Azimuthal, conical projections and d.rast, r.proj

[Seppo Kaitala]

Hello list


The help conserning Lambert Azimuth. Equal area projection  I got from
Morten Hulden was extreamly useful for me.  I also support the his
suggestion to include more projections to be compiled directly from the
sourse.  For instance, some DEM files that you can find from GRASS
documentation pages, are in Lampert Azimuth. Equal area projection and
are so useless without
support for that projection. The problem, that Morten has mentioned
underneath if the south is above north I have solved first converting my
data to ll projection and after that to Lambert Azimuth or what so
ever.  So I can get allways same coordinates for my projections (and I
think if I would not they would be wrong). Am I right ? It has worked so
far.

Seppo Kaitala

Morten Hulden wrote:
> 
> Hello List
> 
> I recently added some code to GRASS' g.setproj to get support for Lambert
> Azimuthal Equal Area projection.
> (http://earth.uni-muenster.de/~eicksch/GRASS-Help/msg02001.html)
> 
> After trying out imports to the new projection I have noticed following
> problem with the r.proj and d.rast modules: The modules refuse to
> convert/display data in areas where south is above north or east is to the
> left of west. This is not a problem for the corresponding
> conversion/display of vector data with v.proj/d.vect.
> 
> As an example, take a LAEA projection of the Eurasian continent with the
> standard parallel and longtude axis put over Europe. (lon_0=20, lat_0=55
> in my case). The Eurasian continent then forms a kidney bean shaped block
> with Europe standing straight up in the lower part, and Kamchatka and East
> Siberia almost upside down in the upper part. This beautiful and very
> useful projection is used in the land coverage maps from the usgs.gov
> sites.
> 
> The r.proj and d.rast modules makes the (false?) assumption that north
> must always be on top of south and east must always be to the right of
> west, even in non-ll projections. In the example above, going eastward
> over the continent a point is eventually reached (somewhere in middle
> Asia), where north is pointing straight to the left and east is pointing
> straight upwards. From that point onwards to the east r.proj will fail,
> and d.rast will not accept maps in this region. Cellhd for such maps would
> have values (in meter units) where N<S and E<W, which will trigger an
> error message.
> 
> The PROJ4 program, which r.proj also uses for the conversion, does the
> calculation from degrees to Cartesian coordinates correctly when used
> externally. Is it an oversight that the N>S, E>W assumption is activ in
> these modules, when the projection is not ll and proj_unit is meters, not
> degrees? It does not make sense for azimuthal projections. I have not
> tried the Lambert Conformal Conic (lcc), one of the default projections in
> GRASS, but I guess a similar situation may arise here when the location
> covers an area which extends far enough in the east-west direction, and
> the central longitude is put near either end.
> 
> The fact that the modules v.proj (which also relies on PROJ4 code) and
> d.vect work suggests to me that relatively small changes in the source
> code for the corresponding raster modules could solve the problem. I do
> not have time, nor skill, to look closer at this. I can just hope someone
> will take in on from here.
> 
> By the way, I'm using GRASS5.0beta compiled on Linux2.2.x Intel686, at
> home and work. Great program!
> 
> Morten Hulden

-- 
Seppo Kaitala, Ph D
University of Helsinki
Department of Ecology and Systematics
Division of Environmental Biology
P.O. Box 44 (Jyrängöntie 2, Botanical Garden)
FIN-00014 Helsinki, Finland
tel. +358-9-19140059, fax +358-9-191140048
email:seppo.kaitala at helsinki.fi