Why Temperature and Air Pressure Are Important to Survey

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Why Temperature and Air Pressure Are Important to Survey

Post by Johnny »

As I read some total station reference guides this morning, I wondered why it was so important to set the current temperature and air pressure when surveying. For anyone who, like me, hasn't been properly trained in professional surveying this is probably a good question to see answered.

Here's an excerpt from an article by Dr. Ben Buckner in Professional Surveyor Magazine, 09/98:

Natural Errors in EDM

As a "rule of thumb," an error of 1ºC in the air temperature causes about 1 PPM error in a light wave measurement. Similarly, an error of 0.1 inches in pressure affects the measured distance by 1 PPM, as does an elevation difference of 100 feet. Such errors are negligible for topographic surveys but can be significant for boundary surveys.

For example, suppose the field crews do not change the PPM variable in the total station during times when there are wide swings of temperature. If the temperature goes from 5ºC to 24ºC (41ºF to 75ºF) on a spring day, and the PPM correction was not changed from morning to afternoon, the field crew would have a discrepancy of 0.06 feet in a 3,000-foot distance at the two different points in time.

Atmospheric pressure can also change several tenths of an inch over a few hours when weather is unstable. A weather radio broadcasts the pressure at the weather observation station, which may be miles away from the job site, and the weather may be significantly different there, causing another discrepancy. Considering that 100 feet in elevation difference causes 0.1 inches pressure difference, surveyors working in hilly or mountainous areas have an additional problem of constantly watching the pressure as they change elevation.

The biggest oversight concerning pressure occurs when the field crew uses the cited pressure directly from the weather broadcast. This is not an error, but a mistake because the pressure cited is "sea level" rather than local. Because the pressure drops with altitude, approximately 0.1 inches per 100 feet of altitude above sea level must be subtracted from the sea level pressure. The 1,900-foot contour passes through my back yard. If I were surveying in this subdivision, I would probably start with the broadcast pressure, then subtract 0.1 times 19, or 1.9 inches from that for keying into the total station. Neglecting this difference would cause 19 PPM error in the distances. Thus, a distance of 3,000 feet would have an error from this source of 0.06 feet, the same amount caused by the 19ºC error. If such differences are cumulative, not compensatory, we have a total error of 0.12 feet as a result of overlooking two commonly misunderstood aspects of temperature and pressure.

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Re: Why Temperature and Air Pressure Are Important to Survey

Post by Tiompan »

Although nowhere near as important as parallax and refaction, air pressue and temperature should also be taken into account when making astronomical calculations .

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