Thank you all so much!  That was great and very helpful information!


On Fri, May 20, 2011 at 9:09 PM, Michael Sumner <mdsumner@gmail.com> 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 <clint@ecy.wa.gov> 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:       clint@ecy.wa.gov
> > Air Quality Modeler             INTERNET:       clint@math.utah.edu
> > 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 <megotte@gmail.com>
> 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: mdsumner@gmail.com
>
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