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Topcon GPT-3105 Prism offset.

Discussion in 'General Land Surveying' started by Al Chace, Oct 28, 2010.

  1. Al Chace

    Al Chace 4-Year Member

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    OK..the manual for this gismo reads:

    "The prism constant value of Topcon is set to zero. When using prisms other than Topcon's, it is necessary to set the prism correction value of that specific prism."

    OK..then the gun is indeed set to zero (easily changed.)

    First question----I thought Topcon prisms were -30mm.

    We've got Leica prisms...-34.4 here.

    Going crazy trying to figure this out. I seem to be missing things on the SPCS by about 1 cm. (Pretty close to twice the 4.4 difference between the Leica's and what I thought was the Topcon value.) Yes, I'm taking scale and elevation factors into account.

    Going to get back to basics and lay down a short baseline with a steel tape, I've got a cheap -30 prism somewhere....just thought I'd get your feedback before I spent another few fruitless hours trying to get this straight.

    Thanks in advance.
     
  2. Bill93

    Bill93 5-Year Member

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    >close to twice the 4.4 difference

    I can't see any way that twice the distance is relevant. The EDM measures two-way path length and divides by 2. The prism offset for everybody but Leica is specified as the distance between the pivot point and the effective point of reflection. That affects the measurement on the way in and on the way out, and the EDM divide by two takes care of it.

    Leica chose an offset that gives slightly better results when off angle, and then built their unique constant into their instruments as the normal "zero offset" point. For anybody else's instrument it is a -34.4 mm prism.
     
  3. Paul in PA

    Paul in PA 5-Year Member

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    TEST TIME

    There is no better way to understand this than to physically test it yourself. Set up your instrument, 10' away set up a tripod. Fist use a tape measure, then put a prism in the tribrach and shoot a distance. If you have variable prism mounst swirct from 0 to 30 and shoot again. In fact shoot all your prisms. I adjust any that are more than 0.005' from true. That might involve masking tape preventing the prism from screwing in too far, once I filed some material to allow it to screw in farther. I also check my peanuts on a plumb line at the same time.

    Once you have consistent measurements then check your instrument with a data collector. Instruments can adjust for the offset, data colectors can adjust for the offset, but you sure do not want both. Personally I want my data collector to record exactly what my instrument did. If you have a replectorless instrument put a target over the tribrach center and check. You want to verify that when your instrument goes into reflectorless taht the offset automatically goes to xero and is reset to whatever when you return to rpism mode. my dta collector has no idea which mode my isntrument is in which is why I want no offset adjsutment done in the data collector.

    You should do this anyway at least twice a year at the same time you check all your pole bubbles.

    Paul in PA
     
  4. Mark Mayer

    Mark Mayer 4-Year Member

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    > Going crazy trying to figure this out.

    Article from the dark side

    Test procedure to determine prism offset. It does not involve a steel tape. Also, EDM's can malfunction. We once had one consistently measuring 0.04' off.
     
  5. Paul in PA

    Paul in PA 5-Year Member

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    Testing Requires A Base Comparison

    A steel tape being the most convenient at hand.

    A baseline being less convenient.

    Or one could set up and do 2 separate 5 hour OPUS observations and wait for precise orbits.

    I proposed my test as a way of getting the surveyor as comfortable with the instrument results in as quick a time as possible. Set up equipment, measure 10.00' with steel tape, measure 10.04' with instrument, quickly understand the problem. Or measure 9.96' with instrument, different problem? yes or no?

    Time to understanding problem is minimal. Resolving the problem takes much longer.

    Paul in PA
     
  6. Bill93

    Bill93 5-Year Member

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    Testing Requires A Base Comparison

    A known baseline is not required to determine the prism constant. Use the
    AB+BC=AC+offset
    method on 3 points in a straight line.

    This doesn't address instrument offset vs prism constant, but neither does using a baseline.
     
  7. Kris Morgan

    Kris Morgan 5-Year Member

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    First

    Cool, you're back! Second, you have no choice but to set up a base line and check. I seem to remember a system of determining the exact offset for each mirror based on three set ups. One at one end, one in the middle, one at the other end, and measuring the snot out of it. At the end, you would have the answer for THAT mirror, not all mirrors.

