“Sketch-based annotations in Google Earth”

Painting in a 3D geobrowser is interesting both for artistic uses such as virtual graffiti and for commercial applications such as architectural sketches. It seems straightforward to turn Google Earth into a 3D painting tool: Store the 3D mouse data that the software returns and construct polylines from them. However, this approach has two vexing drawbacks: First, when the user paints past an edge, the end of the stroke will be placed at an incorrect depth, see Figure 1; second, it is not possible to sketch in mid-air, for instance to indicate a planned height extension of a building.