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<TITLE>Re: [GRASS-user] Creation of a DEM</TITLE>
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<FONT FACE="Verdana, Helvetica, Arial"><SPAN STYLE='font-size:12.0px'>Ed,<BR>
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It sounds like your shape file is made up of polygons, but that it needs to be cleaned.<BR>
<BR>
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). <BR>
<BR>
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.<BR>
<BR>
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).<BR>
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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.<BR>
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Michael<BR>
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On 6/29/07 7:48 PM, "Edward Tomlinson" <edwardjtomlinson@gmail.com> wrote:<BR>
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</SPAN></FONT><BLOCKQUOTE><FONT FACE="Verdana, Helvetica, Arial"><SPAN STYLE='font-size:12.0px'>Hi,<BR>
<BR>
I am also trying to create a DEM from a shapefile.<BR>
<BR>
Once again i have some questions about the cell resolution and the interpolation.<BR>
<BR>
The use of this is for precision agriculture.<BR>
<BR>
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... <BR>
<BR>
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??<BR>
<BR>
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. <BR>
<BR>
<BR>
Currently i am having trouble reading the file in using v.in.ogr.<BR>
command:<BR>
v.in.ogr 'dsn=shapefile name' output=elevation09 'layer=Elevation Map 9'<BR>
Result:<BR>
GRASS_INFO_WARNING Area size 9e..., area not imported<BR>
....<BR>
0 areas imported<BR>
<BR>
THere are many columns in the .df file. How can i only import the ones i want? Clearly i have missed something.<BR>
<BR>
Any help would be greatly appreciated.<BR>
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THanks,<BR>
Ed<BR>
<BR>
<BR>
---------- Forwarded message ----------<BR>
From: <B>quinonero juan manuel</B> <jumaqu@yahoo.es><BR>
Date: 25-Mar-2007 02:45 <BR>
Subject: [GRASS-user] Creation of a DEM<BR>
To: grassuser@grass.itc.it<BR>
<BR>
<BR>
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<BR>
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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. <BR>
<BR>
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!!!!<BR>
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4.- Finally you have to transform the results to the correct units usin r.mapcalc 'final_DEM=cm_DEM/100.00'<BR>
<BR>
<BR>
If you want to generate a DEM usin another interpolation methods please ask!!<BR>
<BR>
Sorry for my english<BR>
<BR>
Juanma<BR>
<BR>
<B><I>megha shyam <capt_lms@yahoo.co.in></I></B> escribió:<BR>
</SPAN></FONT><BLOCKQUOTE><FONT FACE="Verdana, Helvetica, Arial"><SPAN STYLE='font-size:12.0px'>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<BR>
<BR>
shyam<BR>
<BR>
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