[GRASS-user] Creation of a DEM

Sat Jun 30 15:10:56 EDT 2007

Ed,

It sounds like your shape file is made up of polygons, but that it needs to
be cleaned.

The OGR layer warning is for one or more polygons that are extremely tiny.
An area of 6.5e-11 means 6.5 with an exponent of -11. It is probably just
ignoring these incorrect "polygons" (I bet that they are actually points
instead of real polygons).

Ideally, you need to create your DEM from a point file. Probably a point at
the center of each polygon is the best representation of your data. You can
get these from area centroids after you import the polygons (use v.type to
convert from centroids to points; in the vector>develop map>convert object
types menu). Someone mentioned a v.in.OGR option to just import area
centroids. I haven't used this, but it might save a step.

The number of columns are irrelevant to interpolation, but they might slow
down your import. They are stored in a *.dbf file along with your *.shp
file. You can get rid of columns before your import with any program that
will read DBF files (e.g., open office).

Once you have a point file with elevation values imported into GRASS, there
are many interpolation methods available to you. The easiest way to see what
is possible is to look at the raster>interpolate surfaces menu. You can
interpolate from the vector points you have imported, or you can change the
vector points to raster points (files>map type conversions) and use the
raster points.

Michael

On 6/29/07 7:48 PM, "Edward Tomlinson" <edwardjtomlinson at gmail.com> 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
>>  
>> 
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__________________________________________
Michael Barton, Professor of Anthropology
Director of Graduate Studies
School of Human Evolution & Social Change
Center for Social Dynamics & Complexity
Arizona State University

phone: 480-965-6213
fax: 480-965-7671
www: http://www.public.asu.edu/~cmbarton

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