Temperature correction for elevations?

[Bill93]

I have been trying to measure a building foundation for uneven settling, and that has got me thinking about all the error sources in level measurements (My working draft writeup). I have one that isn't discussed where I've looked in my old surveying book, that may only be of theoretical interest for most applications. But it still bugs me.

 

What about thermal expansion of the structure a benchmark is on? Of course for precise work they use an invar tape on the rod or else temperature correct the rod readings. But do they correct for the structure temperature?

 

Suppose you have two marks within a few hundred meters of each other, one flush with the ground and the other on a large bridge abutment with an exposed 6 meter face. Between seasons you might get a 60 degree F change in the temperature of the abutment. A mark flush in the ground would move little; temperature changes are minimal below a meter depth. So careful measurements would show the relative heights of these two marks changing.

 

Since concrete is usually reinforced with iron I'm assuming their expansion coefficients aren't dramatically different. With iron's coefficient 6.45e-6 / deg F, 60 F, and 6 meters, we get a differential expansion of 2.3 mm.

 

The NGS reports elevations to 1 mm resolution, so the value would change. Even 3rd order marks, if they are within a few tenths of a km, would need to be held to 1 mm (I think).

 

So the question is: does NGS correct for the temperature of the supporting structure, and if so what temperature is standard?

 

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Edit: example

NJ0547 (GC) . . NJ0547 (NGS) . . Photo

is on the abutment of the upper of two former rail lines, Second order Class 0 at 247.366 meters 811.57 ft.

 

Within 50 feet horizontally and 35 feet below on the other former railroad there was another Second/0 mark, probably destroyed NJ0583 (GC) . . NJ0583 (NGS) at 236.702 meters 776.58 feet

Edited April 24, 2007 by Bill93

[68-eldo]

I can’t answer your question, but I can offer this tidbit of information.

 

When we did battery alignment on the ships to align the guns, directors, and other parts of the weapons system we do it by sighting on a star to eliminate parallax. Obviously this has to be done during darkness. But it is always done during the early morning hours because we had to wait until the temperature of the ship evened out. The sun shining on one side of the ship warps the ship enough to make the readings of little value if done too soon after sunset. Not to mention the readings would change as the ship cooled off.

 

With that in mind one would have to wonder about stations like this one. As the sun shines on one side of the building I would think the mark would move because of expansion of the sunny side of the building. So how can this be a first order mark? And of course the height of this mark must change too, but it is scaled so its height is not accurate.

 

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