[R-sig-Geo] calculating area

[Don MacQueen]

At 12:14 AM -0700 1/26/07, 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.

In addition to the good advice already sent by others:

A quick and simple improvement would be to get what's called the 
convex hull. See
   help.search('convex hull')
   ?chull
and some others.

The complex hull is a polygon, instead of the rectangle you're now 
using, that will more closely surround your locations. Then all you 
need is a function to calculate the area of a polygon. I know one or 
more exists in R or a package, but I don't recall the name (or names).

>   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.
>
>Pedro
>
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>
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-Don

-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA