Google "Haversine formula" to get the great circle distance between two points.
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That assumes the earth is a perfect sphere rather than the more accurate model of an oblate spheroid. If you need more accuracy than that, there are more complex formulas.
I cheat by using the line drawing tool in Google Earth. Can also be used to illustrate LOS between two points. (change line altitude parameters) Very cool.
This will get you down to the millimeter level, not micrometer. ;-)
For example, from Toronto to Chicago is 702.9810543723332 km elliptic, but the spherical solution gives only 700.9181433797103 km. That's an error of more than 2,000 meters.. that could put you on the wrong runway or something.
I've programmed this for checking distances between zip codes using a converison table, there are three formulas depending on what you need for accuracy. Acroos town, the difference between the formulas is like less than a tenth of a mile, it's more significant at longer distances.
What I use: If lat1 lat2 And long1 long2 Then X = (Sin(lat1 / 57.2958) * Sin(lat2 / 57.2958)) + (Cos(lat1 /
57.2958) * Cos(lat2 / 57.2958) * Cos(long2 / 57.2958 - long1 /
57.2958)) Y = Sqr(1 - X ^ 2) miles3 = 3963# * Atn(Y / X) Else miles3 = 0 End If Sensitive to where Lattitude and longitude are equal(same zip codes), hence the check. Also runs a lot longer if you've got thousands of entries to check.
From a tutorial out on the web from about 10 years back. HTH
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