d. Avoid observations during thunderstorms, squalls, or other unstable conditions.
e. For best results, the initial and final reading of any run should not be over four hours apart.
Prior to the recording run, all altimeters to be used must be observed simultaneously at intervals over a
period of at least one day.
g. If possible, the difference in elevation between the bases should not exceed 75 meters, and a lesser distance
h. The distance between the base stations should not exceed 12 miles, (1 mile = 1.609 kilometers.)
Observations should not be taken outside the range of the base stations, as they may not be reliable.
Single-Base Method of Altimetry
Field surveys using the single-base method of altimetry are conducted as follows:
a. For the base station, select a point of known elevation, centrally located if possible, among the points for
which elevations are to b)e determined. Allow the instruments to rest at the base station for approximately 10 minutes prior
to taking the first observation, to allow them to become fully adjusted to the prevailing conditions of pressure and
temperature. After this period, they may be set in agreement, if desired. During this period, a check should be made to
insure that all observers watches are in agreement.
b. The first observation is made at the base station, and particular care should be taken because it is upon this
observation that the accuracy of all