Hi Anthony & dmhoneycutt, Both of your reply are very helpful! From Anthony's replay I learned that the problem is referring as "great circle distances". I did further Google search and found the math formula for solve it in this site: http://en.wikipedia.org/wiki/Great-circle_distance. Because I plan to do the calculation with c# code anyway (for special selection and post processing), I can use the formula to calculate the great-circle distance. dmhoneycutt's replay prompt me to do a search in the ESRI SDK help, I found there is some interface to get the great-circle distance of two points. So I can skip the formula and direct use the interface. Thanks both of you for opening the door for me! Ming
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