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. 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 <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> wrote: > > On Mon, 9 Apr 2007 [EMAIL PROTECTED] 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 > > > > > [EMAIL PROTECTED] 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, [EMAIL PROTECTED] <[EMAIL PROTECTED]> > > > 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) > > > > > > > > > > [EMAIL PROTECTED] 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 [EMAIL PROTECTED] 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 <[EMAIL PROTECTED]> 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, [EMAIL PROTECTED] > > > > > <[EMAIL PROTECTED]> > > > > > > > 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) > > > > > > > > > > > > > > > > > > [EMAIL PROTECTED] 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@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: [EMAIL PROTECTED] > > > > > > > > > > > > _______________________________________________ > > > > > > R-sig-Geo mailing list > > > > > > R-sig-Geo@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@stat.math.ethz.ch > > > > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > > > > > > _______________________________________________ > > > R-sig-Geo mailing list > > > R-sig-Geo@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: [EMAIL PROTECTED] > > > > > > > > > -- > 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/
whemi.projs.png
Description: PNG image
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