[GRASS-user] Creation of a DEM

[Dave Kent]

Hi Ed:

I have had better luck creating DEMs using point data rather than  
area data.  Can you get your RTK to export point data (xyz)  rather  
than the swath segment data.  Alternatively, use the "type=point"  
feature of v.in.ogr.  I have never used this feature  but the manual  
says it imports area centroids as points.

The next step is to interpret the point data to a DEM raster  
surface.  I have used v.surf.rst to create a raster DEM from  
irregularly spaced points.  It works well.  You can use the   
"maskmap=name" feature to limit the DEM to a restricted area.   
v.surf.rst expects a raster map for maskmap.  You can create one from  
your field outline shape file with v.to.rst .

I can't help on the import problem.  Perhaps importing the centroids  
only with "type=point" will help.

I think that precision agriculture is a natural application for  
Grass.  It would be helpful and interesting  to hear of your  
challenges and successes in its use.

Regards

Dave



On Jun 29, 2007, at 8:48 PM, Edward Tomlinson wrote:

> Hi,
>
> I am also trying to create a DEM from a shapefile.
>
> Once again i have some questions about the cell resolution and the  
> interpolation.
>
> The use of this is for precision agriculture.
>
> The shapefile was generated from a 2cm accuracy RTK GPS. Height  
> variation is about 1 metre (over 1200metres). The shapefile isn't  
> exactly what you would call a contour map. The resulting shapefile  
> is made up of rectangles of equal height. Each rectangle is 8m wide  
> (always as this is the swath width used). The length varies as it  
> keeps ~equal heights together. Technically i suppose this is a  
> contour map, however equal height rectangles are not connected and  
> are not part of the same shape...
>
> My other problem is trying to work out what gradient splits should  
> be used. As in i beleive i should use the standard deviation of  
> height as the gradient step value. Does this sound correct??
>
> Also i need to restrict the boudary of the DEM to another  
> shapefile. It is just a single shape of the boundary of the field.  
> Mostly all the data is there, however some interpolation will be  
> needed.
>
>
> Currently i am having trouble reading the file in using v.in.ogr.
> command:
> v.in.ogr 'dsn=shapefile name' output=elevation09 'layer=Elevation  
> Map 9'
> Result:
> GRASS_INFO_WARNING Area size 9e..., area not imported
> ....
> 0 areas imported
>
> THere are many columns in the .df file. How can i only import the  
> ones i want? Clearly i have missed something.
>
> Any help would be greatly appreciated.
>
> THanks,
> Ed
>
>
> ---------- Forwarded message ----------
> From: quinonero juan manuel <jumaqu at yahoo.es>
> Date: 25-Mar-2007 02:45
> Subject: [GRASS-user] Creation of a DEM
> To: grassuser at grass.itc.it
>
>
> 1.- You have to transform the vector information to a rast layer,  
> you can do it using the comand v.to.rast, baut before you have to  
> determine the cell resolution using for example g.region res=meters
>
> 2.- If you wsnt to create a good DEM you have to change the units  
> of the rast layer created, Only have to convert this layer from  
> meter to centimeter using r,mapcalc 'cm_layer=rast_meter*100.00'.  
> That's for obtain a good relation betwen horizontal and vertical  
> resolution.
>
> 3.- Now you have to use an interpolatión method to create a DEM  
> continous surface. There are several methods, the commonest is  
> r.surf.contour tat use a lineal interpolation. If you use this  
> command you must use the option -f and it'll build faster, dont  
> forget this point!!!!
>
> 4.- Finally you have to transform the results to the correct units  
> usin r.mapcalc 'final_DEM=cm_DEM/100.00'
>
>
> If you want to generate a DEM usin another interpolation methods  
> please ask!!
>
> Sorry for my english
>
> Juanma
>
> megha shyam <capt_lms at yahoo.co.in> escribió:
> Please help me in creation of DEM.i have a vector layer contaning  
> contour information.How to create a DEM model  of this.Can you tell  
> me the stpes to be followed
>
> shyam
>
> Here's a new way to find what you're looking for - Yahoo! Answers  
> _______________________________________________
> grassuser mailing list
> grassuser at grass.itc.it
> http://grass.itc.it/mailman/listinfo/grassuser
>
>
>
>
> LLama Gratis a cualquier PC del Mundo.
> Llamadas a fijos y móviles desde 1 céntimo por minuto.
> http://es.voice.yahoo.com
>
>
>
> _______________________________________________
> grassuser mailing list
> grassuser at grass.itc.it
> http://grass.itc.it/mailman/listinfo/grassuser
>
> _______________________________________________
> grassuser mailing list
> grassuser at grass.itc.it
> http://grass.itc.it/mailman/listinfo/grassuser

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.osgeo.org/pipermail/grass-user/attachments/20070630/5b71b594/attachment.html