Or maybe the Haversine Formula? Which? In VFP, If I have two coordinates on a map with x/y for 0/0 like the examples below and a radius to draw a circle around that position (let's say 500 ft), how can I tell if any other x/y coordinates are within the radius? I know I need to check and see if the distance to the 2nd x/y coordinates is less than the radius, but it escapes me -- math in general tonight. I've tried checking within the radius, just returning the distance in feet, but just cannot get it right....

None of these are accurate--the distance should be about 392 ft (a mish-mash of attempts from me, the web, etc):

*circumference is 360 degrees times 60 minutes/degree times 1.852 km/minute = 40003.2 km. 
*The implied radius is the circumference divided by 2 pi: R = 6367 km = 3956 mi 
lon1=  1698101.25   &&1713881.3750
 lat1=   813831.69 && 833601.8125
lon2 =    1698215.13 &&1697432.8750
lat2 =    813458.25 && 810161.0625
 R = 3956 && 3963 && 3956
 dlon = lon2 - lon1
  dlat = lat2 - lat1
  a = (sin(dlat/2))^2 + cos(lat1) * cos(lat2) * (sin(dlon/2))^2
  ? a
  c = 2 * atn2(sqrt(a), sqrt(1-a)) 
  ? c
  d = R * c
  ?[Example1: ]
  ??d

   
*--is point within a radius
radius = 500  &&(what would 500 ft be?)
centerx =  1698101.25   &&1713881.3750
centery =   813831.69 && 833601.8125
x =    1698215.13 &&1697432.8750
y =    813458.25 && 810161.0625

?[Example2: ] 
?? (x-centerx)^2 + (y - centery)^2 
?[     Inside radius: ] 
?? (x-centerx)^2 + (y - centery)^2 < radius^2



*--Get distance between two points
#DEFINE KMRAD 6377
#DEFINE MIRAD 3956 && 3963  
#DEFINE RADIANS DTOR


tlKM = .F.

lon1=  1698101.25   &&1713881.3750
lat1=   813831.69 && 833601.8125
lon2 =    1698215.13 &&1697432.8750
lat2 =    813458.25 && 810161.0625

    lnDistance=IIF(tlKM, KMRAD, MIRAD)*;
     ACOS(COS(DTOR(90-lat1))*COS(DTOR(90-lat2))+;
     SIN(DTOR(90-lat1))*SIN(DTOR(90-lat2))*;
     COS(DTOR(lon1 - lon2)))

?[Example3: ]
??lnDistance  && *5280 && 5280 feet in a mile



*--Select all within the radius  &&Courtesy Carl Karsten 
#DEFINE cdbl
#DEFINE _PI 3.1415926535893
#DEFINE _StatuteMile 5280 &&' 5280 feet = 1 statute mile
#DEFINE _NauticalMile 6076.11549 &&' 6076.11549 feet = 1 nautical mile
#DEFINE _Seconds 60 && ' 60 seconds = 1 nautical mile
lon1=  1698101.25   &&1713881.3750
lat1=   813831.69 && 833601.8125
lon2 =    1698215.13 &&1697432.8750
lat2 =    813458.25 && 810161.0625

? InRadius(lat1,lon1,500,lat2,lon2)
return

FUNCTION InRadius( tlat, tlong, tnRadius, tlat2, tlong2 )
    LOCAL lnResults, lnSeconds, ;
      lnLatRange, lnLonRange,;
      lnLowLat, lnLowLon, ;
      lnHighLat, lnHighLon
      
   *' 1 degree of latitude = 60 nautical miles or 1 minute = 1 nautical mile
    lnLatRange = tnRadius / ((_NauticalMile / _StatuteMile) * _Seconds)

    *  Longitude is a bit more complicated; 
    * 1 degree of longitude at the equator = 60 nautical miles, 
    * At the poles 1 degree = 0 nautical miles.
    lnLonRange = tnRadius / (((COS(cdbl(tlat * _PI / 180)) * ;
				_NauticalMile) / _StatuteMile) * _Seconds)

    lnLowLat = tlat - lnLatRange
    lnHighLat = tlat + lnLatRange
    lnLowLon = tlong - lnLonRange
    lnHighLon = tlong+ lnLonRange

?[Example4: ]
?? tlat2 <= lnHighLat;
      AND tlat2 >= lnLowLat;
      AND tlong2 >= lnLowLon;
      AND tlong2 <= lnHighLon
?[Example5: ] 
?? getdistance2(lon1,lat1,lon2,lat2)

RETURN

      
FUNCTION getdistance2  &&Courtesy Carl Karsten
LPARAMETERS lon1,lat1,lon2,lat2

*--Get distance between two points
#DEFINE KMRAD 6377
#DEFINE MIRAD 3956 && 3963  
#DEFINE RADIANS DTOR


tlKM = .F.

lon1=  1698101.25   &&1713881.3750
lat1=   813831.69 && 833601.8125
lon2 =    1698215.13 &&1697432.8750
lat2 =    813458.25 && 810161.0625

    lnDistance=IIF(tlKM, KMRAD, MIRAD)*;
     ACOS(COS(DTOR(90-lat1))*COS(DTOR(90-lat2))+;
     SIN(DTOR(90-lat1))*SIN(DTOR(90-lat2))*;
     COS(DTOR(lon1 - lon2)))

?[Example6: ]
??lnDistance  

return lndistance

I'm brain dead! I've mucked it all up.....