Below are two segments of the "manual", provided with the Leica GHM007 and GHT196 attachments, which elicited some debate amongst crew chiefs today.
These attachments provide a means of measuring a "slant" measurement from the instrument to the survey point. The tape is "modified" to provide the true vertical height from the slanted measurement without the necessity of computation.
A pretty clever device.
Surprisingly (to me), there is some difference of opinion as to what these two segments of the "manual" mean.
So, at +10 Celsius and 1.6 meters, what is the correction applied to the measured distance?
It would appear that the temperature caused 0.33 millimeter shrinkage (-0.33).
I'd say the correction is along the lines of a BCH.
Odd. Very odd, indeed.
-0.33mm; but that's too small for my old eyes to see anyway.
Thats Leica precision for you. Even the tapes for measuring the HI are done in the mm accuracy.
That's 0.33mm subtracted from your measured slant to get the true HI.
To my mind, they are saying the tape is 0.33mm short due to the temperature, so, you must add the 0.33mm.
The sentence could use some clarification, no?
As it gets colder than 20° (C) the tape contracts by -0.33 mm. The True H.I. is 0.33mm higher.........
Variation seems to be the key operative word.
I was a minute too late for the glory........
Have I lost my mind?
If the tape shrinks, it reads longer between two fixed points, so you have to SUBTRACT the correction ...
Son of a ....
> As it gets colder than 20° (C) the tape contracts by -0.33 mm. The True H.I. is 0.33mm higher.........
Think carefully. The tape shrinks by 0.33 mm, so the value read is too high by 0.33mm, so the value must be corrected by -0.33mm.
An old rule of thumb is "hot-set-subtract."
And by corollary, "hot-measure-add", "cold-measure-subtract", "cold-set-add".
Always change two.
:star:
So what was the difference in opinion? Some thought it should be subtracted and some thought added?
To be sure, at +10 degrees celsius and 1.6 meters, the correction to be applied is -0.33 meters. It is also clear that the reason this quick reference is listed, is because the coefficient of thermal expansion of the Leica measuring tape is not constant. For instance, the coefficient of thermal expansion for an invar tape is 0.0000002/degrees farenheit and can be used in the equation
Ct = L[tex]a[/tex](T-Ts)
Where Ct is the correction in feet or meters, L is the measured length, ? is the coefficient of thermal expasion, T is ambient temperature and Ts is standard temperature. The segements posted by Scott shows all of the knowns except for [tex]a[/tex].
Thusly, if one solves for the coefficient [tex]a[/tex] for all of the temperature values in the chart, except at 20 degrees celcius where the correction to length is zero, one can see a trend:
Ct L T Ts Ct/L (T-Ts) [tex]a[/tex]
-0.19 1 10 20 -0.190 -10 0.019000
-0.38 1 0 20 -0.380 -20 0.019000
-0.76 1 -20 20 -0.760 -40 0.019000
0.19 1 30 20 0.190 10 0.019000
0.38 1 40 20 0.380 20 0.019000
0.57 1 50 20 0.570 30 0.019000
-0.28 1.4 10 20 -0.200 -10 0.020000
-0.56 1.4 0 20 -0.400 -20 0.020000
-1.13 1.4 -20 20 -0.807 -40 0.020179
0.28 1.4 30 20 0.200 10 0.020000
0.56 1.4 40 20 0.400 20 0.020000
0.85 1.4 50 20 0.607 30 0.020238
-0.31 1.5 10 20 -0.207 -10 0.020667
-0.61 1.5 0 20 -0.407 -20 0.020333
-1.22 1.5 -20 20 -0.813 -40 0.020333
0.31 1.5 30 20 0.207 10 0.020667
0.61 1.5 40 20 0.407 20 0.020333
0.92 1.5 50 20 0.613 30 0.020444
-0.33 1.6 10 20 -0.206 -10 0.020625
-0.66 1.6 0 20 -0.413 -20 0.020625
-1.31 1.6 -20 20 -0.819 -40 0.020469
0.33 1.6 30 20 0.206 10 0.020625
0.66 1.6 40 20 0.413 20 0.020625
0.98 1.6 50 20 0.613 30 0.020417
-0.35 1.7 10 20 -0.206 -10 0.020588
-0.7 1.7 0 20 -0.412 -20 0.020588
-1.4 1.7 -20 20 -0.824 -40 0.020588
0.35 1.7 30 20 0.206 10 0.020588
0.7 1.7 40 20 0.412 20 0.020588
1.05 1.7 50 20 0.618 30 0.020588
-0.42 2 10 20 -0.210 -10 0.021000
-0.84 2 0 20 -0.420 -20 0.021000
-1.68 2 -20 20 -0.840 -40 0.021000
0.42 2 30 20 0.210 10 0.021000
0.84 2 40 20 0.420 20 0.021000
1.26 2 50 20 0.630 30 0.021000
It is easy to see that the coefficient starts at 0.019 and creeps to 0.021 with some anomalys along the way.
So, it would seem that the correction quick guide is necessary since there is not a constant value for the coefficient of thermal expansion.
AS3
I'm sorry; 0.33 mm ?????
Is this really significant?
> I'm sorry; 0.33 mm ?????
>
> Is this really significant?
Did you measure to the bottom of the dimple of the punch-mark or the "high-point" of the cap that you would level to?
It is funny. Knowing all of that expansion and shrinkage information is great, but it is still easy to think you should subtract when you should add and add when you should subtract....doubling the error you are trying to eliminate. Yes...you have to take a double-take. If a measure device is too long....you have to add to the measurement you read to get the 'true' distance....but if you are staking something out (or setting you HI at the 'same' height in this case, you need to set the point at a seemingly longer distance (add your correction) to get the right point.
We lose this automatic remembering with the loss of chaining. We have the same trouble with figuring out which way to make the compass correction from/to magnetic north. We do need to deal with applying the correct corrections, however, in "Grid to true" or in some electronic techniques and it is a good idea to remain sharp at it.