how do I correct measurement on an imported image?

I am using the trial of 2011, have worked a little bit with 06 in the past but forgot much of it. The issue I am having is I copied an arial image from google earth of the property I am working on, within the pic I drew a line along a building that I know is 50'. I imported the image into model space, I would like to use that image to trace the property, buildings, etc. I am having trouble drawing a line along that known measurement and getting it to read 50'......is there a way to trace that point, rename the measurement to 50' and have my following lines that are traced measure true? I only have that one known place for the 50' measurement for scale.

My units are in architectual.

Any insight would be great.

Thanks.

Scale or Align the image.

http://www.cadtutor.net/forum/showthread.php?p=327867#post327867

Had one of the other engineers from another department visit today and he showed me how to rescale an image which I never new about. You must have a known length when you do the scale command pick object then "ref " pick two points then enter correct measurement rescaled picture.

Forwarding to our crew tomorrow

Awesome! Will try that tonight.....I couldn't get the handle on how the first response suggested.