Good projection for N/S America?
On Mon, 9 Apr 2007 White.Denis at epamail.epa.gov wrote:
Roger's solution makes sense to me. The sinusoidal does have the appearance of pinching poleward, a consequence of allocating equal area by spacing equally in both x and y.
Both Canada and Norway - especially including Spitzbergen - suffer from this, but good compromises are hard to find if one needs Patagonia too. I can see that getting rgdal binaries for popular platforms is a real issue, any suggestions? Roger
Also the Lambert cylindrical sent in before should have had standard parallels set to +/- 30, i.e., (See attached file: whemi.projs.png) Tim Keitt <tkeitt at gmail.com> wrote on 2007-04-09 13:38:33:
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
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/
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