Calculating GPS difference formula

[moone009]

I was wondering if anyone has a formula to calculate feet between two gps coordinates?

Latitude Longitude
42.7642733333333 -87.8000916666667
42.764265 -87.7996183333333

From the formulas that I have put together and the ones I have found online my results have varied. I have found a difference of anywhere from 20 feet all the way to 123 feet. I know for a fact these distances cannot be more than 15 feet apart. Any suggestions

 

[shg]

This function computes the central angle between two points on a spherical earth:

Function CentralAngle(ByVal lat1 As Double, ByVal lon1 As Double, _
                      ByVal lat2 As Double, ByVal lon2 As Double) As Double
    ' shg 2008-1111
    ' Returns central angle between two point in RADIANS
    ' using Vincenty formula
 
    Const pi    As Double = 3.14159265358979
    Const D2R   As Double = pi / 180#
    Dim dLon    As Double
    Dim x       As Double
    Dim y       As Double
 
    ' convert angles from degrees to radians
    lat1 = D2R * lat1
    lat2 = D2R * lat2
    dLon = D2R * (lon2 - lon1) ' delta lon
 
    x = Sin(lat1) * Sin(lat2) + Cos(lat1) * Cos(lat2) * Cos(dLon)
    y = Sqr((Cos(lat2) * Sin(dLon)) ^ 2 + (Cos(lat1) * Sin(lat2) - Sin(lat1) * Cos(lat2) * Cos(dLon)) ^ 2)
    CentralAngle = WorksheetFunction.Atan2(x, y)
End Function

Multiply by the mean earth radius in the units of your choice, e.g., for feet,

=20902000*CentralAngle(A2, B2, A3, B3)

      ----A---- ----B----- --C--
  1      Lat       Lon     Dist 
  2   42.764273 -87.800092 126.8
  3   42.764265 -87.799618

I know for a fact these distances cannot be more than 15 feet apart.

Sanity check: Your points are at essentially the same latitude and differ by 0.000473 degrees in longitude. A degree of longitude is 60 nautical miles at the equator, and decreases with the cosine of latitude.

= CONVERT(0.000473 * 60 * COS(RADIANS(42.76)), "Nmi", "ft") returns ~ 126.6 feet

 

[moone009]

Thank you very much! I guess it could be the actual GPS unit that is recording the results.