Effects of barometric pressure

Most instruments have validity ranges that prevent entering a?ÿvalue that's physically impossible; the ranges for temperature and pressure in Trimble Access are -94 - 158 degrees Fahrenheit and 14.76 - 103.35 inHg, respectively. If you enter the extremes (158 and 103.3) you get a ppm correction of -530ppm, which would be an error of -0.106' @ 200'.

If one set an average setting appropriate for the urban Portland area (elevation 200' +/-) and then went an hour drive east to Mt. Hood to do a job (road pass elevation 5000' +/-) , there would be some noticeable effects.?ÿ

I find that temperature differences at each end of the calibration baseline (sun/shade) can affect the ppm correction enough to make the difference between passing and failing. But that is for 4000' distances.?ÿ

We set the atmospheric ppm where I work, I usually do it either every set up or at the top of a new page. It only takes a minute and we do a lot of monitoring, use long shots for resection etc.?ÿ

We also peg the instruments every day, these little steps do not take long and help strengthen the overall program.?ÿ

It changes throughout the day. I would think it depends on a number of factors; whether you're just taking a bunch of natural ground shots, or doing boundary and/or control, the temperature swings throughout the day, distances you're measuring. Even a C&A early in the morning won't cover the warming throughout the day.?ÿ

Our Trimble S6 and SX10 have built in barometric sensors, which I have checked and found to be accurate. I use my phone to check the nearest temperature every setup. For high accuracy work I use a handheld digital sensor.?ÿ

The following are roughly 1 ppm;

temperature: 1?ø C (1.8?ø F)

pressure:?ÿ 3 mb (0.1 in HG)

Knowing the sea level pressure in the area you can quite accurately compute the station pressure if you know your elevation

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humidity can be ignored except for long distances

Note that barometric pressure varies regularly with elevation (~0.1 in HG per 100 feet) and only changes slowly over areas

Temperature varies a lot more but it also varies with elevation (6.4?ø/1000 m)

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Some places on earth and the project are hardly affected by the settings.

Then one day you find yourself on a certain project in a place that constant monitoring the settings is very crucial.

I work in a area where the elevation change at an extreme would be 300 feet, and usually less than a 100 feet day to day.?ÿ Temperature would have much more of an affect.?ÿ Also if you look at most weather data, the pressure is usually corrected to sea level, so you need to take that into account.

For most of the conventional work we do you're never going to see 5 - 10 ppm but I still try to get the guys to check the T & P. With the SX-10 I want everything as tight as possible so I check the temperature regularly. As John said, the instrument senses the pressure.

Thanks to everyone for your knowledge and experience and willingness to share it As usual this forum has not let me down?ÿ