TM 5-6675-250-10
2-14. OPERATION IN DUSTY OR SANDY AREAS
Special care must be given to the theodolite when the SIAGL is being used in dusty or sandy areas, since
both dust and sand are highly abrasive. Keep the rain and dust cover over the theodolite at all times,
except when making actual sightings. The theodolite should be brushed frequently and carefully wiped
clean. Be extremely careful not to scratch lens and prism surfaces when cleaning.
2-15. OPERATION UNDER RAINY OR HUMID CONDITIONS
Exercise care to avoid getting the cable connectors wet or muddy when setting up the equipment. Keep
the rain and dust cover over the theodolite at all times, except when making actual sightings. Use lens
cleaning tissues to remove moisture from the glass surfaces of the level vials and the theodolite lenses. A
slight lowering of temperature in a humid area will cause condensation of moisture and fogging of optics.
Internal fogging can usually be removed by placing the instrument in a warm, dry place. Corrosion due to
high humidity can be partially eliminated by using warm, dry storage areas and desiccant. Clean and dry
the equipment with a soft cloth after use.
2-16. OPERATING IN SALT WATER AREAS
When operating in salt water areas, wipe the instrument often with a soft, clean cloth. Keep the rain and
dust cover over the theodolite at all times, except when making actual sightings. Do not allow the theo-
dolite to be exposed to direct salt water spray. Cleaning intervals should be shortened considerably for
theodolites subjected to salt air exposure.
2-17. OPERATION IN AREAS OF LARGE MAGNETIC ANOMALIES
Anomalies in the earth's magnetic field have no effect on the ability of the SIAGL to determine true north.
However, they do affect the magnetic compass used to preorient the GRU unit to north. The servo system
is free to travel +40; only anomalies of more than 40 affect the operation of the instrument and require
special operating procedures. It is not necessary to know the amount of anomaly at the operating sight.
The action taken by the operator during the first gyrocompass is to observe the position of the follow-up
index mark on the servo operating band, which is displayed on top of the reference unit at the base of the
theodolite. If the index mark moves to the end of the servo operating band, the pendulum is to be caged;
rotate the GRU in the tripod 60 in the direction that the index mark was moving. Re-level and re-bias the
instrument and proceed with the gyrocompass operation.
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