On 06/05/2010 09:45 AM, Hanlie Pretorius wrote:
2010/6/4, Micha Silver<[email protected]>:
On 06/04/2010 04:20 PM, Hanlie Pretorius wrote:
Hi,
I've been following the procedure at
http://grass.osgeo.org/wiki/Import_XYZ to import DEMs into GRASS.
It has worked for two DEMS, but I have a problem with the third one at
the step where one "verifies that the number of rows in the ASCII file
corresponds to the number of cells in the enlarged region".
At this point, g.region reports 1146474 cells in the region, while I
have 1146370 lines of coordinates in my file.
Assuming you already did the "r.in.xyz -s ..." step to get and set your
region to match the input data.
So it looks like there are about 100 coordinates missing from the ASCII
file. Maybe "holes" in the data?
One way to work around this might be to import the point data as a 3D
vector, then run the usual v.surf.rst interpolation.
v.in.xyz -z in=ascii.txt out=elev_pts z=3 fs=,
then
g.region vect=elev_pts res=<choose appropriate resolution>
v.surf.rst elev_pts elev=dem layer=0
Thanks for the suggestion, Micha.
I've tried the interpolation route before and found that the
difference between the resulting interpolated surface and the original
DEM was up to 7m. I want to use the DEMs for a flood application, so
they need to be as accurate as possible.
I wonder if playiing with the "tension" and "smooth" parameters would
help? Higher tension (>40) means that the interpolation acts more like a
rubber sheet, so each point influences a maller area, and smooth=0 means
that the final dem must go exactly thru each point.
I was thinking perhaps importing the points as vectors, converting
them to raster and then doing a nearest neighbour or IDW interpolation
to fill the gaps. At least then I'll be able to see where the gaps are
and limit the interpolated pixels using a mask?
Worth a try. But as you probably know, nearest neighbor won't take
trends in the surface into account. IDW the same. And IDW usually gives
worse terrain results than RST.
The output of g.region is:
-----
projection: 99 (Transverse Mercator)
zone: 0
datum: ** unknown (default: WGS84) **
ellipsoid: wgs84
north: -49312.49
south: -74587.5
west: -3015862.5
east: -2987512.5
nsres: 25.00000989
ewres: 25
rows: 1011
cols: 1134
cells: 1146474
-----
The first three and last three coordinates in my text file are:
-----
-74575.000000,-3015850.000000,1548.830000
-74575.000000,-3015825.000000,1548.330000
-74575.000000,-3015800.000000,1547.500000
.
.
.
-49324.990000,-2987575.010000,1510.980000
-49324.990000,-2987550.010000,1511.240000
-49324.990000,-2987525.010000,1511.470000
-----
Can someone help me to figure out what's going on?
Thanks
Hanlie
_______________________________________________
grass-user mailing list
[email protected]
http://lists.osgeo.org/mailman/listinfo/grass-user
This mail was received via Mail-SeCure System.
--
Micha Silver
Arava Development Co. +972-52-3665918
http://www.surfaces.co.il
This mail was received via Mail-SeCure System.
--
Micha Silver
Arava Development Co. +972-52-3665918
http://surfaces.co.il
_______________________________________________
grass-user mailing list
[email protected]
http://lists.osgeo.org/mailman/listinfo/grass-user
