Trimble SLR Prism Constant
I am trying to use an old SLR prism, but can't find a prism constant. It is Trimble Part No. 571204360. Anybody know off hand?
I thought most of those old ones were 2mm, but cannot confirm...
Find it with three measurements on points in a straight line (B in the middle).
Correction = AC - (AB+BC)
@bill93 You are WRONG to think that this method will give you the prism constant. What you show (3 points on line) will give you the Instrument constant or sometime referred to as the IOC (Inst. offset correction). If the manufacture does not give you the prism constant then it can be determined but not by your method without stipulating many more things.
It gives the constant for this prism and instrument combination.
That is not exactly the prism constant, but I think most people assume that the instrument offset has already been taken care of, and just use the number that comes with the prism if it is given. So this procedure is just actually better than using the catalog value.
@bill93 It just as you say in the first 7 words (Your method as posted above: " That is not exactly the prism constant" is just what I said; you are wrong.)
As I said in my last sentence you would have to Stipulate more things for your method to work. In surveying we do not "Assume" as you state above.
you are wrong.)
As I said in my last sentence you would have to Stipulate more things for your method to work.
And you, are less than kind.
Please John, “stipulate” for us.
I think I'll just shoot a prism with known configuration set up on a tribrach then shoot the SLR initially set up with a 0mm constant. Then figure the offset by slope distance difference.
I'm just surprised that I couldn't find anything online. Even the local Seiler service department didn't know.
Maybe I should have asked the ex-wife. She knows everything.
In surveying we do not "Assume"
But I would bet that 99 out of 100 users DO assume the instrument offset has been calibrated to zero.
What you show (3 points on line) will give you the Instrument constant
No, it will not. That is a misstatement of your point. It does not give it, it includes it.
I have not tried a thorough search of this site, but my foggy brain seems to recall some fairly detailed discussions of this process somewhere in the past years…
…EDIT (I just found this on my pc):
John, I wonder if you are willing to hold this dumb old dirt surveyor's hand a little regarding my prism offsets? Per your posts (one of which is copied below) I think you are the best guy at beerleg.com to help me "offline".
I typically do small site topo boundary surveys for engineers to do site design from. So I have been using my 360° prism since 1998 and been fairly happy with the 0.03 feet floating around in my control. (This is the part I am not excited about posting "online").
So lately that 0.03 feet has been bugging me. Maybe some anchor bolts are coming in my future, I don't know for sure what that tingling sensation is on the back of my neck.
Today, I dug out one of my old "control type" circular prisms that my dealer sold me with my setup in 1998. Haven't used it hardly at all since then. I am lazy, and it sure is nice to not worry about what direction the 360° is pointing to maintain lock.
Doing a quick comparison in my TDS SurveyPro software on some points I set out at Dad's barn lot in 2010, again today between the 360° prism (0.0mm offset in the software) it looks like I have to tell the software to use something close to -18mm offset for my "control type" circular prism, today I named "ORANGE".
So my question for you is (1) does this seem reasonable, and (2) should it be -18mm, or -23.1mm, or (30 minus 11.3 = (-)18.7mm, or...?
I also think I will try Bill93's AC - (AB+BC) = prism offset field test too.
Thanks a lot, Brad
Your recent post:
Prism offset of unknown prism by John Putnam , Forest Grove, OR, Wednesday, July 30, 2014, 16:12 (53 days ago) @ Jp7191 No, The circular is still held as zero even though it is -34.4mm. It is just that given the different types of glass they have different offsets. I'm sure it is the same thing for Trimble and Topcon. The nice thing is that the guns, from 1100 series on, let you choose the type of glass you are using and then make the proper adjustments. The 1200 even show an icon of the glass type on the screen and even allows you to specify what type of glass you use for setups versus side shots. The only problem is user error. -- John Putnam, PLS OR, CA, WA & ID -
@brad-ott I would be glad to help.
Using bill93 post with 3 points on line, gives you the combined constant for the Inst. and prism (but the method is overkill) plus you have more errors. The errors are each time you set up; setting up the Inst. is one and setting up the prism is another. Then you have Inst. (EDM) error. Each one is a + or - something. Your optical plummet is +/- 0.5mm and that will be for the prism end also. Then how good is the EDM in the Inst. (lets say you are using a DI2002 EDMI then the error in measurement is +/- 1mm , EDMI only).
Now by statistics we know that the total error is the square root of the sum of the squares of the errors(we here are only talking about setup error at this time). So in 3 points on line there is 6 setup's. sqrt. of 0.5^2 +0.5^2+0.5^2+0.5^2+0.5^2+0.5^2 = +/-1.7mm (and that's just for setting up).
Can we do better? Yes; time and money will do the job, but will help in the long run.
1. The easiest way with not time and money is the way Scott Bordenet is trying to do and that's by using the manufactures quoted offset for Trimble Part No. 571204360. I don't know when Trimble made his prism but the offset should be better than in the 1970's. AGA made very good prism in the day and their offset was +/- 1mm at most.
2. I think the best and most accurate way is to use an absolute distance measuring Inst.. This would be a Laser tracker. I have used this back many years ago (~15 y) and the way it works is the Laser tracker (Faro,API,Leica, etc.) will measure to their SMR prism (which has around a +/- 0.002mm offset) then you remove the SMR and put up your prism and measure again and you get the offset (very very accurate). I needed this for some interferometer work I was doing. The person that had the FARO Laser tracker I gave him $150.00 for that measurement.
3. Another way is to use 2 points (at the same elevation) Measure between them with your calibrated 100 foot tape or 50 meter invar tape (which would be the best BUT who has either of of them, except me?). (Your Total station must be calibrated with a Known prism). You then can use the 2 points to determine another prism constant; to some degree of accuracy.
4. From here on it gets more complicated and the time and money come in to the equation; but it can be done.
Hope this helps.
PS for your 360 prism I would 1st take your Inst. to a NGS base line and calibrate it with a prism of known offset. Then use your 360 prism. This will check the 360 offset and should get rid of the annoying 0.03 foot error. One problem would be IF you can find an up-to-date NGS calibration base line.
Some of the errors in John's description can be minimized if you set up 3 tripods with tribrachs so you have forced centering. Shoot AC, move instrument to B and shoot prism, move prism to other end and shoot it there. That should get you near the accuracy of the EDM.
If you are obsessive about it you could repeat with the tribrachs at a different distance to see how repeatable the EDM is.
The "prism constant" per se, for the unit shown in the OP, is described on page 1-14 of the attached manual. Note that the height of the prism to the pole top, is Trimble's standard 135mm, to match its other targets in that series (and the distance between the tracker and the optical scope on the instrument)
Also attached is what appears to be a proprietary paper discussing prism constants.