Correct way of finding distance between two coordi

Question

I am trying to calculate distance between two locations using spatial functions in both Mysql and PostgresSQL. I have taken the latitude and longitude from Google. The details are below

Location one - Lat: 42.260223; Lon: -71.800010

Location two - Lat: 42.245647; Lon: -71.802521

SQL Query used:

SELECT DISTANCE(GEOMFROMTEXT('Point(42.260223 -71.800010)'),GEOMFROMTEXT('Point(42.245647 -71.802521)'))

The both databases are giving the same result 0.014790703059697. But when I calculate distance in other systems the results are different. Please refer the below links

http://www.zip-codes.com/distance_calculator.asp?zip1=01601&zip2=01610&Submit=Search = 1.44 miles

http://www.distancecheck.com/zipcode-distance.php?start=01601&end=01610 = 1.53 miles

So I want to know whether my calculation method/query is right or not. And if it is wrong, then what is the right way of querying the db for the distance.

Answer 1

The simple answer is to use the Haversine formula. This assumes the earth is a sphere, which it isn't, but it's not a bad approximation. This, with lots of other details are described in this presentation:

http://www.scribd.com/doc/2569355/Geo-Distance-Search-with-MySQL

Answer 2

In the case above, MySql is simply applying the pythagorean theorem: c2 = a^2 + b^2. In this specific case SQRT((42.245647 - 42.260223)^2 + (-71.802521^2 - -71.800010)^2) = 0.014790703.

There are actually two problems with using the MySql distance functon for distance with coordinates on a sphere. (1) MySql is caclulating distance on a plane, not a sphere. (2) The result is coming back in a form of degrees, not miles. To get a true, spherical distance in miles, km, ft etc, you need to convert your lat and long degrees into the units you want to measure by determining the radius from a line through the center of the earth for the latitude(s) you are measuring.

To get a true measure is quite complicated, many individuals and companies have made careers out of this.