All--
In response to a private query, I sent Laura a draft revision of
-vincenty-early this morning that will use any of these ellipsoids; in
Stata, type
net from http://www-personal.umich.edu/~nicholsa/stata
to get it. She told me she was not using Stata today, so she would
try it out another day.
I doubt the issue is really one of choice of ellipsoidal or spherical
approximation; I suspect she is computing distances from farms to
random points (one on each lake), judging from other questions on the
Statalist. As I told her privately, that will often be worse than
using the centroids of lakes, but if she has the shapefile she can
compute the distance to the nearest vertex. That approach would also
work for rivers, if she had a shapefile for rivers.
Perhaps her calculations are failing because the latitudes and
longitudes are not in the same format in both datasets, i.e. signed
decimal degrees--I have not investigated that possibility.
On Sun, Sep 13, 2009 at 8:22 AM, Laura Platchkov <[email protected]> wrote:
> Dear all,
>
> I am trying to find out abotu those distances, and apparently somethign went wrong with the conversion and the distmatch formula, so I am thinkin abotu trying the vincenty.
>
> I am not experienced wiht programming,... may I ask if someone knows how to modify the vincenty command for Africa, using Clarke 1880 ellipsoid value?
>
> From:http://www.movable-type.co.uk/scripts/latlong-vincenty.html
>
> The most accurate and widely used globally-applicable model for the earth ellipsoid is WGS-84, used in this script. Other ellipsoids offering a better fit to the local geoid include Airy (1830) in the UK, International 1924 in much of Europe, Clarke (1880) in Africa, and GRS-67 in South America. America (NAD83) and Australia (GDA) use GRS-80, functionally equivalent to the WGS-84 ellipsoid.
> WGS-84 a = 6 378 137 m (±2 m) b = 6 356 752.3142 m f = 1 / 298.257223563
> GRS-80 a = 6 378 137 m b = 6 356 752.3141 m f = 1 / 298.257222101
> Airy (1830) a = 6 377 563.396 m b = 6 356 256.909 m f = 1 / 299.3249646
> Int’l 1924 a = 6 378 388 m b = 6 356 911.946 m f = 1 / 297
> Clarke (1880) a = 6 378 249.145 m b = 6 356 514.86955 m f = 1 / 293.465
>
> Laura
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