st: New version of -geonear-; updated helpfile for -geodist-

[Robert Picard <picard@netbox.com>]

Thanks to Kit Baum, a new version of the -geonear- package is now
available from SSC. The -geodist- package has also been updated with a
new help file; the -geodist- program remains unchanged.

-geonear- finds the nearest neighbors using geodetic distances, i.e.
the length of the shortest curve between two points along the surface
of a mathematical model of the earth. Both great-circle and ellipsoid
distances are available.

The new version of -geonear- adds a long form option with the ability
of finding all neighbors within a specified distance.

-geonear- uses a divide and conquer strategy to significantly reduce
the number of distances that must be computed. The approach involves
splitting the initial set of locations into progressively smaller
geographic regions while at the same time safely reducing the set of
potential neighbors for each region. Generally, geonear will
predictably find the nearest neighbors in a linear time even though
the size of the overall problem increases exponentially. In other
words doubling the size of both location sets simply doubles the
number of regions and thus doubles the run time.

-geodist- provides a standalone version of -geonear-'s distance
routines. The program remains unchanged but the help file has been
updated with new examples as well as a link to a new distance
calculator that does not have limitations for ellipsoid distances
between near-antipodal points. There's also a photo of my GPS lying on
the Prime Meridian line at the Royal Observatory in Greenwich,
England!

Both -geonear- and -geodist- require Stata version 9.2 or higher. To
install, type in Stata:

ssc install geonear
ssc install geodist

If you are updating, use -adoupdate- or type:

ssc install geonear, replace
ssc install geodist, replace

Cheers,

Robert
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