upper base (U) and the lower base (L'). Since the difference in the elevation between the bases (U and L) is known, a

simple proportion calculation may be used to determine the true difference in elevation between the field station (F) and

the lower base (L). In the application of the two-base method, follow the general considerations and observe the

precautions previously described in this section, and proceed as follows:

a. Select the lower and upper base stations in the area to be surveyed and establish their elevation.. These

bases should be within 10 miles distance of each other, and closer if possible. These bases should be chosen so that

most of the points to be surveyed fall within the elevation range between them. Best results are obtained when the

difference in elevation between bases is 60 to 90 meters. In this case the average error will be less than one meter.

When the vertical distance between bases is 600 meters, the average error will be less than two meters. During the

survey, one altimeter will be located at each base. However, any number of instruments may be used in conducting the

field survey.

b. Before starting the survey, all altimeters to be used are adjusted at one of the base stations so that they are

exactly in agreement. While all the instruments are at this base they should be read simultaneously five times at two-

minute intervals, with readings taken on the even minute. The mean of the five readings is established, and is fled as the

correct reading of all the individual altimeters for this point. The instrument which is selected to remain at this base is

considered the comparison