On Jan 20, 2009, at 3:16 PM, Markus Neteler wrote:
Helena,
I guess this is a question for you. If you post from hmitaso@unity.ncsu.edu it
reaches grass-user directly.
Markus
On Tue, Jan 20, 2009 at 8:39 PM, Moskovitz, Bob
<Bob.Moskovitz@conservation.ca.gov> wrote:
Hello Grass Users,
I was just wondering why aspect is measured from the east and increased
counterclockwise instead of the north and clockwise? It seems to run
counter to what is done in other GISs.
at the time when it was implemented there was no other major raster GIS (no Spatial Analyst, only
some research software and each used different rule, based on the background
of the developer), so it was done to conform how the angles are measured in math (east is your +x
and the values increase counterclockw).
I vaguely remember discussions on whether to change it to conform with
the "aspect as azimuth" concept with 0 pointing towards North but there were already too many
people used to the way it was originally implemented that it was safer to keep it.
(we had to make the decision for aspect in v.surf.rst as well and we decided
to keep the math convention to make it consistent with r.slope.aspect).
GRASS7 provides opportunity to change it and make sure that all modules that compute and use
aspect or flow direction conform to the same rule. But it would be a major undertaking that could
break a lot of scripts - I am not sure it is worth it and whether anybody would actually be
willing to do it - it really depends on what your background is.
So if there are people who think this is really important and there is some official standard that
says what it should be and there are some volunteers to actually work on it, please post
to the grass-dev list,
Helena
Bob
Bob Moskovitz
Research Analyst I
Seismic Hazard Zonation Project
California Geological Survey
http://www.conservation.ca.gov/cgs/shzp
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On Tuesday 20 January 2009, Helena Mitasova wrote:
On Jan 20, 2009, at 3:16 PM, Markus Neteler wrote:
> Helena,
>
> I guess this is a question for you. If you post from
> hmitaso@unity.ncsu.edu it
> reaches grass-user directly.
>
> Markus
>
> On Tue, Jan 20, 2009 at 8:39 PM, Moskovitz, Bob
>
> <Bob.Moskovitz@conservation.ca.gov> wrote:
>> Hello Grass Users,
>>
>> I was just wondering why aspect is measured from the east and
>> increased
>> counterclockwise instead of the north and clockwise? It seems to run
>> counter to what is done in other GISs.
at the time when it was implemented there was no other major raster
GIS (no Spatial Analyst, only
some research software and each used different rule, based on the
background
of the developer), so it was done to conform how the angles are
measured in math (east is your +x
and the values increase counterclockw).
I vaguely remember discussions on whether to change it to conform with
the "aspect as azimuth" concept with 0 pointing towards North but
there were already too many
people used to the way it was originally implemented that it was
safer to keep it.
(we had to make the decision for aspect in v.surf.rst as well and we
decided
to keep the math convention to make it consistent with r.slope.aspect).
GRASS7 provides opportunity to change it and make sure that all
modules that compute and use
aspect or flow direction conform to the same rule. But it would be a
major undertaking that could
break a lot of scripts - I am not sure it is worth it and whether
anybody would actually be
willing to do it - it really depends on what your background is.
So if there are people who think this is really important and there
is some official standard that
says what it should be and there are some volunteers to actually work
on it, please post
to the grass-dev list,
Helena
Good topic for discussion. I think that GRASS7 should stick with the current
conventions to retain backwards compatibility. Since the reference of the
aspect calculation is clearly spelled out in the manual there should be no
problem- as long as people read the manual. Perhaps a careful check of all of
the manual pages associated with directional calculation should be checked to
make sure that there is a note.
Cheers,
Dylan
>> Bob
>>
>>
>> Bob Moskovitz
>> Research Analyst I
>> Seismic Hazard Zonation Project
>> California Geological Survey
>> http://www.conservation.ca.gov/cgs/shzp
>>
>> CONFIDENTIALITY NOTICE: This communication is intended only for
>> the use of
>> the individual or entity to which it is addressed. This message
>> contains
>> information from the State of California, California Geological
>> Survey,
>> which may be privileged, confidential and exempt from disclosure
>> under
>> applicable law, including the Electronic Communications Privacy
>> Act. If the
>> reader of this communication is not the intended recipient, you
>> are hereby
>> notified that any dissemination, distribution, or copying of this
>> communication is strictly prohibited.
>>
>> _______________________________________________
>> grass-user mailing list
>> grass-user@lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/grass-user
>
> --
> Open Source Geospatial Foundation
> http://www.osgeo.org/
> http://www.grassbook.org/
_______________________________________________
grass-dev mailing list
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--
Dylan Beaudette
Soil Resource Laboratory
http://casoilresource.lawr.ucdavis.edu/
University of California at Davis
530.754.7341