[R-sig-Geo] Azimuthal equidistant projection and rgdal

[Ashton Shortridge]

Dear Johan (and list),

you are right in the first case: I think the result is expected. Using global 
data with this sort of projection will look kind of awful - the equatorial 
azimuthal equidistant illustrations in Snyder do not try to map data for the 
whole planet - they employ a hemisphere.

When I clip the results to a hemisphere:
dta <- map("world",  xlim=c(-180, 0), ylim=c(-90,90), plot=FALSE)
...
dta2 <- spTransform(dta1, CRS("+proj=aeqd +lat_0=0 +lon_0=-0"))
plot(dta2, axes=TRUE)

the result looks pretty similar to what I see in Snyder's work.

That said... it is strange to me that using your global data the projection 
does NOT wrap around when doing a polar azimuthal projection, but then it does 
when you do not. This may not be on-topic for this stats list - I wonder if 
you could reach out to the GDAL community if you still have questions?

Yours,

Ashton




On 11/13/12, Johan Mazel, wrote:
> Thanks for your answer.
> 
> Regarding your last paragraph, I am not really sure to understand.
> Do you mean that the result is expected in regards to the script (and
> rgdal use) ?
> Or do you mean that azimuthal equidistant projection is not
> possible/valid on any other points than the two poles ? Because, if it
> is the latter, Synder's book seems to disagree. If you look at figure
> 41.C, it looks like azimuthal equidistant projection centered on a
> different point than on of the two pole is possible.
> 
> Thanks a lot for your time.
> Johan
> 
> 2012/11/12 Ashton Shortridge <ashton at msu.edu>:
> > Dear Johan (and list),
> > 
> > I'm not a professional cartographer, but I do find myself flipping
> > through Snyder's book of map projections every once in a while.
> > 
> > Looking at your results, I don't see anything 'wrong' with the code or
> > the results. Yes, the map looks awful, but that is what it should look
> > like. Imagine a glass globe of the Earth with outlines of the land
> > masses etched on it. Now imagine you have oriented that globe so that
> > you are looking down at 45N, 90W (somewhere not far from my neck of the
> > woods). What you see as "Australia right next to Canada" is from looking
> > 'through the globe', which is why Australia appears to be inverted, and
> > why South America seems to fold under itself (also Africa).
> > 
> > Now this projection is not really a glass globe, but I hope the metaphor
> > makes it a bit clearer. It should also make clearer why people don't
> > normally use azimuthal projections for global mapping. Coincidentally it
> > works sorta kinda ok from the north pole, but don't plan on it working
> > well from other places.
> > 
> > Hope this is helpful.
> > 
> > Ashton
> > 
> > 
> > -----
> > Ashton Shortridge
> > Associate Professor                     ashton at msu.edu
> > Dept of Geography                       http://www.msu.edu/~ashton
> > 235 Geography Building          ph (517) 432-3561
> > Michigan State University               fx (517) 432-1671
> > 
> > On 11/12/12, Johan Mazel, wrote:
> >> Here is the code that I am using and the result.
> >> The problem on the result is that, on the fourth and last page, it
> >> looks like some parts of the map are not located where they are
> >> supposed to be (Australia is thus right next to Canada).
> >> 
> >> My only modification is that I removed the xlim and ylim parameters in
> >> the line "dta <- map("world", xlim=c(-120, -60), ylim=c(0,90),
> >> plot=FALSE)" in the original script. It obviously has a impact on the
> >> final output but I can`t figure out what is happening.
> >> 
> >> Thanks for your time.
> >> Regards.
> >> Johan
> >> 
> >> 2012/10/25 Johan Mazel <johan.mazel at gmail.com>:
> >> > Hi
> >> > I am trying to use the solution rpesented here:
> >> > https://stat.ethz.ch/pipermail/r-sig-geo/2007-December/002939.html.
> >> > 
> >> > However it looks like the shifted (from the north pole) map is
> >> > displayed on top on the original azimuthal equidistant projection
> >> > centered on the north pole.
> >> > Is there any way to correct this ?
> >> > 
> >> > Thanks for your time.
> >> > Regards.
> >> > Johan


-----
Ashton Shortridge
Associate Professor			ashton at msu.edu
Dept of Geography			http://www.msu.edu/~ashton
235 Geography Building		ph (517) 432-3561
Michigan State University		fx (517) 432-1671