Brooklyn College
City University of New York

STEP 3. CONSTANT TARGET DISTANCE:   You can now use your Step 2 results to measure objects remotely as long as an object you are measuring is at a distance from you equal to the distance to the wall in Step 2. If you used 2 meters as this target distance in Step 2, then this is the distance you must use when measuring. If you use a different distance figure, your measurement results will be wrong. To illustrate how measurents are made, consider the tyrannosaur scale model shown at the right.
      We wish to know the height of the model at the hips. Standing 2m away from it, with plastic ruler held at arm's length, produces the view seen at the right. The blue plastic ruler can be seen vertically superimposed on the tyrannosaur model so that the height of the model at the hips can be readily evaluated. We see that at this position, the base of the foot is at the "0" division of the ruler and the upper surface of the body is at the "4 and 1/2" mark on the ruler. Thus, the model extends 4 and 1/2 major divisions of the hand held ruler. Since we have already established in Step 2 that at a target distance of 2m, each major division on the ruler is equivalent to 10 cm of actual distance on the target object - the tyrannosaur model in this case - we can calculate the height of the model as:

            MODEL HEIGHT = (4.5 divisions) x (10cm/division) = 45 cm

This example illustrates the general rule that:

            Actual distance = (No. of ruler divisions) x (distance for 1 ruler division)

Remember that this relation is only correct when the target distance is equal to the distance at which you calibrated your hand-held plastic ruler (2m in the example described here).