Going the Distance

fran_parrett Couple thoughts, let's say that one of your postcodes is blank and that is a substitute for City in the example. You could do this:

c = 
    VAR __FromCity = SELECTEDVALUE('From City'[City])
    VAR __ToCity = SELECTEDVALUE('To City'[City])
    VAR __FromLat = LOOKUPVALUE('Table'[Latitude],'Table'[City],__FromCity)
    VAR __ToLat = LOOKUPVALUE('Table'[Latitude],'Table'[City],__ToCity)
    VAR __FromLong = LOOKUPVALUE('Table'[Longitude],'Table'[City],__FromCity)
    VAR __ToLong = LOOKUPVALUE('Table'[Longitude],'Table'[City],__ToCity)
    VAR __distanceLong = RADIANS(__ToLong - __FromLong)
    VAR __distanceLat = RADIANS(__ToLat - __FromLat)
    VAR __a = (SIN(__distanceLat/2))^2 + COS(RADIANS(__FromLat)) * COS(RADIANS(__ToLat)) * SIN((__distanceLong/2))^2
    VAR __y = SQRT(__a)
    VAR __x = SQRT(1 - __a)
    VAR __atan2 = 
        SWITCH(
            TRUE(),
            __x > 0, ATAN(__y/__x),
            __x < 0 && __y >= 0, ATAN(__y/__x) + PI(),
            __x < 0 && __y < 0, ATAN(__y/__x) - PI(),
            __x = 0 && __y > 0, PI()/2,
            __x = 0 && __y < 0, PI()/2 * (0-1),
            BLANK()
        )
    VAR __c = 2 * __atan2
    VAR __Result = IF( __FromCity = BLANK() || __ToCity = BLANK(), 0, __c)
RETURN
    __Result