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1)
Atmospheric correction
e
The SET2C uses a beam of infrared light to measure the distance.
The velocity of this light in the atmosphere varies according t o the
temperature and pressure.
The distance will be changed b y 1 ppm by:
- a variation in temperature of lo
- a variation in pressure of 3.6 m b
(A 1 ppm change means a I m m difference for every I k m of
measured distance).
To obtain distance measurement, of the highest accuracy, the
temperature and pressure must be carefully measured by accu-
rate equipment.
e
The p p m correction should be applied when the calculated ppm
value is over k5ppm or if the slope distance is more than 200m.
2) Average temperature and pressure between 2 points in different
atmospheric conditions:
e
In flat terrain, measure the temperature and pressure at the
midpoint of the line as there is little variation in the values.
e
In mountainous terrain, midpoint values should be used. If those
values cannot be measured, take the temperature and pressure at
the instrument and target stations, then calculate the average
values.
t l
+
t2
Average temperature
=
-
Average pressure
Average
sea level
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