[GRASS-user] High resolution dem

[Micha Silver]

On 05/08/2010 09:38, Frank Broniewski wrote:
> Yesterday I received a note off list which pointed me to an error I 
> made when calculated the elevation
>
>> If you used r.mapcalc, make sure to divide by 100000.0 and not 100000
>> in order to get DCELL output, otherwise the result will be integer
>> with terraces, as in the aspect map, particularly on 5m horizontal
>> resolution.
>
> Now I have a nice aspect map :-)
>
> What would be the best approach to calculate a dtm for a contour map 
> where large parts miss elevation data? I 've uploaded an example on 
> our server:
>
> http://www.gis-hosting.lu/static/img/grass/no_contours.png
>
> Since the algorythm from r.surf.contour takes much longer when there 
> are large gaps or no elevation information is present, there should be 
> soem kind of solution. My idea is to take elevation data from srtm or 
> aster data and create contours from that data and patch it into the 
> finer contour map. Any other ideas?
>
I would be very surprised if you could get any match in elevations 
bewteen your high resolution data, and the SRTM or ASTER data. I've 
tried with some local, high quality DTM and was always disappointed.  
THere was always a step at the edge where the overlap ends. And it was 
not necessarily uniform, so it wasn't possible to just add i.e. x meters 
to the ASTER DEM to get them to match.

In any case, let us know how it goes...

>
> Many thanks
>
> Frank
>
> Am 04.08.2010 09:53, schrieb Frank Broniewski:
>> Hi,
>>
>>> Hamish:Feb 28, 2010; 03:15am Re: High resolution dem
>>> I am curious to know what the problem with r.surf.contour was. Usually
>>> it does a great job as long as you take a moment to work around the
>>> integer and NULL/0 issues.
>>
>> Sorry that I quitted this discussion so silently, but I had to drop this
>> and could not persue the issue further. Sometimes I really could use a
>> 48h day :-)
>> Of course my problems are still present. To take the thread up again, I
>> want to answer to Hamish's question about my problems with
>> r.surf.contour first:
>>
>> DEM interpolation seems to work nice, but when you start deriving maps,
>> especially the aspect map looks simply not right ;-)
>> I've uploaded some example images on our server, so that you can see
>> what I mean:
>>
>> http://www.gis-hosting.lu/static/img/grass/contours5m.png
>> http://www.gis-hosting.lu/static/img/grass/dtm5m.png
>> http://www.gis-hosting.lu/static/img/grass/dtm3d.png
>> http://www.gis-hosting.lu/static/img/grass/expo5m.png
>>
>> The data source is a shape file with contour lines with 5 meters
>> interval. Conversion to raster went well and the pixel values match the
>> height values for the contours. Before using the raster map, I
>> multiplied the map with 100000, just like it is mentioned in the wiki.
>> Interpolation took approx. 5 minutes. The region is 10,000m * 10,000m
>> wide and has a 5m resolution. Before deriving the aspect map, I divided
>> the elevation map by 100000 to gain real height values again ...
>>
>> Now I wonder what could cause the gaps in the aspect map - since the
>> terrain model itself looks OK to me ...
>>
>> Many thanks
>>
>> Frank
>>
>
>


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