On Tue, 26 Jan 2010, Glynn Clements wrote:
First, the current region's grid should at least match that of the
input map, and ideally of the original data.
Glynn,
The number of rows and columns has been changed but not the resolution. At
least, not deliberately.
If the data has been resampled (including projection with r.proj), this
may produce artifacts, particularly if it used method=nearest (which is
the default).
Nope. Source metadata:
Map_Projection:
Map_Projection_Name: Lambert Conformal Conic
Lambert_Conformal_Conic:
Standard_P arallel: 43.000000
Standard_P arallel: 45.500000
Longitude_of_Central_Meridian: -120.500000
Latitude_of_Projection_O rigin: 41.750000
False_Easting: 1312336.000000
False_Northing: 0.000000
Planar_Coordinate_Information:
Planar_Coordinate_Encoding_Method: row and column
Coordinate_Representation:
Abscissa_Resolution: 32.828670
Ordinate_Resolution: 32.828670
Planar_Distance_Units: User_Defined_Unit
Geodetic_Model:
Horizontal_Datum_Name: North American Datum of 1983
Ellipsoid_Name: Geodetic Reference System 80
Semi-major_Axis: 6378137.000000
Denominator_of_Flattening_Ratio: 298.257222
No reprojection necessary.
Essentially, slope calculations can be quite sensitive to resampling
artifacts.
So I understand.
1. Better (i.e. "uncooked") source data. Reprojecting from the
original grid to the desired target grid in one step is better than
reprojecting to lat/lon then again to the target grid.
The source originally-imported DEM produces appropriate slope and aspect
maps. The sub-region I cut out with v.in.region (I believe that's the module
I used) doesn't calculate properly. Therefore, the problem is with the
sub-map.
2. Upsampling the source data with e.g. r.resamp.interp.
I'll try this on the sub-map.
3. Filtering the source data with e.g. r.neighbors or r.mfilter.fp.
Source data are OK.
4. Re-projecting using method=cubic rather than method=nearest.
Data came as LCC; no reprojecting needed.
You wouldn't want to use *all* of these. #1 is nice if you can get it
(and can use it; you need to know the source "projection"),
See above.
Rich
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