How temperature affects EDM measurements

When the judge asks "Did you apply temperature/pressure/ppm corrections to your measurements?" you had better be able to answer "Yes, M'am!"

> When I checked the ppm, it did indeed change (+ above 68 degrees or so, - below)as it should, but what's the use of setting it frequently if it doesn't make a difference?
>
> What am I missing?

Here in Florida, as far as the EDM is concerned it's always 80°F and 30inHg.

"DITTO"

Those little numbers of difference as applied on each leg of a traverse will combine and can turn a 1:25,000 closure into a 1:80,000 closure.

B-)

Are there very many judges there that make a habit of asking about temperature and pressure corrections applied to EDM measurements in California?

ppm is, by definition, proportional. small distances make small differences. big distances make big differences.

If one wanted to use it, isn't there an in-line command in Starnet that one could use to scale a group of distances?

> "DITTO"
>
> Those little numbers of difference as applied on each leg of a traverse will combine and can turn a 1:25,000 closure into a 1:80,000 closure.
>
> B-)

Of course. But how do you explain that the residuals are random. I can take 10 observations and get the same results.

I guess what you're suggesting is that regardless of my informal test, I can rely on using the formula for atmospheric correction applied to my observations that were taken with the gun incorrectly set.

> ppm is, by definition, proportional. small distances make small differences. big distances make big differences.

Shawn: yes, I guess that's part of my point. at 500' or so, the greatest distance I shot that day, the correction is something like .00002'. I've got so much else going on contributing to the overall precision that I'd be thrilled to be able to detect that.

Like any other error source, you need to compare its magnitude to the other errors you expect, and put a proportional amount of effort into correcting/minimizing each one.

My old Topcon manual gives the formula that the correction it uses is

k = 279.6 - (106.0 P)/(273.2 + T)
with
k in parts per million
T in degrees C
P in mm Hg equivalent

OR

k = 279.6 - (2692.4 P)/(273.2 + 5/9(T-32)
k in parts per million
P in inches Hg
T in degrees F

True length = measured length * (1+ k * 10^-6)

The constants may depend very slightly on the light wavelength your unit uses.

Note that you want absolute pressure, not the sea level equivalent that is usually reported on the local weather report, but fortunately the result isn't very sensitive to pressure. It is quite significant if you are at high elevation vs low elevation, though (Don't quote me, but I think it's around 1 ppm per 100 ft elevation if you use the wrong kind of pressure.)

As an approximation at average conditions, this is about 1 ppm per degree C or 2 degrees F, or 0.1 inch Hg pressure.
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So in your case of relatively short lines, it doesn't make a huge difference:
45 to 80 degrees is about 18 ppm and is about 0.01 ft in 550 ft. If you are holding the centering error to 0.01 then the temperature correction is important at that distance.

Depending upon limits of your instrument, 0.001ft difference between shots is an expected occurrence.

The fact that your display readout will extend to multiple places past the decimal, does not actually represent that the instrument is capable of that precision.

The principal of using PPM adjustment based upon temperature and atmospheric pressure and EC (earth curvature) adjustments is to approach the best possible results your instrument is capable of under the conditions and purpose you are surveying under.

In a short traverse these numbers hardly affect the outcome. On a 15mi cross country survey the little numbers will add up when you have shot hundred traverse legs or have a closed traversed along a road around a 250ac lake.

It is like the difference of surveying locally for me where city surveying is much the same as rural surveying with no towns over 10K population and the surveying in Austin, Houston or DFW and NYC where those precisions required are at or above 1:50,000.

You are making a relative measurement and to attempt that to be as consistent as possible, you enter those values as they change between setups and shots.

😉

Had a shot today over 2800 feet. Even if the temp setting was off a bunch that would still be far more accurate than practically any other measurement system used routinely for common boundary surveys. The width of the crosshair probably introduced more angular concern than temp setting would have on distance measurement.

> So in your case of relatively short lines, it doesn't make a huge difference:
> 45 to 80 degrees is about 18 ppm and is about 0.01 ft in 550 ft. If you are holding the centering error to 0.01 then the temperature correction is important at that distance.

Thanks. This time out, before running any LSA, I can probably look at my traverse and apply a factor (a small amount of distance)to those legs that I did later in the day. Theoretically at least, that should improve the closure. It will be interesting to see.

I've got to get one of those little "skier's jacket thermometers" to hang on the instrument handle. (and add temperature to my checklist).

I was recently involved in the measurement/recording of a new EDM calibrated baseline. It was fascinating to see NGS's EDMs. They used two identical EDMs, with accuracy specs that I didn't previously know existed: 0.1mm +/- 1ppm.

Prior to coming out to Maryland, Steve Breidenbach, from NGS, ran a calibration on the Corbin baseline in Fredericksburg VA at the NOAA NGS geomagnetism lab.

Drive out to Hancock MD to the new line, run the measurements. Occupy each of the four markers, measuring each of the other three legs. We used thermistors and barometers and recorded temperature and pressure before and after each block of distance measurements. Mr. Breidenbach briefed us on many of the particulars; one of which was that a one degree celsius temperature difference can cause a one part per million change, and it is an EDM like theirs that can 'notice' that.

He then drove all his gear back out to Fredericksburg to run the calibration routine again. Buckets of work.

It is ironic to me that finding a site to build a baseline can be more difficult than anything else.

" When the judge asks "Did you apply temperature/pressure/ppm corrections to your measurements?" you had better be able to answer "Yes, M'am!"

I would tell the judge that I didn't understand her question and ask her to please expound upon it and explain to me exactly what she was asking and give me a possible example of the question.

After having finished being cross examined at a trial one time, the opposing attorney had finished asking me all her questions, when I saw one of the lawyers with her lean over and whisper something to her. She then threw out one last question to me that really didn't have anything to do with the line of questioning she had just finished - something about as absurd as asking "did you use a bearing rotation on this job" or some such. I stated that I didn't quite understand her question and asked if she could explain it in more detail and maybe give me an example. Watching her trying to maneuver around trying to ask the seated attorney what she was talking about and seeing that he didn't have an answer was priceless. The question got removed and I got to sit down.

I use Trimble survey controller on a TSC2 as a data collection platform. It stores everything RAW. It also stores the T+P that were entered so that the results displayed in the field can be replicated in the office.

However, I always download weather info from the nearest weather station, correct the readings for elevation of the instrument station, and load all of that into a database with the observed total station data. I then rum a program that corrects the distance for PPM (with a choice of using either the field entered values or the time tagged weather station values), and reduces the distance and the zenith angle to mark-to-mark. The instrument I use (S6) outputs into the DC file the values for use in the atmospheric reduction formula (J and N).

The other instrument I occasionally use (a Zeiss S10) stores the corrected distance, and also the T and P used in the reduction. I "unapply" the PPM correction in this case to get the raw distance, and then apply corrections.

I have a portable weather station (Brunton ADC Pro), but usually rely on the local weather station if it is not very far away. However, to do this it is critical to correct the observed pressure to a pressure at the elevation of the observations, as the values are at sea level. Similarly, I correct temperature for elevation difference as well, although that is usually very small. Another advantage to using the weather station values is that humidity is also available. This is also small, but why not use it?

Here is a blurb from the SC manual:

Damn straight Jeff. That's a great story.

When thinking in terms of parts per million, it helps me to remember that 1 ppm is 0.01' in 10,000 feet.

It only takes one to spoil your survey.

I think it may have been eluded already, but you should read your manual and be aware of which measurements the total station actually applies the corrections.

I believe many only apply the corrections to distances that have been reduced to horizontal.