[GRASS-user] Triangles polygons with Arc*

GRASS GRASS user
1

Hi,
Hi,

Just curious if anybody encountered this issue with Arc*:
When converting raster to vector using ArcGIS/View triangular polygons
are generated especially with small areas (1-3 pixels from the
original raster).

See image:
http://farm3.static.flickr.com/2127/2510680112_25cc16686e.jpg

I think I made a similar thing before (using ArcGIS "Hydro
extension"). Is this correct?
As far as I'm concerned, the output should be regular polygon blocks
since raster grid is the input. Unless in GRASS, you use the -s flag
of r.to.vect

maning

--
|---------|----------------------------------------------------------|
| __.-._ |"Ohhh. Great warrior. Wars not make one great." -Yoda |
| '-._"7' |"Freedom is still the most radical idea of all" -N.Branden|
| /'.-c |Linux registered user #402901, http://counter.li.org/ |
| | /T |http://esambale.wikispaces.com|
| _)_/LI
|---------|----------------------------------------------------------|

2

Well, maybe Arc is smoothing those 3 pixels...

On Tue, May 27, 2008 at 12:07 AM, maning sambale
<emmanuel.sambale@gmail.com> wrote:

Hi,
Hi,

Just curious if anybody encountered this issue with Arc*:
When converting raster to vector using ArcGIS/View triangular polygons
are generated especially with small areas (1-3 pixels from the
original raster).

See image:
http://farm3.static.flickr.com/2127/2510680112_25cc16686e.jpg

I think I made a similar thing before (using ArcGIS "Hydro
extension"). Is this correct?
As far as I'm concerned, the output should be regular polygon blocks
since raster grid is the input. Unless in GRASS, you use the -s flag
of r.to.vect

maning

--
|---------|----------------------------------------------------------|
| __.-._ |"Ohhh. Great warrior. Wars not make one great." -Yoda |
| '-._"7' |"Freedom is still the most radical idea of all" -N.Branden|
| /'.-c |Linux registered user #402901, http://counter.li.org/ |
| | /T |http://esambale.wikispaces.com|
| _)_/LI
|---------|----------------------------------------------------------|
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http://lists.osgeo.org/mailman/listinfo/grass-user

3

MS,

On Tue, May 27, 2008 at 8:21 PM, M S <mseibel@gmail.com> wrote:

This sounds like a question for the Arc List, or their tech support?

I know, are there any public Arc list out there (can't seem to find
one with Google)? I'm not an arc user but my colleague sent me this
shapefile so we can verify both processing results with GRASS and
ArcGis. I was hoping somebody on the list encountered this issue.

cheers,
maning

--
|---------|----------------------------------------------------------|
| __.-._ |"Ohhh. Great warrior. Wars not make one great." -Yoda |
| '-._"7' |"Freedom is still the most radical idea of all" -N.Branden|
| /'.-c |Linux registered user #402901, http://counter.li.org/ |
| | /T |http://esambale.wikispaces.com|
| _)_/LI
|---------|----------------------------------------------------------|

4

Just curious if anybody encountered this issue with Arc*:

it's a common problem.

It does seem to be.

short answer: TINs stink. Raster maps created from TINs stink more.

My colleague used SRTM DEM converted to grid format of Arc
He mentioned that it is fairly common when you convert raster grids in
arc to vectors (shapefiles). Especially with very small areas like a
2-3 pixels. Will look into this further.

To all the others who gave me a list of Arc mailing list thanks! I am
reading a few right now.

cheers,

maning

--
|---------|----------------------------------------------------------|
| __.-._ |"Ohhh. Great warrior. Wars not make one great." -Yoda |
| '-._"7' |"Freedom is still the most radical idea of all" -N.Branden|
| /'.-c |Linux registered user #402901, http://counter.li.org/ |
| | /T |http://esambale.wikispaces.com|
| _)_/LI
|---------|----------------------------------------------------------|

5

maning sambale:

My colleague used SRTM DEM converted to grid format of Arc
He mentioned that it is fairly common when you convert
raster grids in arc to vectors (shapefiles). Especially with very small
areas like a 2-3 pixels. Will look into this further.

ok, then try and use GRASS's r.contour on the original data to create the vector instead of Arc's version and see if it is any nicer.

Hamish

6

To add info to this thread:
Some arc users said it its dependent on the snapping threshold or
smoothing option in the polygon creation. Polygons will be "blocky"
when this options are not enabled.

cheers,
maning

--
|---------|----------------------------------------------------------|
| __.-._ |"Ohhh. Great warrior. Wars not make one great." -Yoda |
| '-._"7' |"Freedom is still the most radical idea of all" -N.Branden|
| /'.-c |Linux registered user #402901, http://counter.li.org/ |
| | /T |http://esambale.wikispaces.com|
| _)_/LI
|---------|----------------------------------------------------------|