Curious to know what other people's practice is regarding setting atmospheric PPM values in your total station.
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All boundary work here is reduced to ellipsoidal distances (~sea level distance) and given where I work is 300-600m above sea level we always reduce using the geometric PPM to "sea level"; much easier now the GNSS gives you the elevations to within a few metres! Typically anything over 250-300m is shot with GNSS these days so not huge corrections but still necessary.
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With the atmospheric PPM I don't bother to carry a thermometer or barometer just plug in an average temperature for the season/day and elevation which the instrument (Lecia/Geomax) converts to hPA and be done with it. Figure that is good enough for the 5-10mm precision I'm after.
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Let's hear what everyone else is doing.
I have a kestrel for temp and humidity and enter the data when I set up each time.
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We always, always input atmospheric readings, both because it gets confusing telling the crews to do it for one job where it's critical (like deformation monitoring) but not to worry about it for other jobs (like a lot & block survey).
We have many crews working at higher elevations and they have a bad habit of thinking that the altitude-adjusted pressure from the nearest airport weather station is correct.
Also, considering some jobs will go on for well over a year, and many over several years, it is possible to see temperature variations from 20F to 100F over the duration of a project, along with pressure changes from weather systems. A lot of our offices are near the coast so changes can happen fast.
All that is to say that even though the changes are minor compared to a lot of the other sources of error, it's still systematic error, and something we can account for, so we do it.
It's nice having the pressure sensors built into the total station - all we have to do is get the temperature with a small thermometer in the instrument case.
YMMV
We keep a thermometer and a barometer in the instruemnt case. It might not make much of a difference on most jobs, but the entries appear on the printouts, so if there is a challenge on anything else to do with the job it looks as though we have been very careful and done everything by the book.
Probably have not touched it in 24 years.
I do it always for my Topcon PS. Generally once a day per site, but sometimes update it at midday if temperature is changing. I used to carry a barometer, but I have found that my iphone (with Barometer app) will give me the station pressure - if set to do so. I use the temp in the weather app.
Trimble S5 will adjust itself, automatically.
There are those errors that you can't do much about, and there are those that you can easily compensate for at no cost, money or time. The more you do about the latter the less you have to worry about the former.?ÿ ?ÿ ?ÿ
I have carried both a thermometer and barometer/altimeter in the instrument case since the early 1970s. SOP was to ALWAYS read both and make the ppm corrections both in the notes and (usually) the instrument at EACH setup, and if the occupation spanned more than a couple of hours (or conditions changed), then repeat. On "long" EDMI shots (greater than mile or large elevation changes), Temp/Pressure readings were also observed and noted at the remote station. On LONG EDMI observations, the Instrument ppm was set @ 0 (zero), and the mean ppm (EDMI + Remote/2) were computed and applied to the "raw" EDMI Slope Distance. Considering that we did a lot of trilateration work with lines ranging from about 10,000 to 50,000 feet back in the the 70s & 80s, and elevation changes could be several thousand feet, these corrections were nearly always non-trivial. Obviously size matters. On short (less than a mile), atmospheric corrections at the EDMI were more than adequate for all practical purposes. The fact that I might be working at "sea level" one day, and 10,000 msl the next (usually 5,000 to 8,000 day to day), required us to pay closer attention to such things than those who work in smaller distance/elevation scenarios. Good procedures are always better than...well...not so good procedures (good habits better than bad habits). Of course the advent of GNSS was pretty much supplanted long EDMI observations, but the law of 7 P(s) is still applicable (Proper Prior Planning Prevents Piss Poor Projects).?ÿ ?ÿ?ÿ
Our instruments have onboard pressure sensors, which I have found to be accurate enough. I usually get the temperature from my phone, which is an "estimated" value for my location based on surrounding stations. We also use Brunton and Kestrel weather sensors. Depends on what the job is, for deformation surveys I am more careful.
I am giving a workshop at Trimble Dimensions talking about this exact topic, and also how to properly use a CBL to calibrate an EDM and how to properly reduce the observations.?ÿ
Things to remember...0.1" of pressure is 1 ppm. That corresponds to 100 feet of elevation. So it is good to know your elevation to better than 100' if using sea level pressure in order to properly reduce it. For a good approximation you can figure the correction as -0.1" per 100 feet above MSL. There is a more exact formula but that works for most users unless you are high up or trying to get mm accuracy. 1 degree C (1.8 degree F) is 1 ppm. I tell people that temperature is by far the most critical, it is easy to get barometric pressure (often on board) and it only varies by elevation and is pretty stable over large areas unless there is a front moving through. Temperature measurements are affected by sunlight, and heat effects close to the ground, like whether the line of sight is over asphalt, concrete, grass, etc. Temperature also decreases with elevation but not at a constant rate, there can be inversions, etc. It varies a lot more over small areas. Humidity can be safely ignored with distances we measure being much shorter than the old days before GNSS.
One nice thing about living in a "flat" state is that we rarely have an elevation difference across a boundary survey in excess of 100 feet.?ÿ Temperature changes of 40 to 50 degrees F occur somewhat regularly between start and finish times, though.
One nice thing about living in a "flat" state is that we rarely have an elevation difference across a boundary survey in excess of 100 feet.?ÿ
Flat, yes. But at close to 2000' ASL. You shouldn't rely on standard pressure settings.
More like 850 to 1150 in my corner of the state.?ÿ I'm sitting at about 1050 right now.?ÿ Using 1000 feet is close enough on small projects.?ÿ I was referring to the need of some to account for elevation differences between setups. Less than 3500 feet between the lowest and the highest points in the entire state, nearly 400 miles apart
Just 66 miles north of the highest point in Kansas is the lowest point in Colorado.?ÿ A difference of 700 feet downhill.
When Kansas Territory was created the western border was to be the Continental Divide near the centerline of what is now Colorado.?ÿ Thus, several of the fourteeners would have technically been in Kansas prior to 1861.
?ÿJust 66 miles north of the highest point in Kansas is the lowest point in Colorado.?ÿ A difference of 700 feet downhill.
According to Wikipedia, this is the high point of Kansas at elevation 4039 ft. Not much topography there.?ÿ
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Iowa has a somewhat similarly unimpressive high point, but there at least you can see that water would flow away from it.
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@bill93?ÿ
One can actually see that the Colorado border about one-quarter mile to the west is, in fact, downhill.?ÿ It's when you look around at other hills that you can't tell which one is the highest.?ÿ Really takes away from the illusion of grandeur.
I've never gone to the Oklahoma highpoint.?ÿ One because it is remote.?ÿ Two because to access it from Oklahoma you have to climb Black Mesa which extends into Colorado and New Mexico and only occupies a tiny bit of Oklahoma.
