Limiting the number of GPS locations in MySQL query [duplicate] - php

Question

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MySQL Great Circle Distance (Haversine formula)
(9 answers)
Closed 8 years ago.
I'm using the following query to get the next local events using their location. My database is growing and I've got events from all over Europe. So basically, the query calculates distances from events of two different countries (like Spain and Germany) which is really not useful since I'm looking for the next events within 20 KM.
# latitude & longitude are the fields in DB
# 43.57 & 3.85 are given values representing the local test point
SELECT ( 6366 * ACOS( COS( RADIANS( 43.57 ) ) * COS( RADIANS( latitude ) ) * COS( RADIANS( longitude ) - RADIANS( 3.85 ) ) + SIN( RADIANS( 43.57 ) ) * SIN( RADIANS( latitude ) ) ) ) dist
FROM events
HAVING dist >0 AND dist <=20
ORDER BY event_time ASC
LIMIT 0 , 5
So basically, this query is going to get all the distances of all the events before being able to use the HAVING.
Is there a better way?

Answer 1

Calculate the longitude and latitude of the four points of a box that encloses the radius surrounding the test point. Over distances of a few kilometres you can ignore errors due to the curvature of the Earth.
We can use a rough approximation to limit an initial search, then refine things later.
At 50 degrees North (roughly Paris) a degree of latitude (North/South) is approximately 111.229km. A degree of longitude (East/West) is approximately 71.695km (I got these numbers from this page From this 20Km is approximately 10.8 seconds of latitude, and 16.74 seconds of longitude. Calculate a bounding box by adding and subtracting these numbers from the latitude and longitude of your test point.
Query for locations in that box using longitude and latitude.
If you're worried that your bounding box may be too large, or that an event in a corner of the box is outside the radius, calculate an accurate distance for those points you've already identified and filter out the few you don't want.
Note
These figures approximate for 50deg North. As you move north of that your value for km of longitude will become too long, and south of that it will be too short. You could use a small table of lookup values, or calculate the actual distance from the latitude, or simply increase the initial bounding box in the longitude direction. The box is just a first approximation, so as long as it's bigger than the target area it's exact size doesn't matter too much.