So I searched and found a few threads here that addressed similar topics, but nobody that I have ever surveyed with has ever explained to me any differently, so I will ask here for the consensus on proper procedure for this...
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I have Trimble Access 2017.23 on my data collector and an older Trimble S3 robot (passive tracking). I have noticed with each new setup that during the instrument corrections portion, that the data collector will offer some ever so slightly different pressures (temp has not changed from what I have noticed). In doing some reading in the Trimble Access General Survey Help PDF for version 2017.23, it mentions instrument having internal pressure sensors? But it doesn't specify which instruments might have that either. Looked in the Trimble S3 Total Station User Guide PDF that I have & did a control F but didn't manage to find anything about internal pressure sensors.
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My question is what's the proper procedure here? And if my total station does have its own internal pressure sensor, do I let Trimble Access just use whatever the robot spits out when I get to the corrections screen where it auto populates that field? Has this sensor been checked during the most recent calibration at the Trimble dealer? I ask all this because sadly, in all of my previous experience, most party chiefs just used the weather apps on their phones which might not necessarily be indicative of what's going on right there on the site while others just blew right through this portion of the station setup (scary to think about now in hindsight). Just wanted to correct my procedure here as best as possible and make sure I'm not repeating mistakes myself
The first thing to know is the weather report gives equivalent sea level pressure, and the instrument needs actual absolute pressure at your elevation.
The difference may not matter much for lots of people?ÿ but would be significant at higher elevations.
My first robot was a Zeiss S10. Very accurate instrument. It had on-board T and P which I found to be pretty accurate, although the temperature might be a bit higher than the ambient temperature when out in the sun.?ÿ
Next was a Trimble S6. Pressure only. I found that to be very accurate. I usually either have a thermometer (analog or a digital handheld weather sensor such as a Brunton ADC Pro) or look on a weather app and get the temperature there.
When I do a high accuracy survey (for example a deformation survey), I will download data from the nearest weather station when I am in the office, and get the temperature from there for the time of each observation throughout the day (this is done in a program, not manually).?ÿ
We did a survey last week at a dam where the instrument was at the bottom, and we were shooting 100 monuments along the embankment portion of the dam, which was about 170 feet higher. In that case I corrected the pressure at the instrument to make it the pressure at the halfway elevation (i.e. the mean pressure along the line). The correct temperature and pressure for a line with a large difference in elevation would be at the midpoint elevation.?ÿ
I had this discussion yesterday with a friend, there are a lot of surveyors who just get the SLP from an app and use it to correct EDM distances. This is fine if you are close to SL, but but note that for every 100 feet in elevation the pressure decreases by about 0.1" Hg, which is 1 ppm. So if you are at 2000 feet, and use a SLP, your edm shots are off by about 20 ppm. Unless a front is moving through, Sea Level Pressure varies very little over large areas, here is the current map of pressures across PA:
The formula for the reduction is (H in meters)...
Here is how it varies by elevation...
There are also formulas for correcting temperature for elevation effects (roughly 3.5?øF per 1000 feet), but this is less accurate and there are inversions, etc that make it even less so.?ÿ
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Where I??m at it rarely varies from more than ~70?? to maybe 550?? above sea level as we work our way west 1-2 counties. Probably not a huge pressure difference due to elevation for me.?ÿ
but again I just wasn??t sure if the Trimble S3 came with an onboard/internal sensor for pressure. Trying to find out if it does have one or if I should just go another route to determine pressure
It really is pretty small as far as ppms are concerned. Assuming 30" at sea level and reducing for your elevation would be all that you need the vast majority of the time. So working at 500' abv sea level and entering 29.5" Hg with a rough temperature would get you within a few ppm of what your edm adjustment should be.
So if you are working in an area with an elevation around 1000 feet you don't need to worry much about the two reportings for barometric pressure.?ÿ Right?
@holy-cow I'm not sure of your question but setting your pressure to 29.0"Hg with the current temperature would put your ppm adjustment at an appropriate value when surveying 1000' abv sea level.
This link may help to understand what you're getting from the National Weather Service:
https://www.weather.gov/bou/pressure_definitions
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@lurker?ÿ
Where I am sitting is at 1050 feet above sea level.?ÿ Probably 98 percent of my projects fall somewhere between 900 and 1100.?ÿ A few will be lower but not many higher.?ÿ No need to worry about it in general usage.?ÿ Adjusting for temperature is much more important as it's in the -5 to 115 range over a years time.?ÿ Last January we hit -15 and lower in places for the first time in years.?ÿ No field work was done at that time.
here in my part of NC, the lowest it might get is in the high teens maybe in January & February. That being said, we went duck hunting once in a tiny farming community in Sumner, Missouri. The day we left it was -18* with a decent wind and I had never before in my life felt that sort of biting cold (for a southerner like me). The day before we had gone hunting on one of the rivers out there & the guide had an 8lb sledge hammer busting up the 3-4? thick ice to put out decoys. Again - I was amazed lol
Low temperature combined with wind and humidity, especially if the wind is crossing ice, can be miserable. My general thought is that if I must experience such conditions it's because I am getting paid to be there.
