The first step is to create your new map. Go to Google Maps and click on the new “My Maps” tab.

Now you create your map. You can create polygons, lines or just points and include whatever you want in the descriptions because it will all be included in the KML.

Once you have completed your map you want to go to click on the KML link at the top right to save the KML for this map to disk.

Just last month Google added support for KML overlays to their map API. It isn’t very well documented on the GGeoXml documentation page but there is only one real issue you have to know about. The main thing to understand is that the KML file must sit on a public webserver somewhere and when you reference it Google needs to be able to access it. For example if you are testing on your local box and you put something like “http://localhost/mykml.kml” in for the URL it will not work. After you get past that the path to using your saved KML file is easy. Here is a full example:

    <script src="http://maps.google.com/maps?file=api&amp;v=2&amp;key=ABQIAAAAcktFen-Z7CQCe0dxIKmxeRReUzuwBhfUVuhk2aRjVRv1avb-hhQdlkzQnQ1P5PComj1CZx2jeOOyIw" type="text/javascript"></script>
    <script type="text/javascript"></script>

Notice that you have to zoom and center to somewhere since that isn’t controled by the KML file but other than that it makes creating embedded maps very simple.

And here are the results from the example:

Enjoy!

First it helps to have a little background. Most people are familiar with latitude and longitude but you may not be familiar with their exact definitions. I have included the links to the wikipedia pages for both so you can research them more if you want. The main things to know are: latitude runs north/south and each degree of latitude is about 111 km; longitude runs east/west and is 111 km times the cosine of the latitude. The next thing that is helpful to know is the radius of the earth: 6,378.135 km in this case I’ll just use the equatorial radius.

Now that we have the earth taken care of we need to go back to basics. We start by looking at the definition of a circle and the unit circle. The mathematical definition of a circle is the important part here. The following equations will play an important part: x = a + r cos(t) and y = b + r sin(t) where a,b are the center of the circle, x,y are points of the unit circle, r is the radius of the circle, and t is the angle of a line from the origin to x,y in radians. It helps to think of the circle as broken up into triangles.

The last thing to keep in mind is that latitude and longitude are in degrees but when working with cos and sin you need to use radians and not degrees. The conversion between radians and degrees is easy: radians = degrees * (PI / 180) and degrees = radians * (180 / PI).

So now for the main formula in pseudocode:

 earths_radius = 3963 // number of miles in the radius of the earth
 longitude = 80          // longitude of the center of the circle
 latitude = 90            // latitude of the center of the circle
 points = 32              // number of points in the polygon
 circle_radius = 1       // miles of radius

 // find the raidus in lat/lon, units per latitude, units per longitude
 r_latitude = radius_to_degrees(circle_radius / earths_radius)
 r_longitude = r_latitude / cos(degrees_to_radius(latitude))

 // create a point for each edge, we need one extra point to connect the
 // end to the begining
 for point_count = 0 to points + 1
 {
    theta = PI * (point_count / (points / 2))    // find the angle for the current triangle
    circle_x = lng + (r_longitude * cos(theta))  // center a + radius x * cos(theta)
    circle_y = lat + (r_latitude * sin(theta))     // center b + radius y * sin(theta)

    // circle_x, circle_y represents a point on the circle
 }

That is all there is to it. In this example I just picked 32 points for the circle but you may want more or less depending on how large your radius is and how closely you want to approximate the circle. One thing to note about this function is that it produces the points in a counter-clockwise direction.

Now for an example using google maps to display a circle around a point with a radius of one mile:

Missiondata World HQ, Louisville, Ky

And here is the javascript source you need to do it:

var d2r = Math.PI / 180;   // degrees to radians
var r2d = 180 / Math.PI;   // radians to degrees
var earthsradius = 3963; // 3963 is the radius of the earth in miles

var hqpoint = new GPoint(-85.578852, 38.215601);
var map = new GMap(document.getElementById("map"));

map.addControl(new GSmallMapControl());
map.addControl(new GMapTypeControl());
map.centerAndZoom(hqpoint, 4);
map.addOverlay(new GMarker(hqpoint));

drawCircle(-85.578852, 38.215601, map);

function drawCircle(lng, lat, map)
{
   var points = 32;
   var radius = 1;             // radius in miles

   // find the raidus in lat/lon
   var rlat = (radius / earthsradius) * r2d;
   var rlng = rlat / Math.cos(lat * d2r);

   var extp = new Array();
   for (var i=0; i < points+1; i++) // one extra here makes sure we connect the
   {
      var theta = Math.PI * (i / (points/2));
      ex = lng + (rlng * Math.cos(theta)); // center a + radius x * cos(theta)
      ey = lat + (rlat * Math.sin(theta)); // center b + radius y * sin(theta)
      extp.push(new GPoint(ex, ey));
   }

   map.addOverlay(new GPolyline(extp, "#000000", 2));
}

• http://www.esri.com/software/awspublicservices/index.html
• http://developer.yahoo.com/maps/index.html
• http://www.mapquest.com/openapi
• http://maps.google.com

For google I viewed the resulting values for latitude and longitude that were generated from a search for the address. For yahoo and ESRI I used their REST geocoding apis and for mapquest I used their java api to their commercial service since their openapi service is only in beta currently.I took 5 addresses located at different points in the US and one in Canada and mapped the returned latitude and longitude from each service. Here are the results:

Missiondata, 2300 Hurstbourne Village Drive, Suite 1100, Louisville, KY, 40299
mapquest: 38.215303, -85.578698
google: 38.215601, -85.578852
yahoo: 38.215496, -85.578669
esri: 38.216273, -85.579028

Googleplex, 1600 Amphitheatre Parkway, Mountain View, CA, 94043
mapquest: 37.4238, -122.0901
google: 37.422845, -122.085035
yahoo: 37.42386, -122.090332
esri: 37.42386, -122.090332

IBM New York, 590 Madison Ave, New York, NY, 10022
mapquest: 40.7623, -73.972399
google: 40.762267, -73.972535
yahoo: 40.762245, -73.972644
esri: 40.762245, -73.972644

Lockheed Martin, 6304 Spine Rd, Boulder, CO, 80301
mapquest: 40.0672, -105.206711
google: 40.067084, -105.206555
yahoo: 40.067063, -105.20654
esri: 40.067063, -105.20654

Red Hat Canada, 2323 Yonge Street, Suite #300, Toronto, Ontario M4P 2C9, Canada
yahoo: none
mapquest: 43.708137, -79.3985
google: 37.062500, -95.677068
esri: 79.398592, -43.7081

For the most part all of the services provide very similar results for geocoding in the US. From the limited number of locations I tested it seems that the larger cities have a more reliable set of outputs. In NY for example they almost completely stack on top of each other. I find it interesting too that in 4 out of 6 cases yahoo came back with the exact same results as ESRI provided.

The last thing to notice of course is that the results for Canada are not as good. First off I couldn’t get yahoo’s service to work with a Canadian address for some reason. Also notice that the ESRI latitude and longitude results are swapped. And viewing the results from google for latitude and longitude gave me some wildly incorrect values but the map was actually correct (thus the two maps). The only completely reliable service for Canada was mapquest and google (although google doesn’t do geocoding) with ESRI coming in next just because the results were swapped. All in all I think all 3 did a good job.

I’m not sure why google hasn’t produced something in the way of geocoding yet. I think they have the mapping nailed down but their lack of geocoding will put them at somewhat of a disadvantage at some point since the other 3 services are in the open now and both ESRI and mapquest already have commercial versions of their services that people use widely.