One more time

Whatever the local barometric pressure is, does not matter, as long as you just dial in the correct altitude. While sitting on the tarmac.

(You are welcome!)

N

> Local airport sits at 1000 ft MSL. It reports a barometric pressure of 29.95 and an altimeter setting of 30.95. Do you set the total station pressure setting to 30.95, 29.95 or 28.95?

The last go-round was a learning experience for me, but here's what I took away from it:

29.95 is the actual measured barometric pressure at the airport. The altimeter wants to be calibrated to MSL, so you set it to 30.95. Your gun, which is going to be working at the airport elevation, should be set to 29.95.

The last time this was discussed turned out to be fortuitous timing for me, as a few weeks afterward I got a project in the foothills at 3,600'. I made good use of the information I gleaned from the discussion.

For Nate: my Leica asks for pressure and temperature rather than elevation and temperature, so I need to get the pressure right. There may be a way to make it ask for elevation, but if so I don't know what it is. (I could use a chart, but it's easier and generally more accurate to look up the day's pressure at the local airport.)

I agree with Jim.

Their "barometric pressure" is the actual pressure (and thus the TS setting) at 29.95 . By telling the altimeter the equivalent sea level pressure 30.95, it then can convert the actual pressure to an elevation. I don't think 28.95 gets involved at all.

Did they actually call it "barometric pressure" and not something similar? It is common for people (including weather reports) to apply that term to the value reduced to sea level, that is, the altimeter setting number, so that everybody is comparing things on the same basis. This is explained in the Wikipedia article "Atmospheric Pressure" under the section "Mean Seal Level Pressure".

My Garmin tells me right now that I'm at 830 ft, the barometer reading is 30.02, and the ambient pressure is 29.36 That seems consistent with my interpretation of the airport data.

In aviation ...

altimeter settings are reported as Sea level pressure. That is what you dial into the Kolsman window on your altimeter. That way your altimeter is indicating the actual altitude.

Barometric pressure decreases one inch for every thousand feet you go up. So, where SLP is 30.95 at 1000' MSL your actual pressure would be 29.95.

The exception to this is that once you get above 18,000' all air planes set there altimeters to standard SLP of 29.92.

Elevation does not translate directly to actual pressure because barometric pressure is constantly changing at any given location. That is why airplanes need to have correctable altimeters. See my additional explanation below.

It seems to me if you set in the airport elevation that is not exactly the correct altimeter setting; the needle will descend (if I remember right) about 50' as you reach takeoff speed. The altimeter is adjusted for airflow over the static port which is on the side of the nose.

Thanks to all

A bit of support is always appreciated.

Jim,

Waiting for the family to wake up. You can specify either pressure or elevation on the TPS1100 series. The F4 key in the atmospherics page will toggle between the two.

As for using the barometric pressure, you what to use the uncorrected value. Back in the days of doing GPS work for NOAA it was SOP to head on down to the National Weather Service and calibrate my barometer to uncorrected.

Merry Christmas.

John Putnam

Set a prism just beyond the longest distance you need to shoot. Then shoot 3 or 4 distances at each pressure setting. If the distances are different you need to pick one, if they are the same it doesn't matter which you use.

I believe "Station Pressure" is the proper term for the information you need.

James

> Local airport sits at 1000 ft MSL. It reports a barometric pressure of 29.95 and an altimeter setting of 30.95. Do you set the total station pressure setting to 30.95, 29.95 or 28.95?

If you are surveying at the airport you would likely use 29.95. The "altimeter setting" would be corrected to zero elevation. The pressure at 1000 ft would be lower. You use the local "station pressure" in your instrument.

Being a conscientious surveyor you would be using the local reading on the barometer you are carrying anyway. You need one, because you aren't usually surveying at the airport. A very good one costs about $70. Keep it in the instrument case.

:good:

> Being a conscientious surveyor you would be using the local reading on the barometer you are carrying anyway. You need one, because you aren't usually surveying at the airport. A very good one costs about $70.

I have mixed feelings about this. Most of the time I'm close enough to an airport that the pressure variation *probably* isn't sufficiently different to introduce any significant error in distances. On the other hand, being able to directly measure station pressure would be reassuring. On the third hand (so to speak), I'm not sure a $70 barometer is going to provide a more accurate determination of station pressure than will using the value from the nearest airport.

The Thomas Traceable Handheld Digital Barometer looks nice, but it's a bit spendy at $210. Since it seems likely that it uses an aneroid cell, and my understanding is that those need a fair amount of time (hourish) to stabilze, I don't know if it would be any more useful than either a cheaper barometer or the airport method.

Still mulling my options...

You don't need super accuracy. An error of 0.1 inch in pressure is about 1 ppm in distance.

> ...... I'm not sure a $70 barometer is going to provide a more accurate determination of station pressure than will using the value from the nearest airport.
Having carried and used one for years I can assure you that the reading from a small barometer will be much better than estimating from airport readings, and much simpler. That's the main thing, it's just simpler.

Over the years I've checked my Brunton Nomad against other barometers and against airports and it's always been good. Furthermore, much cheaper digital barometers have also been good. I'm pretty sure that the guts of these things all come out of the same Chinese factory. I was skeptical at first, too. But they do work.

Often you will be a long way from the nearest reporting airport and pressure does vary even when elevation doesn't. Sure, most of the time an estimation based on the airport reading is good enough. But why go through that work when a glance at a modestly priced gadget will tell you what you need to know?

I would remember it this way. To measure through the air, you need the actual barometric pressure of the air you are measuring through. If you are above sea level, the barometric pressure will be lower than the sea level pressure, so if you see two pressures, the higher one would be at altitude. (around here we are never below 4,000 feet, so I always think in terms of above sea level.)

One other thing, when measuring in the mountains and shooting distance that are going through major elevation changes, we always took pressure readings at both ends of the line and averaged them. Also, if you typically don't shoot both ways, you should when going through major elevation changes. I don't understand why, but have seen screwy things go on when doing this, and you want to make sure and get all the data you can.

> ...., so if you see two pressures, the higher one would be at altitude....
Please Review. MJM