[R-sig-Geo] calculating area

[Michael Sumner]

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
>   

This sounds to me like what marine biologists (seals, penguins, etc.) 
refer to as "time spent", and what seems to be
more generally referred to in terrestrial tracking as a "utilization 
distribution".

We routinely use simple interpolation and grid cell counts to provide a 
simple map of time spent (or diving effort or other foraging-proxy),
and I have functions to perform this for sets of individual tracks in 
the 'trip' package.  Another common method is to use
kernel density to try to smooth over the gaps between location estimates 
in tracks and provided a density
map of time spent - but both linear interpolation and kernel density 
provide only a very simplistic model of the
inferred motion.  

It's fine for a rough first pass, and we often use the count of grids 
cells for comparing different groups - the 'sp' package makes this very 
simple to do.

In terms of calculating area I see it as a matter of choosing the 
appropriate projection for your coordinates, and then defining the
grid of cells to make comparisons between times or groups.

I can provide an example of doing this in the trip package if that would 
help.

Cheers, Mike.