[R-sig-Geo] Distances in geoR package

[Tim Keitt]

Great. This looks useful. We need a "geoToolbox" package to collect
these sorts of utility functions...

THK

On Thu, 2005-06-16 at 09:26 -0700, White.Denis at epamail.epa.gov wrote:
> In higher latitudes, if the study area has a relatively small range of
> latitude (say less than 5 degrees), then simply scaling the longitudes
> by the cosine of the mid-latitude, will give a satisfactory coordinate
> system.  For example, if the study area ranges from latitude 40 to 45,
> multiply the longitudes by cos (42.5 * pi/180) = 0.73..., or use the
> function below.  This projection is neither equal-area nor conformal.
> 
> marinus <- function (lon, lat)
>   # Arguments are vectors of longitude and latitude,
>   # that is, geographic coordinates in decimal degrees.
>   # Projection center defined as midpoint in lat-lon space.
>   # Returns a two column matrix with column names "x" and "y",
>   # in units of kilometers.
>   # Line identifiers in lon[is.na (lat)] retained in x.
>   # This is the equidistant cylindric map projection,
>   # here named after Marinus of Tyre.
>   # (ref JP Snyder, USGS Prof Paper 1395, p 90).
> {
>     R <- 6371 # authalic radius of Clarke 1866 rounded to km
>     rlat <- range (lat, na.rm=TRUE)
>     rlon <- range (lon, na.rm=TRUE)
>     clat <- diff (rlat) / 2 + rlat[1]
>     clon <- diff (rlon) / 2 + rlon[1]
>     x <- R * (lon-clon)*pi/180 * cos (clat*pi/180)
>     y <- R * (lat-clat)*pi/180
>     x[is.na (lat)] <- lon[is.na (lat)]
>     xy <- cbind (x=x, y=y)
>     xy
> }
> 
> r-sig-geo-bounces at stat.math.ethz.ch wrote on 2005-06-16 07:43:22:
> 
> > For low-latitudes and small distances, the error should be
> > insignificant. You can check using proj.4 to compute some distances in
> > both Euclidean and geodesic distances. For large distances or high
> > latitudes, you can 1) project to a Cartesian coordinate system or 2)
> > compute geodesic distances. I think there is an "earth.distance"
> > somewhere in R.
> >
> > THK
> >
> >
> > On Thu, 2005-06-16 at 15:58 +0200, giovanna jona lasinio wrote:
> > > Dear all, this may be a stupid question, but I wasn't able to find
> an
> > > answer in the FAQ and in the mail archive.
> > > I have a huge amount of  data, 644 sites for 30 different  times,
> which
> > > coordinates are expressed in decimal longitude and latitude, I'd
> like to
> > > simply compute the empirical variogram and use "variofit" to
> estimate
> > > variogram's parameters for each time. The "variog" function uses
> > > euclidean distances and then it is not appropriate for lon-lat
> > > coordinate. Is there any way to change the distance type in
> "variog"? Or
> > > which information are necessary to "variofit" in order to estimate
> > > variogram's parameters?
> > >
> > > Thanks for any help
> > > Giovanna Jona Lasinio
> > >
> > > _______________________________________________
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> > > R-sig-Geo at stat.math.ethz.ch
> > > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> > --
> > Tim Keitt
> > Section of Integrative Biology
> > http://www.keittlab.org/
> >
> > _______________________________________________
> > R-sig-Geo mailing list
> > R-sig-Geo at stat.math.ethz.ch
> > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> 
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-- 
Tim Keitt
Section of Integrative Biology
http://www.keittlab.org/