If you want to use polygons clamped to ground or relative to ground that display information rather than architecture, here are some tips when preparing them in SketchUp:

Rough out your polygons on the flat image from GE, with imported terrain turned off.

Then elevate the rough draft above the terrain, now turned back on.

Uncheck Perspective in the Camera menu, so that you are in paraline view, and work in top down mode with the polygons.

For any given polygon, work within the center of the view pane, using the hand grabber to position the view.

When you are satisfied with what you see in SketchUp, export to GE. Then in GE, change the location of the geometry from absolute to clamped to ground. Check the positioning and reiterate as necessary with SketchUp until the positioning is satisfactory.

If you want polygon drop shadows, in GE duplicate the clamped to ground file, then edit both so that one shows as semi-transparent shadow and the other as relative to ground by 1 meter.

This method is a workaround, compensating for a few quirks. If you simply construct and export polygons on the flat terrain image in SketchUp, the polygons will not be visible in GE when clamped because they are, in a sense, on the UNDERSIDE of the terrain rather than upon it. Working above the terrain in SketchUp allows you to see where the polygons are relative to the terrain layer, but if you used the camera in perspective view instead of paraline, you would be misperceiving the polygon positions. This would become painfully apparent when the polygons were exported to GE.

I don't know if polygons exported with absolute elevations less than terrain values would be correctly positioned if they were visible, but I suspect this is so. As it is now, such polygons are visible only if the original absolute values are above the terrain. Thus, the workaround detailed previously.

Bear in mind that when there are a lot of polygons clamped to ground they are automatically rasterized. Polygons relative to ground are not affected this way, but the minimum height relative to ground is 1 meter, which can affect the perception of where they are placed. Therefore, a collection of polygons, such as the Deer Valley bus route in this Sundance file, will show up nice and clean in the relative to ground set, while their drop shadow version is coarsely rasterized.

From a technical point of view, the rasterization is understandable, as clamped polygons are tesselated into many more polygons so that they conform to the terrain. This can slow down performance in GE. I just wish there were some option to reduce the coarseness of the rasterization.

SMcQ