Thank you all so much! That was great and very helpful information!
On Fri, May 20, 2011 at 9:09 PM, Michael Sumner <[email protected]> wrote: > Choosing a projection has its own new set of complications that are > even trickier. If you want to keep it simple and work in Lon lat see > ?spDistsN1 in the sp package to calculate distance on the WGS4 > ellipsoid. > > Mike > > On Saturday, May 21, 2011, Clint Bowman <[email protected]> wrote: > > I think I would convert your coordinates to a rectangular one (e.g., UTM > or a Lambert Conformal) and compute distance relationships there. > > > > Clint > > > > -- > > Clint Bowman INTERNET: [email protected] > > Air Quality Modeler INTERNET: [email protected] > > Department of Ecology VOICE: (360) 407-6815 > > PO Box 47600 FAX: (360) 407-7534 > > Olympia, WA 98504-7600 > > > > > > USPS: PO Box 47600, Olympia, WA 98504-7600 > > Parcels: 300 Desmond Drive, Lacey, WA 98503-1274 > > > > > > On Sat, 21 May 2011, Thomas Lumley wrote: > > > > > > On Sat, May 21, 2011 at 10:07 AM, Megan Marcotte <[email protected]> > wrote: > > > > Hello All > > > > > > I am new to the list and am hoping to get some advice. I am working with > > statisticians who are doing the programming of a new analysis in R where > we > > are comparing animal tracks with environmental parameters. I was > wondering > > if anyone knows or has an opinion on the following: > > > > 1) a) A rule of thumb or even a hard rule for when you need to start > > using great circle instead of Euclidian calculations for distances? Just > a > > bit more information for perspective: depending on the animals tracked > the > > distances between fixes could be as small as 20 m or up to 1.5 km. The > > tracks may be up to 20 km of cumulative length. > > > > > > When you say "Euclidian" it's not clear whether you mean > > a/ treating the earth as flat > > b/ treating latitude and longitude as a rectangular coordinate system > > c/ treating the degree grid as square. > > > > (a) should be fine on this scale, for (b) and (c) it depends on the > latitude > > > > > > 2) b) Are functions based on Euclidian math okay to use with WGS84 > > coordinates if the total distance of the tracks are <20 km, or a max of > 0.5 > > degrees of latitude (but usually much less, at latitudes of 25 or or -36 > > degrees). I know that with latitude the length of a degree of longitude > > changes but at this scale is it a factor? Any rules for this? > > > > > > > > I would have said that it was perfectly ok to treat the earth as flat > > and the degree grid as rectangular on this sort of scale and at these > > latitudes, but that you can't treat the degree grid as square. That > > is, a degree of longitude is about 10% less than a degree of latitude > > at 25 degrees and about 20% less at 36 degrees. > > > > -thomas > > > > > > > > -- > Michael Sumner > Institute for Marine and Antarctic Studies, University of Tasmania > Hobart, Australia > e-mail: [email protected] > > _______________________________________________ > R-sig-Geo mailing list > [email protected] > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > [[alternative HTML version deleted]] _______________________________________________ R-sig-Geo mailing list [email protected] https://stat.ethz.ch/mailman/listinfo/r-sig-geo
