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Azimuthal equidistant projection and rgdal

5 messages · Ashton Shortridge, Johan Mazel

Johan Mazel
# 
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>:
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Ashton Shortridge
# 
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:
Johan Mazel
# 
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>:
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:
-----
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
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>: