[GRASSLIST:2552] Re: Moire effect in aspect map

[Christof Bigler]

The data that I originally imported are from the U.S. Geological 
Survey. The input files are in the directory 'codem1nw', which contains 
the actual data 'w001001.adf', a file 'prj.adf' and some more files. 
The projection settings in 'prj.adf' correspond to my default settings:

g.region -pd
> projection: 1 (UTM)
> zone:       13
> datum:      nad27
> ellipsoid:  clark66
> north:      4480020
> south:      4380000
> west:       235000
> east:       340000
> nsres:      30
> ewres:      30
> rows:       3334
> cols:       3500

Import of the raster data (without overriding the default projections):
r.in.gdal in=codem1nw out=codem1nw.r
r.support -r codem1nw.r
r.slope.aspect el=codem1nw.r as=codem1nw.asp.r

r.info codem1nw.asp.r
> Type of Map:  raster              Number of Categories: 360
> Data Type:    FCELL
> Rows:         3668
> Columns:      3850
> Total Cells:  14121800
> Projection: UTM (zone 13)
> N:    4480029    S:    4379982   Res: 27.27562704
> E:     340003    W:     234992   Res: 27.27558442
Note that there's a slightly different N-S resolution compared to the 
E-W resolution.

Import of the raster data (with overriding the default projections):
r.in.gdal -o in=codem1nw out=codem1nw2.r
r.support -r codem1nw2.r
r.slope.aspect el=codem1nw2.r as=codem1nw2.asp.r

r.info codem1nw2.asp.r
[Only the following values changed:]
> N: 4480028.5797883    S: 4379982.28159183   Res: 27.27543571
> E: 340002.76575265    W: 234992.33825635   Res: 27.27543571

There's a moire effect on both aspect maps. The region doesn't change, 
and when I zoom in, n, s, e, w are clean multiplies of resolution (= 
30).

Did I miss some essential information (I'm new to GRASS...)?

Christof

On 10.02.2004, at 17:42, Hamish wrote:

>> I imported a DEM with r.in.gdal and derived aspect with r.slope.aspect
>> (floating point). These raster maps have a slightly different
>> resolution (27.28 m) than the default settings (30 m).
>> When I look at the aspect map, it looks ok on the screen and the
>> printout as well. However, when I zoom in, I can detect a sort of 
>> moire
>> effect (diagonal bands crossing the map). Interpolation didn't have 
>> any
>> effects.
>> Any ideas how to avoid this?
>
>
> are you sure the region didn't change?
>
> have a look at 'g.region -p' and make sure the n,s,e,w are clean 
> multiples
> of "res".
>
>
> Hamish