On Google Earth, can I vary the altitude on a lineString/LinearRing programmatically? - google-earth

Question

I am using the Google Earth plugin on an HTML page. In this context, say you have a line string or polygon like this
// Create the placemark
var lineStringPlacemark = ge.createPlacemark('');
// Create the LineString
var lineString = ge.createLineString('');
lineStringPlacemark.setGeometry(lineString);
// Add LineString points
lineString.getCoordinates().pushLatLngAlt(48.754, -121.835, 0);
lineString.getCoordinates().pushLatLngAlt(48.764, -121.828, 0);
// Add the feature to Earth
ge.getFeatures().appendChild(lineStringPlacemark);
I got the sample from https://developers.google.com/earth/documentation/geometries
Now, say you would like to vary the altitude (height) programmatically, after you append the lineString, how would you do it?
I saw you can retrieve the features through ge.getFeatures(). However, the returned object can not be inspected and I am struggling with the syntax to change the altitude.
I could remove the whole object and redraw it but that is hacky and the user can see the redraw. This is the code to remove
var features = ge.getFeatures();
while (features.getFirstChild())
features.removeChild(features.getFirstChild());
I got the code from https://developers.google.com/earth/documentation/containers
Does someone know the right syntax?

Answer 1

If you have a reference to the LineString (you can hold on to it, or walk the KML DOM and get it again), you can change the altitude of the entire LineString via
lineString.setAltitudeOffset(offsetFromCurrentAltitude);
If you want to change the altitude on a per coordinate basis, you can access them basically as you constructed it above. lineString.getCoordinates() returns the KmlCoordArray, and then you can read values from individual coordinates from there. One kind of awkward thing about KmlCoordArray is that it returns copies of its KmlCoord children, not its children directly. So you can do lineString.getCoordinates().get(0) and then read the lat/lng/alt values from the KmlCoord it returns, but if you set those values on that coordinate, it won't automatically be reflected in the LineString. Instead, you have to readd that KmlCoord to the KmlCoordArray. It's somewhat awkward, but useable.
So you might do something like this, if you're usually only altering one altitude at a time:
function setNewAltitude(lineString, coordIndex, altitude) {
var coords = lineString.getCoordinates();
if (coordIndex >= 0 && coordIndex < coords.getLength()) {
var coord = coords.get(coordIndex);
coord.setAltitude(altitude);
coords.set(coordIndex, coord);
}
}
Check out the KmlCoordArray reference page for its other methods to see if they would be more helpful for the exact use case you have in mind.

Answer 2

I found the answer. My insight was requesting the type as I navigated through the objects. See below
// read the number of features in GE
var length = ge.getFeatures().getChildNodes().getLength();
// get the first feature
var feature = ge.getFeatures().getFirstChild();
// for debugging get type - expecting KmlPlacemark
var featureType = feature.getType();
console.log(featureType);
// get KmlPlacemark geometry
var geometry = feature.getGeometry();
// for debugging get type - expecting KmlLineString
var geometryType = geometry.getType();
console.log(geometryType);
// get KmlLineString coordinates
var coordinates = geometry.getCoordinates();
// for debugging get type - expecting KmlCoordArray
var coordinatesType = coordinates.getType();
console.log(coordinatesType);
var altitude = Math.random()*10000;
var coordinatesLength = coordinates.getLength();
for(var i=0; i< coordinatesLength; i++){
var coordinate = coordinates.get(i);
console.log(coordinate.getType());
coordinate.setAltitude(altitude);
coordinates.set(i,coordinate)
}
for(var i=0; i< coordinatesLength; i++){
var coordinate = coordinates.get(i);
console.log(coordinate.getAltitude());
}