use the tangent ratio. for example ...
Pick a tall structure (tree, house, billboard, etc.). Take a look at it at 3 points during the day (eg. 9 am, noon, 3 pm), and estimate the length of the shadow it makes at each time (stand on the side that has a shadow). Stand at the end of the shadow, and estimate the angle between the top of the structure and your viewpoint (90º is straight above you, 0º is same level as you, 45º is half way between, so 25 is about a ¼ of the way, etc.). Using the length of the shadow and the estimated angle, determine the height of the structure. Are your 3 answers similar? Post a message to the discussion group with your calculations and what you found.
Any help on this problem would be more then appreciated! Thanks guys.