[GRASS-user] Creation of a DEM

Sun Jul 1 22:05:53 EDT 2007

Thanks Michael and Dave,

It appears than my minimum area size was too big. I thought it said minimum
area in square meters (using v.in.ogr) however on second reading it was
square units. So after adjusting this it imported. However that is as far as
i got. I am away for a week then i will atempt the rest of it
(interpolation, DEM, etc).

Thanks again.
Ed

On 01/07/07, Michael Barton <michael.barton at asu.edu> wrote:
>
> 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
>
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