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

[Johan Mazel]

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 <ashton at 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 <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