Re: lat lon function

"Edward Peloke" <epeloke@echoman.com> wrote in message news:NFBBIIKNALMFFNANEKEFGEAGDMAA.epeloke@echoman.com... > Hello all, > > I currently have a php function that takes two airport codes and returns > their distance in miles. The function gets the longitude and latitude for > the airports from a mysql db. My current problem is I have to show the user > all airports within a certain mile radius of the one they selected. I can't > run this function against all 7000+ rows in the db and return only the ones > needed, how can I do this? I know I have seen this several places online. Is the radius a constant value, or chosen from a short list of constant values? If the user can enter whatever radius they want, you pretty much have to calculate this dynamically; otherwise I would really consider precalculating and storing this for each airport (it's not like the airports are likely to move on short notice). For fun, I will now derive the surface-distance formula; this requires trig and basic vector math. For each airport, we must convert the polar coordinates of latitude and longitude to rectangular coordinates. We will assume the Earth is a perfect sphere. Let R = the Earth's radius, then for each x = R cos(lon) cos(lat) y = R sin(lon) cos(lat) z = R sin(lat) Because the sphere's center is the original, we can also consider these coordinates as vectors from the center to the surface; so we can find the angle between the two vectors. By the dot product, A1 (dot) A2 = |A1| |A2| cos(theta), so theta = acos( ( A1 (dot) A2 ) / ( |A1| |A2| ) ). We know that |A1| == |A2| == R; also, A1 (dot) A2 = x1 x2 + y1 y2 + z1 z2 and each of these components has an R. Simplifying, we get theta = acos( cos(lon1)cos(lat1)cos(lon2)cos(lat2) + sin(lon1)cos(lat1)sin(lon2)cos(lat2) + sin(lat1)sin(lat2) ) = acos( [sin(lon1)sin(lon2) + cos(lon1)cos(lon2)] cos(lat1)cos(lat2) + sin(lat1)sin(lat2) ) then if theta is expressed in radians, the distance between the airports = R theta, and if R is expressed in miles, so will the distance be. From here on I will assume that latitude and longitude are angles in (-90..90) and [-180..180) degrees. <?php define("R", 3960); // Earth's radius in miles // Calculate distance between two points: // lat/lon, lat/lon in decimal-degrees. // Returns distance in miles. deg_dist($lat1, $lon1, $lat2, $lon2) { return( rad_dist( deg2rad($lat1), deg2rad($lon1), deg2rad($lat2), deg2rad($lon2) )); } // Calculate distance between two points: // lat/lon, lat/lon in radians. // Returns distance in miles. rad_dist($lat1, $lon1, $lat2, $lon2) { return( R * acos( ( sin($lon1)*sin($lon2) + cos($lon1)*cos($lon2) ) * cos($lat1)*cos($lat2) + sin($lat1)*sin($lat2) )); } ?>