Inside the KML Placemark

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 Inside the KML Placemark
Projects
Written by Mike James
Tuesday, 05 October 2010
Article Index
Inside the KML Placemark
LinearRing
Style
3D concerns
More than one geometry
Polygon generator in Javascript

## 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.

## 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>`

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

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 )