Inside the KML Placemark
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Written by Mike James   
Tuesday, 05 October 2010
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Inside the KML Placemark
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More than one geometry
Polygon generator in Javascript

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An example

As an example the following draws two rectangles one in green and one in red:

<Document>
<Style id="green">
<LineStyle>
<color>ff00ff00</color>
<width>5</width>
</LineStyle>
</Style>  
<Style id="red">
<LineStyle>
<color>ff0000ff</color>
<width>5</width>
</LineStyle>
</Style>
<Placemark>
<styleUrl>#red</styleUrl>
<LinearRing>
<coordinates>
0.0,0.0,0.0
1.0,5.0,0.0
5.0,5.0,0.0
5.0,0.0,0.0
0.0,0.0,0.0
</coordinates>    
</LinearRing>
</Placemark>
<Placemark>
<styleUrl>#green</styleUrl>
<LinearRing>
<coordinates>
0.0,6.0,0.0
1.0,15.0,0.0
5.0,15.0,0.0
5.0,6.0,0.0
0.0,6.0,0.0
</coordinates>    
</LinearRing>
</Placemark>
</Document>

Notice that now as we have two Placemarks we have to bundle them up into a single ??Document.

 

redgreen

 

A Polygon generator in Javascript

One of the uses of geometric objects is to create a custom place marker without the need to use a custom bitmap icon. To do this you generally need to generate the KML using a script.

For example to generate an n-sided polygon of radius r and centered on x,y you might use:

function polygon(x,y,r,n){
var inc=2*Math.PI/n;
var KML=x.toString()+","+
(r+y).toString()+",0.0\n";
for (var i = 1; i <n; i++){
KML=KML +
(r*Math.sin(i*inc)+x).toString() +
   "," +
(r*Math.cos(i*inc)+y).toString()+
",0.0\n";
};

As this only generates the necessary co-ordinates you have to embed it in sutiable KML tags.

For example:

alert(polygon(-1,55,1,3));

generates the co-ordinates for a triangle centered on -1,55 and radius 1.

These can be copied and pasted into the co-ordinate section of a polygon to give:

<Placemark>  
<Style>
<LineStyle>
<color>ff0000ff</color>
<width>5</width>
</LineStyle>
<PolyStyle>
<color>ff0000ff</color>
</PolyStyle>
</Style>
<Polygon>
<outerBoundaryIs>
<LinearRing>
<coordinates>
-1,56,0.0
-0.1339745962155613,54.5,0.0
-1.8660254037844383,54.5,0.0
-1,56,0.0
</coordinates>    
</LinearRing>
</outerBoundaryIs>
</Polygon>
</Placemark>

triangle1

 

You can create any polygon marker in the same way and even approximate a circle if you set n big enough. For example, n=25

circle

 

The reason the "circle" is in fact an ellipse is that we are working with latitude and longitude as if they were measures of linear distance. How much distance a degree of longitude represents depends on the latitude. To make this shape generator position-independent you need to scale the latitude and longitude so that you are working in ground distances - not difficult but something for another article.

 

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Last Updated ( Tuesday, 05 October 2010 )
 
 

   
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