Good projection for N/S America?
Canada looks pinched in this projection. S. Am is perfect. THK
On 4/9/07, Roger Bivand <Roger.Bivand at nhh.no> wrote:
On Mon, 9 Apr 2007 White.Denis at epamail.epa.gov wrote:
For preview graphics and for large areas such as continents, large countries, hemispheres, or the whole earth, spherical projections are often adequate. I can provide some of the ones I have used. For detailed work at sites and small areas, ellipsoidal projections such as UTM are usually used, and then the coding gets more complicated with choices of datums and so forth.
The attached script shows how to do the interrupted sinusoidal projection using spTransform in rgdal, for the whemi.lin data posted with the free-standing functions by Denis White a couple of days ago. Once the lines are converted into SpatialLines objects, the rest is robust and simple, as is the use of gridlines() in sp. The one catch is calculating the offset, here in an x_0= offset along the Equator in metres between the two central longitude values. The output is attached as a PNG image. The point about the sp objects is that they contain enough metadata (here a PROJ.4 projection description) to let them be moved to other R packages or external software. The half-dozen basic projections are easy to specify in PROJ.4, for example from the geotiff list: http://www.remotesensing.org/geotiff/proj_list/ which is what I used here. The other projections mentioned are: Lambert Cylindrical Equal Area "+proj=cea +lon_0=-80" Lambert Azimuthal Equal Area "+proj=laea +lat_0=0 +lon_0=-80" while the Northern hemisphere sinusoidal is: Sinusoidal "+proj=sinu +lon_0=-100" So I'd argue that PROJ.4 projection descriptions are not difficult to use, and with sp objects, do stay stuck to the data (has anyone else ever forgotten what projection was used when revisiting data, not just me?). Using the maptools map2SpatialLines() interface function, or the Rgshhs() interface to GSHHS shorelines, even getting the lines is quite easy, qualified by clipping and bounding box issues in extremities for projection from geographical coordinates. Of course, it would help to have MacOS X and selected Linux binaries of rgdal, we're very lucky that Uwe Ligges is so helpful with the Windows binaries. Roger
r-sig-geo-bounces at stat.math.ethz.ch wrote on 2007-04-08 07:56:03:
Denis, That's really useful. It occurs to me that we only really need a half-dozen basic projections to cover 90% of user cases. Perhaps these could be incorporated into the 'sp' group somewhere and relieve the dependence on proj4. (It could be packaged separately for R for the other 10% of cases where its needed.) THK On 4/6/07, White.Denis at epamail.epa.gov <White.Denis at epamail.epa.gov>
wrote:
Thanks, Roger. There was a request to see the R code for these
figures.
Attached is the script for the second PDF file plus the input
boundary
file I used for the hemisphere. The three projection functions are
for
simple spherical, rather than ellipsoidal, models of the earth. The graticule generating function could be more elegant. I'm not yet up
to
speed with sp and the many new spatial capabilities in R so please excuse the old style "lines()" format encoding and graphics. Tim, I don't know whether proj4 could do the interrupted sinusoidal. (See attached file: whemi.projs.r)(See attached file: whemi.lin) r-sig-geo-bounces at stat.math.ethz.ch wrote on 2007-04-06 04:51:53:
Since this topic is of general interest, I've made an exception
and
allowed (this once!) a posting of more than 200K. In general, if
graphics
are big, please consider either an alternative device (png is
often
OK),
or posting just a URL to the real file. With apologies to list members on dial-up connections in the
field,
Roger On Thu, 5 Apr 2007 White.Denis at epamail.epa.gov wrote:
Yes, for many uses that is my choice also. For the conterminous
US
for
example, the Lambert azimuthal has lower mean distortion than
the
commonly used standard projection, the Albers conical equal
area,
although Albers was chosen by USGS as a standard because of
lower
extreme distortion than many other possible projections. For our hemispherical application, because we were gridding the
data, we
wanted parallels of latitude to be parallel in the projected
coordinate
space, which we wouldn't get with the Lambert azimuthal. (See attached file: whemi.projs.pdf) Tim Keitt <tkeitt at gmail.com> wrote on 2007-04-05 10:56:09:
Thanks. My application is not that demanding. Really, I just
want
it
to look reasonable. My plan is to lay out the postings in the projected coordinates and then back transform into geographic coordinates for analysis. I tried lots of projections and
found
Lamberts Azimuthal Equal Area to be quite good. I like the
look of
the
Azimuthal Equidistant better, but figured equal area was a
good
choice. THK On 4/4/07, White.Denis at epamail.epa.gov
<White.Denis at epamail.epa.gov>
wrote:
Tim, It depends on which kind of distortion is of most concern.
For
many
types of extensive data, especially counts, for example, the
equal
area
property is desirable. We used the Lambert cylindrical
equal
area
projection with standard parallels of +/- 30 degrees for
some
western
hemispherical work, see reference below. (The center
longitude
could be
-80 west, but that is less important than the choice of
parallels.)
Before falling back on the Lambert as an easy to use
projection,
I
tried
to get several ESRI products to implement an interrupted
projection
using the sinusoidal projection, in part for reasons given
in
the
second
reference. I used a separate center longitude for north and
south
of
the equator and the appearance is certainly more
satisfactory
than
the
Lambert in my opinion. I'll attach a PDF of an illustration
of
this
approach generated in R that I hope you will get but not the
rest of
the
list unfortunately. I can send PDFs of the references also
if
needed.
Denis Lawler JJ, White D, Neilson RP, Blaustein AR. 2006.
Predicting
climate-induced range shifts: model differences and model
reliability.
Global Change Biology 12:1568-1584. White D. 2006. Display of pixel loss and replication in
reprojecting
raster data from the sinusoidal projection. Geocarto
International
21(2):19-22. (See attached file: whemi.sinus.pdf) r-sig-geo-bounces at stat.math.ethz.ch wrote on 2007-04-04
12:17:39:
Anyone know of a particularly good map projection for
showing
all
of
North and South America without too much distortion? THK -- Timothy H. Keitt, University of Texas at Austin Contact info and schedule at
Reprints at http://www.keittlab.org/tkeitt/papers/ ODF attachment? See http://www.openoffice.org/
_______________________________________________ R-sig-Geo mailing list R-sig-Geo at stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
-- Timothy H. Keitt, University of Texas at Austin Contact info and schedule at http://www.keittlab.org/tkeitt/ Reprints at http://www.keittlab.org/tkeitt/papers/ ODF attachment? See http://www.openoffice.org/
-- Roger Bivand Economic Geography Section, Department of Economics, Norwegian
School
of
Economics and Business Administration, Helleveien 30, N-5045
Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 e-mail: Roger.Bivand at nhh.no
_______________________________________________ R-sig-Geo mailing list R-sig-Geo at stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
-- Timothy H. Keitt, University of Texas at Austin Contact info and schedule at http://www.keittlab.org/tkeitt/ Reprints at http://www.keittlab.org/tkeitt/papers/ ODF attachment? See http://www.openoffice.org/
_______________________________________________ R-sig-Geo mailing list R-sig-Geo at stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
_______________________________________________ R-sig-Geo mailing list R-sig-Geo at stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
-- Roger Bivand Economic Geography Section, Department of Economics, Norwegian School of Economics and Business Administration, Helleveien 30, N-5045 Bergen, Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43 e-mail: Roger.Bivand at nhh.no
Timothy H. Keitt, University of Texas at Austin Contact info and schedule at http://www.keittlab.org/tkeitt/ Reprints at http://www.keittlab.org/tkeitt/papers/ ODF attachment? See http://www.openoffice.org/