standard. If all the altimeters are not adjusted to correspond with each other at the starting base, an index difference will
have to be used. The altimeters which are to be taken from the starting base (one for the second base station and the
field altimeters) are compared with the comparison standard altimeter selected to remain at the starting base. The index
difference is the algebraic difference between the reading of the comparison standard, or starting point altimeter, and the
readings of each of the other altimeters, taken at the starting point base,
c. During the course of the survey, each altimeter reading with the exception of the reading taken using the
comparison standard altimeter, must be corrected for its index difference. However, if all instruments were adjusted to the
same value for the initial reading at the starting point, no corrections for index difference are necessary.
d. Base altimeters must be shaded, and read at five-minute intervals. Any changes in weather conditions
(especially wind speed) must be noted.
e. Field altimeters must be shielded from the sunlight, and readings taken at each observation point on the even
minute at two-minute intervals. Readings may be started immediately on arrival at the field station unless there has been
an appreciable temperature change from the previous station. If this is the case it may be necessary to wait for five or ten
When reading the altimeter follow the instructions described in paragraph 13.
g. When convenient, it is desirable to compare the field altimeters with each other during the