Hi I don't think that Synder's book actually use a hemisphere for the azimuthal equidistant projection examples. For example, on figures 41.B and 41.C, you can see Australia (respectively left to top-left and bottom-left). I think that in both cases, you can see the whole world. This is quite obvious on figure 41.C. I think it is a little less obvious for figure 41.B because, on figure 41.C the outside of the map is a point located in south pacific, while, on figure 41.B, the outside of the map is a point located in Malaysia (or maybe Indonesia).
But, maybe this is expected behaviour and I misunderstood the behaviour of rgdal. Johan 2012/11/14 Ashton Shortridge <ash...@msu.edu>: > 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 <ash...@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 ash...@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.ma...@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 ash...@msu.edu > Dept of Geography http://www.msu.edu/~ashton > 235 Geography Building ph (517) 432-3561 > Michigan State University fx (517) 432-1671 _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo