CST/BERGER CST202, Instruction Manual

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11. DISTANCE MEASUREMENT
11.1 EDM Settings
Complete the following EDM settings before the distance measurement
：
z
Atmospheric correction factor
z
Target Type
z
Station Height and Target Height if required
z
Prism constant correction value
z
Distance measurement mode
1. Atmospheric correction factor
To perform higher accuracy measurements, it is necessary to find the atmospheric correction
factor from even more accurate temperature and pressure measurements and perform an
atmospheric correction.
The total station measures the distance with a beam of light, but the velocity of this light varies
according to the index of refraction of light in the atmosphere. This index of refraction of light
varies according to the temperature and pressure. In the normal conditions, with constant
pressure, a temperature change of 1C, or with constant temperature, a pressure change of
3.6hPa
，
an index change of 1 ppm . This means the distance measurements will be changed
1mm for one kilometer. So in order to precisely determine the atmospheric correction factor, the
average air pressure and temperature along the measurement beam route must be taken. Take
care when calculating the correction factor in mountainous terrain as the difference in height will
result in differences in atmospheric conditions between two points.
• The instrument is designed so that the correction factor is 0 ppm at an air pressure of 1013
hPa and a temperature of 15C.
z
The atmospheric correction factor (ppm) can be calculated using the following formula and
stored in the instrument’s memory.
0.2904 ×pressure（hPa）
ppm = 278.96 -
1+0.003661
×temperature（C）
z
If the weather correction is not required, set the ppm value to 0.
z
The ppm data can also be entered directly.
z
A quick rule of thumb may also apply when needing barometric pressure. Take 1” of
mercury from 30 for every 1000ft above sea level.
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