*------------------------------------------------------- FUNCTION CalculateDistanceFromLatLong( lat1 AS Double, ; lon1 AS Double, ; lat2 AS Double, ; lon2 AS Double ) *------------------------------------------------------- *-- http://www.meridianworlddata.com/Distance-Calculation.asp *----------------------------------------------------------------------------- *-- The following approximate distance calculations are relatively simple, *-- but can produce distance errors of 10 percent of more. These approximate *-- calculations are performed using latitude and longitude values in degrees. *-- The first approximation requires only simple math functions: *-- *-- Approximate distance in miles: *-- *-- sqrt(x * x + y * y) *-- *-- where x = 69.1 * (lat2 - lat1) *-- and y = 53.0 * (lon2 - lon1) *-- *-- You can improve the accuracy of this approximate distance calculation by *-- adding the cosine math function: *-- *-- Improved approximate distance in miles: *-- *-- sqrt(x * x + y * y) *-- *-- where x = 69.1 * (lat2 - lat1) *-- and y = 69.1 * (lon2 - lon1) * cos(lat1/57.3) *-- *-- If you need greater accuracy, you can use the Great Circle Distance Formula. *-- This formula requires use of spherical geometry and a high level of floating *-- point mathematical accuracy - about 15 digits of accuracy (sometimes called *-- "double-precision"). In order to use this formula properly make sure your *-- software application or programming language is capable of double-precision *-- floating point calculations. In addition, the trig math functions used in *-- this formula require conversion of the latitude and longitude values from *-- decimal degrees to radians. To convert latitude or longitude from decimal *-- degrees to radians, divide the latitude and longitude values in this *-- database by 180/pi, or approximately 57.29577951. The radius of the Earth *-- is assumed to be 6,378.8 kilometers, or 3,963.0 miles. *-- *-- If you convert all latitude and longitude values in the database to *-- radians before the calculation, use this equation: *-- *-- Great Circle Distance Formula using radians: *-- *-- 3963.0 * arccos[sin(lat1) * sin(lat2) + cos(lat1) * cos(lat2) * cos(lon2 - lon1)] *-- *-- If you do NOT first convert the latitude and longitude values in the database *-- to radians, you must include the degrees-to-radians conversion in the calculation. *-- Substituting degrees for radians, the formula becomes: *-- *-- Great Circle Distance Formula using decimal degrees: *-- *-- 3963.0 * arccos[sin(lat1/57.2958) * sin(lat2/57.2958) + ; *-- cos(lat1/57.2958) * cos(lat2/57.2958) * *-- cos(lon2/57.2958 -lon1/57.2958)] *-- *-- OR *-- *-- r * acos[sin(lat1) * sin(lat2) + cos(lat1) * cos(lat2) * cos(lon2 - lon1)] *-- *-- Where r is the radius of the earth in whatever units you desire. *-- r=3437.74677 (statute miles) *-- r=6378.7 (kilometers) *-- r=3963.0 (normal miles) *----------------------------------------------------------------------------- LOCAL lnRetVal lnRetVal = 0 IF VARTYPE( lat1 ) == "N" AND ; VARTYPE( lat2 ) == "N" AND ; VARTYPE( lon1 ) == "N" AND ; VARTYPE( lon2 ) == "N" IF lat1 <> 0 AND ; lon1 <> 0 AND ; lat2 <> 0 AND ; lon2 <> 0 lnRetVal = 3963.0 * ACOS( SIN(lat1/57.2958) * SIN(lat2/57.2958) + ; COS(lat1/57.2958) * COS(lat2/57.2958) * ; COS(lon2/57.2958 -lon1/57.2958) ) ENDIF ENDIF RETURN lnRetVal ENDFUNC
