Good morning,
I am a new GRASS user, and I am trying to create a DEM to do surface
analysis on. We have elevation data that was collected / designed for
use as a TIN with breaklines. I have checked out the various surface
interpolation modules in GRASS 6.0, but I can't find anything that would
incorporate point and line data to create a surface.
Any suggestions. Help!
Thanks,
kes.
--
Ksenia E. Konwicki, RPF
Ecologist
Timberline Forest Inventory Consultants
1579 9th Avenue
Prince George, BC V2L 3R8
P (250)562-2628
F (250)562-6942
E kes@timberline.ca
From: "Ksenia Konwicki, RPF" <kes@timberline.ca>
Good morning,
I am a new GRASS user, and I am trying to create a DEM to do surface
analysis on. We have elevation data that was collected / designed for
use as a TIN with breaklines. I have checked out the various surface
interpolation modules in GRASS 6.0, but I can't find anything that would
incorporate point and line data to create a surface.
E.g. v.surf.rst can do it, but first you have to connect your points and
line layers into one layer, where the elevation value would be stored in the
same column. The problem is that the Grass command for patching vectors,
v.patch, is not completely ported in Grass 6 yet and currently patches only
the geometry and preserves categories of both input layers, so you will end
up with a layer where objects have no attributes and some of them have the
same cat, so it is quite impossible to attach elevation to such a datatable
manually afterwards in case of bigger datasets. The other module, v.append,
available on Grass wiki, should do fine if both input layers have exactly
the same structure of datatables, but is really slow. Eg. I tried to
patch two 3" SRTM tiles converted to vector points. On Athlon-XP 3000,
1GB RAM after the whole night the progress was 2%. Anyway, at least it
works - thanks to Michael Barton.
Yet there is an option to import your layers into Grass 5.4, use v.patch
there and port back into Grass 6 (v.convert) for further processing.
Although the vector engine of Grass 5 is much worse than in 6, v.patch plays
the role fine there, being able to populate attributes from input to output
properly.
Maciek
Hello Maciek
Maciek Sieczka wrote:
Although the vector engine of Grass 5 is much worse than in 6, v.patch plays
the role fine there, being able to populate attributes from input to output
properly.
I'm not sure I agree; can you explain in more detail? As I understand it the 5.4 v.digit is as simple as the 6.0 version. (the following discussion may be confusing as I use the terms categories and attributes as they apply in GRASS <= 5.4; it may or may not be different in >= 5.7). Each element in the map has an associated category number---the dig_att file associates the geometry elements with the category numbers, and the dig_cats file associates the category numbers with the attribute values.
If elements in the two different vector maps being patched have the same category numbers but those category numbers have different attribute values (either strings or decimal numbers) associated with them in each map (i.e. the two maps have different dig_cats files), then the attribute values in the second map are lost and those from the first map are used in the output map. So v.patch does nothing to avoid category number clashes between the two maps.
As I understand it. Haven't worked with this for a while though.
Paul
From: "Paul Kelly" <paul-grass@stjohnspoint.co.uk>
Hello Maciek
Maciek Sieczka wrote:
Although the vector engine of Grass 5 is much worse than in 6, v.patch plays
the role fine there, being able to populate attributes from input to output
properly.
I'm not sure I agree; can you explain in more detail? As I understand it the 5.4
<snip>
If elements in the two different vector maps being patched have the same category numbers but those category numbers have different attribute values (either strings or decimal numbers) associated with them in each map (i.e. the two maps have different dig_cats files), then the attribute values in the second map are lost and those from the first map are used in the output map. So v.patch does nothing to avoid category number clashes between the two maps.
As I understand it. Haven't worked with this for a while though.
It's been a while since I have used 5.4 for the last time. That's how I recall v.patch working in 5.4 but might be in error here. Sorry, you'd have to check it yourself,
it is very unlikely I will have time to do it within next few days.
Maciek