    Anyway, you knew the answer before you asked.

    Good to see you back!
     
  8. MightyMoe

    MightyMoe 4-Year Member

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    We always stick the prism right to the face of the gun and shoot the distance.
    If your offset setting is correct it will shoot the distance measured from the measuring point on the instrument to the corresponding point on the prism assembly. For our topcon gun it's 0.41' (if I remember correctly). It's a simple check but it will tell you if you have an incorrect setting.
     
  9. Mark Mayer

    Mark Mayer 4-Year Member

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    Testing Requires A Base Comparison

    > Time to understanding problem is minimal.

    Evidently you didn't take the time to read the article.
     
  10. Mark Mayer

    Mark Mayer 4-Year Member

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    > We always stick the prism right to the face of the gun and shoot the distance.

    That's a good way to smoke your EDM's components. My Trimble will not measure under 6 feet for that reason.
     
  11. Paul in PA

    Paul in PA 5-Year Member

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    Mark Mayer ?

    Question is about Prism Offset, you post article about Prism Constant.

    A different fish.

    Paul in PA
     
  12. MightyMoe

    MightyMoe 4-Year Member

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    Hey Mark; thanks for the info. I think our Trimble Robot is rated to 0.65' and we have taken very short shots with it. The prism constant for it however is a little simpler and we don't need to check it. It is set with the operation. It will change when it is told what it is looking at, reflectorless, the peanut, the 360 prism, or the tracking prism.

    The operator needs to be careful with all prism options to get the offset correctly set.
     
  13. Paul in PA

    Paul in PA 5-Year Member

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    It Would Blow Out My Trimble 3306

    Especially after I just shot prismless. I am uncertain how many other current guns use only a laser with a filter to reduce the signal to the prism. Last week I did some shots prismless then switched back. On the first shot I was looking and got a quick full laser shot in the eye as the filter was slow to deploy then with what was left of my vision saw the normal reflection.

    Mark I have been able to focus at about 3'. I could not focus on the prism corners but could focus on my object lens which was in effect 6' away to know I was centered on the prism. I would not dare to look in the eyepiece when I pulled the trigger however.

    A few years ago my filter motor went out and jammed the filter half way between prism and prismless. I doubt I would send it back to the factory again, but it is still a worthy warrior.

    Paul in PA
     
  14. Dave Karoly

    Dave Karoly 5-Year Member

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    Testing Requires A Base Comparison

    Have you tried to read a Topcon Manual?

    I think they are translated from Japanese to Vietnamese to Norwegian to English.
     
  15. Dave Karoly

    Dave Karoly 5-Year Member

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    A really simple test:

    Set up three tripods in line at about 50' intervals.

    Put the instrument on one of the outside tripods. Shoot the other far prism. Call this distance A.

    Put the instrument in the middle. Shoot both outside prisms. Call these distances B and C.

    B + C should equal A.

    If you are using a 34.4mm prism I would try that first.

    I hope Topcon has not gone to the Swiss back-assward thinking of making zero equal minus thirty.

    If using 34.4 doesn't work then put in 4.4 and see if that works.
     
  16. Tyler Parsons

    Tyler Parsons 5-Year Member

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    The procedure to determine prism constant was published in POB many years ago, earlier than cited in another post here. The procedure was to set 3 points in a straight line, A, B, and C, NOT equally spaced (cyclic error?). The easiest procedure is to set the instrument prism constant to 0. Then, set on one end, A, and measure to the other end, C. Next, set on B in the middle and measure to A and to C.

    The prism "error" will show up in each measurement, AC, AB, and AC. Since the overall distance AC has to equal the sum of the 2 intermediate distances, you can set up the relationship
    (AC+e) = (AB+e) + (BC+e), where e is the error due to the prism.
    Solving for e, we have e = AC-(AB+BC).

    e is therefor your prism error or constant (I haven't checked for the sign which should be negative for standard prisms). Put that in your instrument and recheck. You should now find AC=AB+BC as e=0, or very close. If you are checking a 0 offset prism, well, you'll just have to figure out which way to go if you don't come up with e=0.
     

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