While I have not been to Sumner in Charlton County, I have visited cousins in Marshall County, south across the Missouri River.?ÿ Both of those counties had early settlers predominately from Tennessee and Kentucky.?ÿ According to Wikipedia their preferred crops were hemp and tobacco.?ÿ Many brought their slaves with them.?ÿ Thus this area was a strong Pro-Confederacy source of soldiers.
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I dunno why this subject comes up so often. If we are just talking about what goes on under the hood of this or that instrument, then it is a good question. But it always seems to go off on a tangent relating to "what difference" environmental corrections really make.
Having spent the vast majority of my 50+ year career working between 4,000 and 10,000 feet "above sea level," AND often measuring distance between about 5,000 and 50,000 feet (with significant vertical differences between the ends of said lines), it's a no-brainer for me.
I have over the years spent a little time working at "sea level," but not as much as I have spent over 10,000 ft. All of that said, having a barometer (altimeter) that reads out in inches of mercury, and a good thermometer has ALWAYS been part or the standard kit (several matched pairs when doing long shots). Since getting into GPS about 25 years ago, it has become something that stays in the Total Station & EDMI boxes, but in the rare cases when that gear comes out, I have (and use) these gadgets. It's just SOP!
Back on subject, I would question the accuracy of an onboard thermometer unless the instrument is in the shade. As far as an onboard barometer goes, I have no clue, and would prefer a good old analog (or reliable) digital barometer UNTIL I have satisfied myself that the onboard gadgets are doing the job correctly.
Just my 2-bits,
Loyal
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Some banks have a fan inside their signs, pulling air through them, to get ambient air temp. (For the bank signs).
Someday, the target will have this, and so will the inst. This part of the deal will become automated. For permanent targets, it'll have to be done manually. All this automation. I've got a little laser pointer, that gives temp. I wonder what range it might still be accurate.
Nate
@loyal?ÿ
Agreed. Remember you need AIR temperature, not instrument temperature or the temperature in the instrument box or the vehicle cab! The thermometer should be SUSPENDED in the air several feet above the ground and in the SHADE.
These corrections are more important if you are a long way above sea level, long lines or very precise work. For long lines have a thermometer and barometer at each end. (Back in the days of Tellurometers you might even have had a set mid-way on a very long line.)
Oh, yes: AND do get them calibrated very occasionally, although they don't tend to go out of calibration very easily.
These corrections are more important if you are a long way above sea level, long lines or very precise work. For long lines have a thermometer and barometer at each end. (Back in the days of Tellurometers you might even have had a set mid-way on a very long line.)
Tellurometer measurements also require a humidity correction,?ÿ usually obtained by using a sling psychrometer.
@mike-marks Yup, I still have a pair of Bendix psychrometers (battery operated with 2 D-cells), that were part of an Electro-Tape DM20 setup. Humidity has a much greater effect on microwave based EDMI(s), than on Light/LASER based units. I made an +/-206,000 foot shot from Utah into Wyoming back about 1982 or 83 with a pair of DM20s. In fact, my first exposure to EDMI(s) was with ElectroTapes back in 1969 or 70 in the High Uinta Mountains (USFS).?ÿ Packing those buggers (and their batteries) around over 10,000 MSL ridges and trails was no picnic.
Ah the good ol days (not).
To answer your question, if you tap the arrow on the right side of the pressure field in the Access screen it should list manual or from instrument. Another way would be to see if the pressure value that shows on that screen changes from day to day.?ÿ
Back when long lines were measured, there were some surveys of high accuracy where they actually flew along the line to measure T and P.?ÿ
Back when long lines were measured, there were some surveys of high accuracy where they actually flew along the line to measure T and P.?ÿ
I remember reading about some of those surveys in Southern California. Fixed wing flying one way, and a helicopter flying the other, kept "on line" by guys with theodolites. Pretty cool stuff.
Loyal
Yes, of course... but we were just discussing temperature and pressure.
On the subject of humidity, in locations where there is a VERY high level it is wise to take it into consideration. When we are certifying a competition swimming pool (which has to be done in full working configuration - ie the air above the pool is almost fully saturated) we carry out a localised calibration of the instrument/prism under those conditions before starting the measurement. This minimises any introduced errors. Accimatisation of the gear normally needs over half an hour before starting.