Calibrating a Total Station

[Sunburned_Surveyor]

I recently read an article explaining some of the errors inherent in measuring horizontal distances with an EDM. I had a couple of questions about this, as we never adjust for either temperature or pressure when using our total stations. Do other survey crews make this a habit? Is it something I should be doing every time I pull out my total station? Where is a good place to look for instruction on how to do this?
I know I can get a thermometer, but can anyone recommend an affordable barometer that I can take in the field for pressure adjustments?
The article also mentioned 4-Point Baselines set up by NGS for use in calibrating a total station. How do they get these baselines so accurate? Is there one near central California?

Thanks for the input.

The Sunburned Surveyor

[bruce hall]

As I remember this, every 10 ppm thrown into a measurement will change the measurement .01' per thousand feet. When I start my day, I enter the temperature, usually a guess. I enter the pressure, usually 29.8 because I work very near sea level. This is also a guess. My shooter figures out the ppm all by its' lonesome. I have a chart put out by topcon about 20 years ago. I shows what the pressure would be at certain elevations. When I survey in areas in the "highlands" I check my chart and use the value shown on this chart. On this same chart it also shows the ppm to be used at at certain elevation and temperature. This is "rough" but not really that bad when I compare the values of points set from one setup verses their value from another setup. If you are measuring long distances at extreme temperatures and altitude this could yield "wrong" values for the distances that you measure. You may get more error in your tribrachs and plumbing pole, unless you have these items in proper adjustment. You could also set two points about 800 feet apart, or any distance, you pick the number. Set up at one end and measure to the other. Set a point somewhere in the middle between these points and measure both ways. Your sum of the two parts should equal the whole. And then of course you have to consider the prism offset. Are all your glass at the same offset. It is possible to measure from one point to the other using two different prisms(both having the some offset) yielding different results. This is not good, but I guess it depends on what the difference is. Maybe I'm out to lunch. Consider the source and file it accordingly. Bruce Hall LS

[Ian Wilson]

I use a Brunton Sherpa to get my barometric and temperature information. I plug the data into my Topcon GTS813AR and it automatically calculates the PPM. I try to do this at least a few times during the day as the day heats up and/or as I feel fronts move through.

The Sherpa runs about $120 and is a nice little compact unit.

As far as the calibration, check with Caltrans in Stockton to see if they have or know of ant local courses. Bob Cunningham, District 10 phtogrametrist is the guy I'd be calling for that info. Bob is a top notch guy and one of my first party chiefs all too many years ago.

[landbutcher]

It won't do you much good, but the county of orange has a baseline along the Santa Ana River.
I use it when I get a new gun or have trouble with mine.
Unless you are doing a lot of long distance stuff or changing altitudes a lot it doesnt make much difference in the msmts.
As Bruce said there are a lot of things that affect msmts more than PPM.

[Craig Stanton]

I use a Suunto X-Lander for both temp and barometric pressure. Properly calibrated it provides values that are fairly reliable. The compass is an added benefit. I also enter current atmospheric conditions at every setup. The instrument (Leica TCRA 1103) calculates the PPM values based on the entered data.

The County of Los Angeles maintains a baseline in the San Fernando Valley. I checked my total station and prism offsets there a few weeks ago. I was able to compare my measured results against precisely measured values performed by the City of Los Angeles.

I also check my tribrach and prism pole centering and level vials for adjustment on a regular basis too.

[Dave Karoly, PLS]

It makes the most significant difference at high elevations and high temperatures.

Humidity probably is also a factor (Water vapor is less dense than regular air which is mostly O2 and Nitrogen) but as far as I know no one considers humidity. I would imagine this is because humidity has a much smaller influence than temp and pressure.