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 <[EMAIL PROTECTED]>
Date: 25-Mar-2007 02:45
Subject: [GRASS-user] Creation of a DEM
To: [email protected]
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 <[EMAIL PROTECTED]> 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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