Google Maps Circle / MySQL query

Question

I would like to draw a circle on a map, and get all rows in my database with geo the lat/lng points that fall into that circle.

I don't want draw the circle using google.maps.Circle as that applies mercator projection distortion. I want a geometric rather than geographic circle.

So I can probably draw this to Google maps using an overlay of some sorts. But is it possible to query my database for these points?

If I were to draw a geographic circle I could use the haversine formula, but that won't work for a geometric circle.

david strachan · Accepted Answer · 2015-01-19T23:21:32.793

You can use Pythagoras to find points within circle.

a^2 + b^2 = c^2

The following SQL uses PDO to find points within the radius of circle

$stmt = $dbh->prepare("SELECT  name, lat, lng, ( SQRT(POW(? - lat, 2)
                      + POW(? - lng, 2))) AS distance FROM mytable   
                       HAVING distance < ? ORDER BY distance ASC ");
    // Assign parameters
    $stmt->bindParam(1,$center_lat);//Center of circle
    $stmt->bindParam(2,$center_lng);////Center of circle
    $stmt->bindParam(3,$radius);

Javascript function if required

function Pythagoras (x1,y1,x2,y2){
    var Dist = Math.sqrt(Math.pow(x1 - x2, 2) + Math.pow(y1 - y2, 2));
return Dist ;
}

score 0 · Answer 2 · answered Jan 19 '15 at 22:51

You can draw the circle using a polygon with enough segments to look ok:

// lat, lng, and radius specify the circle
// CosDeg, SinDeg are functions that accept input in degrees    
mPerDegLat = 110540,
mPerDegLng = CosDeg(lat) * 111320; //-- mPerDegLng CORRECTED FOR LATITUDE
numSegs = 64;
deltaAng = 360 / numSegs;
verts = [];
for (i=0; i<numSegs; i++) {
    angle = i * deltaAng;
    dy = CosDeg(angle) * radius;
    dx = SinDeg(angle) * radius;
    deltaLat = dy / mPerDegLat; //-- METRES PER DEGREE
    deltaLng = dx / mPerDegLng;
    vertex = {lat:(lat + deltaLat), lng:(lng + deltaLng)};
    verts.push(vertex);
}
polyCircle = new google.maps.Polygon({paths:verts, strokeColor...});

As for MySql, could you just run your query for a "geographically rectangular" area using lat, lng values with enough padding to make sure you don't miss any, and then filter the results with a script that does the inverse calculation to see if they are inside the circle?

Google Maps Circle / MySQL query

2 Answers2