Azimuthal equidistant projection and rgdal
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