[R-sig-Geo] Good projection for N/S America?
Tim Keitt
tkeitt at gmail.com
Mon Apr 9 22:38:33 CEST 2007
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
> > 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
> > > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > --
> > > > > > > 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
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> > > > > 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/
> > >
> > > _______________________________________________
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> >
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> >
>
> --
> 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/
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