View > Scale legend
you get a scale bar in the bottom left corner of GEarth. It isn't very usable as it works in the same space as the tour controller. Also, as you zoom in or out the bar itself stays static and just the measure of the distance changes. This doesn't work very well as users can't gauge what 3/4 of 772.1m is quickly.
My Solution: It would be a lot better if the bar itself changed size and the numbers stayed round until a new number was needed. So zooming from a scale where the scale bar initially showed 0 to 100m my idea would be to have the bar gradually expanding until it reaches a critical width at which point the bar snaps back to half its width and the scale numbers now read 0 to 200m.
View > Grid
pulls up a grid of thin white lines that dynamically change as you zoom in and out. Ironically, its a lot like the solution I suggested for the scale bar above but in this case, I don't think it works as its all over the screen and is too visually busy. Better to Allow the user to choose to have a grid of a fixed width visible which has smallish crosses at intersections rather than having a complete grid. This would help with orientation and memory. If you're interested in reading further about landmarks in Virtual Worlds and Virtual Reality I can recomend this paper .
If you have your own suggestions as to how to improve GEarth, Google would like to hear from you. I've added my vote on the above topics...