RF = 1:20,000

MD = 5 inches

x = GD

Using the rule of the means' product divided by the known extreme (1), find the unknown extreme (x),

which, in this case, represents the GD.

(5 x 20,000)/1 = 100,000

The answer is given in inches, which should be converted to the more conventional surveyor's units:

yards, feet, meters, or miles. This requires that you divide the answer by 36, 12, 39.37, or 63,360,

respectively.

e. The use of ratios and proportions can also serve you in other situations. See the following example:

RF of 1:31,680? Set up your proportion using the RF formula (RF = MD/GD). Since the MD is usually

expressed in inches, change the GD to inches by multiplying by 12. Let x equal the unknown mean.

RF = 1:31,680 (scale of progress map)

GD = 7,920 feet (length of surveyed line)

x = MD

1:31,680 :: x:7,920 12

1:31,680 :: x:95,040

Using the rule of the extremes' product divided by the known mean (31,680), find the unknown mean

(x), which, in this case, represents the desired MD.

(1 x 95,040) /31,680 = 3 inches

f. Up to this point, only direct proportion has been discussed. In a direct proportion, both ratios

are direct ratios; that is, they increase or decrease in the same manner. In the map proportion problem

above, the first ratio (1:31,680) increased in the same manner as the second ratio (3:95,040). This

problem would also be a direct proportion if the terms had decreased in a like manner. Most of the

proportion problems you will use are direct proportions and, unless specifically noted, will be treated as

such. A simple clue that will aid you is to analyze each problem carefully to determine whether the

unknown term will be greater or lesser than the known term of the ratio in which it occurs. Thus, each

direct proportion takes the following pattern: