So my crew is in a heated debate on the correct way to measure heights to a prism on a tripod. Since there are so many techniques I was wondering if there was a general consensus among us for determining the height of a prism on a tripod. What do you measure to and why?

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Use a tape measure!

In essence...you are taking a slant measurement...a hypotenuse. Cut a hundredth or so from your slant. I think Leica has a bracket that clips to the tribrach and has a tape measure on it and takes into account the prism height.

If a surveyor or a survey technician has trouble figuring out a triangle, send them away.

JAC

yup...

Amen

Regarding measuring heights....as in a cell tower as an example. Rodman stands under the center of the tower. Measure ups are important, of course. One shot on the prism.....then turn vertically to top center and write down the vertical angle. So you now have the distance and the angle to the top. Do the math. Ta Da!

This is why I love working with a robot by myself. Arguments like this do get heated. Surveyors love to argue about some trivial things that are really just common sense if you know what you are doing. It really all depends on how accurate you need to be. I agree with the first comment here. 5 degrees off plumb over a 5ft rod height is only a .02 difference. I suppose if you are setting control via conventional methods for a construction project requiring tight vertical work and don't intend on running levels through them, it wouldn't hurt to measure to the bottom of transit plate and then add a more precise measurement to the center of the prism to eliminate that error. But if you are setting up a backsight to stake out off that control later after and the precision expected is + or - .05, the backsight just needs to prove that before and after the work is done. So just use your tape.

???  Every once in awhile there is a question on here that makes me scratch my aging head... Mr. Obvious would like to point out that no matter what your prism configuration ( single, triple or ganged ) there will always be an aiming or target point that is fixed for the particular prism/target assembly that is mounted on the tribrach.

In simplest setups, one should measure as near to vertical as practicable from the subject monument to this aiming point.  Of course this is not a perfectly vertical or plumb line.   Therefore a standardized procedure should be established to deal with this.  99 times out of 100, I measured the slant to the Horizontal mark on the edge of the prism target and dropped 0.02 for a nearly perfect "vertical" T.H. on my tripod/tribrach mounted prism/target combo.   This would obviously need to be verified for what ever combination of equipment you would be using.

Sorry but this seems like a problem for the surveyor and/or party chief to simply say "This is how it will be done."  and end the debate among the field crews. 

If it has a hook use it this way you dont have to worry about slant measurements. You want a good representation of how far from the point on the ground is from the center of the crosshairs.so if you use a ruler you have to compensate for the slant as has been explaned. Make sure if you are measuring to the side of a metal target that it is not loose/crooked. Thats all I have to add to the fine answers previously provided.

Don't measure. It drives slow thinkers crazy but the best thing to do is not to measure the height.

1. screw the prism and adjusting screws all the way down

2. read whatever the prism pole says, never mind if it's 1' or 100' or anything in between, never mind that it is not the actual height of the prism

3. whenever you raise the rod; read whatever it says, again never mind what the actual height really is

4. Never measure down to the occupation point, leave he HI at 0.

I've closed 1000's of trig level loops and never took a measurement to the prism or instruement.

Drawbacks: 

1. You won't know the actual height of the 1st occupation point. You may have to set another point close by to Bench Mark off of.

2. All your prisms an poles must be identical; if you use multiple prisms and poles from the same setup.

Advantages:

1. More of your data can be collected electronically. There is no misreading a tape or typing in the wrong numbers. 

I know some will say this is impossible. Maybe somebody who understands and is more articulate can explain. All you actually need to calculate the elevations is the RELATIVE position of the prism.

WTH??!!??  Sorry, but I hope you are joking.  I mean if your Instr. Height is identical to ALL of your target heights you will simply have a floating loop that is not tied to a benchmark.  I know there prism mount systems that can be adjusted to match the height of the instrument from it's own tribrach mount, but I have a $100 bill betting you can't do a dozen leapfrogs in a row to the exact same heights.  Backsight (1) tripod w/prism to Instrument tripod (2) to foresight (3) tripod w/prism.  3 setups, how do you move forward to the next setup (4) and maintain your "perfect" heights with tripods? 

Use plumbob and bullseye on tribach to level it over station. Measure from prism mount plate to station and add on measured height from prism mount plate to centre of prism.

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