of work 
> > another story.
> > 
> > > > But I found out (by trial) that the only usable interpolation method is
> > > > the 'nearest neighbor' (default). The two other methods (bipolar and
> > > > cubic) tend to change ("interpolate") the coastlines and shape of lakes
> > > > too much. 
> > > 
> > > Shall I disable them to prevent user confusion?

No.  Do not disable.  Bilinear/Cubic are appropriate for imagery.

Eric G. Miller <egm2 at jps.net>

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