Mutts wrote:

"Leave everything on the complicated form exactly how it is and the
only thing you have to enter is the UPLOAD field for where your *.GPX
file is on your hard drive. The web site converts it directly to a
*.KML file. Save it then click on it and your Google Earth starts up
and it is in true 3D mode to view the track of the plane."

In Google Earth right click on the track, select "properties"

Go to the alittude tab and select "absolute".

It's much easier to select "absolute" as the altitude mode in GPS
Visualizer's input form before submitting your file. (It's under
"Track options.") That way you don't have to mess around with anything
within the Google Earth application. Also, it allows you to take
advantage of colorization by speed or altitude.

Adam