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 > > [email protected] it > > reaches grass-user directly. > > > > Markus > > > > On Tue, Jan 20, 2009 at 8:39 PM, Moskovitz, Bob > > > > <[email protected]> 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 > >> [email protected] > >> http://lists.osgeo.org/mailman/listinfo/grass-user > > > > -- > > Open Source Geospatial Foundation > > http://www.osgeo.org/ > > http://www.grassbook.org/ > > _______________________________________________ > grass-dev mailing list > [email protected] > http://lists.osgeo.org/mailman/listinfo/grass-dev -- Dylan Beaudette Soil Resource Laboratory http://casoilresource.lawr.ucdavis.edu/ University of California at Davis 530.754.7341 _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
