[Proj] Distance between pair of lat / lons

Charles Karney charles.karney at sri.com
Thu Dec 27 14:09:11 EST 2012

On 12/26/2012 05:38 PM, Clifford J Mugnier wrote:
> When I use ellipsoidal geodesics (normally the inverse case), I always
> use whatever is published by the United States National Geodetic
> Survey.  Regardless of the accuracy returned (which is quite good for
> several hundred miles/kilometers); it is what is legally considered as
> the "National Standard" recognized by /our/ national U.S. government.


Can you point me to documentation for this assertion?

I'm guessing that, given the dispersal of judicial responsibilities in
the US (with its federal, state, and local governments), the situation
is less clear cut than you claim.  Probably a lawyer on one side wheels
out an expert witness who will say he's using the NGS algorithm for
ellipsoidal distances.  The opposing lawyer will have contacted his
expert and knows that he's on the losing side of the distance argument
(probably by a lot) and decides not to contest the distance
computation.  Actually, the bigger question (where expert witnesses will
probably also be involved) is whether to use the ellipsoidal geodesic
distance at all (as opposed to map distance or road distance) and what
datum to use.

The outcome would not be very different if the first expert witness used
any other reasonable algorithm for geodesic distances because legal
questions rarely depend on the discriminating distances to an accuracy
of 1/10 mm (the accuracy of the Vincenty algorithms).

(All of this is based on many hours studying the US legal system
through the medium of TV shows.)


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