[R-sig-Geo] calculating area

[White.Denis at epamail.epa.gov]

r-sig-geo-bounces at stat.math.ethz.ch wrote on 2007-01-26 00:34:22:

> On Fri, 26 Jan 2007, Pedro S. A. Wolf wrote:
>
> >  I am a psychology student who has been using R to analyze
experimental data
> > mainly within the general linear model for about 2 years.  I have
some
> > working knowledge with R, but by no means would call myself
proficient so
> > bear with me.  Currently I'm running a series of experiments dealing
with
> > spatial data.  I'm tracking peoples movements using gps units which
give me
> > latitude, heading, longitude, speed, altitude, and time variables.
I have
> > been using the packages "maps" and "mapproj" to plot the routes
which these
> > individuals take which is a great visualization tool.  I would like
to know
> > if there is a package or set of commands which would calculate the
area
> > these people occupy?  So far the closest I have been able to get to
> > accomplishing this task is basically taking the max and min for both
lat and
> > long coordinates and calculating the size of that area, but I would
like
> > something more accurate.  Another thing I'm trying to accomplish is
creating
> > what I would call a density map. This is probably not the correct
term so
> > let me explain what I mean.  I'm trying to characterize how people
occupy
> > space.  Especially how people visit certain places more than others.
Is
> > there a way to create a map which shows the amount of times a
particular
> > place is visited by these people both within subjects and a second
map
> > between subjects? Help would be greatly appreciated, as my academic
advisors
> > know next to nothing about working with this type of data.
>
> If you think of including humans as animals, you'll find that field
> scientists, for example ecologists, do this a good deal - see the
package
> adehabitat, perhaps function NNCH.area for Home Range Area. The
package
> trip also provides tools for handling spurious GPS reports. You may
find
> that you need to transform the GPS coordinates from geographical to
> projected to make area calculation easier, and that maps and mapproj
will
> limit what you can do, perhaps consider moving to sp, maptools and
rgdal.
>
> (There will be an R spatial workshop at the Association of American
> Geographers conference in San Francisco on Tuesday 17 April; the
"tape" of
> Edzer Pebesma's eSeminar on R spatial classes and methods is not yet
> online, but we'll post when it is ready:
>
> http://www.wun.ac.uk/ggisa/seminars/archive/2006_program/index.html
>
> is where it should turn up).
>
> Roger
>

Depending on where on earth your data are from, the area calculations
may
be confounded by the spherical (or ellipsoidal) shape of the earth.  So
think about using the difference in longitude as you approach the poles.
If your data are from a small area and you are only interested in
relative
differences in area then the geographic coordinates may be sufficient,
otherwise you may want to check out a web site on map projections and
choose one that will give you good area fidelity.  One class of map
projections has the property called equal-area, and preserves, in the
plane of the projection, the area relationships on the spherical (or
ellipsoidal) earth.

Denis

> >
> > Pedro
> >