I'm not sure about how accurate it is, but I've found that with my own kml files, if the points are too close together your track your track is more likely to end up like a wooden rollercoaster.
I think it's because the height data has a certain resolution (80 yards/90m) and if the points are closer together than that, it has trouble getting the height.
When I make a path, I don't worry about getting a smooth curve around each corner. I'll just try to define it with 2 or 3 points, and use 1 point for a kink, and maybe 1 or 2 where I know there's a crest or a dip.
I use 3D Route Builder by Hybrid Geotools. Costs 15e and you can do a lot of different things to either GPS or KML data with it. If you import a KML file into it you can add the elevation data (the software uses Google Earth, which you also must have).
The elevation data is accurate to a point. You have to remember that Google gives you the shape of the mountain, not the road, it's not accurate enough. But you can still use it. I just go throught it by hand and smooth out the lines, for example mountain roads tend to have fairly constant rates of descent, so you even it out a bit. And as you can get contour lines from Google Maps nowadays you can use that as a guideline (set it as a background and match your track to it). I found that according to the contour lines my Irohazaka track layout was accurate to around 0.5-1.5 meters even though I just went by my gut feel. That's about as much as you can ask if you don't have the chance to go there yourself.
all this is well and fine, if you want to build a rally track, but as i stated above i am not remotely intrested in rbr or rally tracks
what i want to do is build closed circuit race tracks......im not rich and i cant drive so that rules out going to a track with a gps system and doing it by hand the only way of doing this at the moment is phographs and you tube vids which at the end of the day is just guess work and slow laborious
guess work at that.
I make the kml in Google Earth and then go point-by-point and edit the flat kml file so it now has height data. I've found that it's not a lot of work for 2-3 mile tracks unless you've defined waaaay too many points in your kml.
The web sites for adding height to kml files are not worth the effort, in my opinion. There's just too few steps between one elevation and the next, so you end up with 10m stairsteps.
The final option, for setting terrain height, is to skip BTB and go directly to 3DSimEd after a detour through Google SketchUp. I've not tried it yet, but it quickly makes the basic terrain on top of your Google Earth image. Now if only BTB could import that file instead of 3DSimEd...:drool: