[GRASS-user] Re: Importing Dems with r.in.xyz

[Hanlie Pretorius]

2010/6/4, Micha Silver <micha at arava.co.il>:
> 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 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?

>
>> 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
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>>
>
>
> --
> Micha Silver
> Arava Development Co. +972-52-3665918
> http://www.surfaces.co.il
>
>
>