> A comparison of some geographic distance calculations is provided at 
> 
http://pineda-krch.com/2010/11/23/great-circle-distance-calculations-in-r/ 

> , along with code for calculating the Vincenty inverse formula, which 
> relies on the WGS-84 ellipsoid approximations.

You know, Scott, I should have included some test results of that method. 
Comparing the distances with Arc 9 indicates that the accuracy varies with 
location and whether there is a longitudinal difference in the two points. 
 Comparing calculation results for points shifted 0 secs to 10 degrees 
North, West and Northwest from a 'base' point, the relative errors 
(defined as (Arc9.distance - Vincenty.distance)/Arc9.distance) range up to 
0.08 in AK, AZ, CA, MT, NE, NM, UT, WA and WY, and range only up to 0.009 
otherwise.  In the special case of zero longitudinal offset (North-South 
distances only), the relative error ranges to 0.006 in those states and to 
2E-7 otherwise.

Let us know if you can do better,
cur

-- 
Curt Seeliger, Data Ranger
Raytheon Information Services - Contractor to ORD
seeliger.curt@epa.gov
541/754-4638

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