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Check rod height w/ Leica 360 prism

Discussion in 'Surveying & Geomatics' started by roger_LS, Mar 20, 2017.

  1. roger_LS

    roger_LS 1-Year Member

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    How do you check the rod height on a Leica rod w/ 360 prism? Where do you measure to? I'm consistently getting .02' to .03' vertical error w/ every backsight check.
     
  2. Conrad

    Conrad 5-Year Member

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    Hello. Assuming it's a GRZ4, measure up to the center of the glass (where the lines cross) on one of the prism faces that does not have a yellow arrow above it (arrow may not still be yellow if it's an old prism).
     
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  3. roger_LS

    roger_LS 1-Year Member

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    Thanks!
     
  4. Grads&Meters

    Grads&Meters Member

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    Hello,

    I called to leica support few years ago about grz4 elevation problem. Depending on which mirror you aim at the height is +2mm or -2mm.

    The explanation was that a large 360 prism like grz4 would be impossible to manufacture because the mirror corners would intersect each other at the exact same center point and the prism would be 0mm diameter at the centre where the mirror corners intersect.
    To avoid this each mirror had to be aligned a little which gives 2mm error to height.

    The same is true with grz121 but because it is so much smaller prism i believe its +-0.5mm or +-1mm height difference.

    A bit hard to explain but if you rotate grz4 around and look at the prism centre it is easier to understand.
    360 prisms should not be used at traverse measurements. They are not meant for that. GRP121 is the way to go.
     
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  5. Jacob Wall

    Jacob Wall 6-Year Member

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    Assuming you are using a Leica robotic total station, I can share my experience with vertical error. I found that when I was using a TCRP1205 on a regular basis that I got noticeably better results if I ran the Check and Adjust program about once per month to calibrate the sensors. I always used the circular prism for that procedure. I was usually getting vertical checks of less than 6mm (0.02') on typical ~100m measurements (with GRZ4 prism), but did see that get up to 3 times that on occasion if I forgot to run Check & Adjust for a few months. Which instrument are you using? How long are your typical measurements? What are you expecting?

    I have seen impressive vertical results from a 1" TS15 that was regularly calibrated and repeat network measurements (Sets of Angles application) adjusted with Star*Net, in the 0.5-1mm range for the 95% error in the vertical. That was with using circular prisms though.
     
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  6. Dave Karoly

    Dave Karoly 7-Year Member

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    I do the same with my Trimble total stations. A long sight on a cold, clear morning is ideal.
     
  7. roger_LS

    roger_LS 1-Year Member

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    It is a Leica robotic TS16 1" that I bought about 6 months ago, it was a demo gun for the Leica rep. I'm talking about shots that are 50', 100', 150' 200' The reason I am suspicious of something is that it gets almost exactly two and a half hundredths error EVERY backsight check and at any distance. I've got an older Topcon GPT series something that's gets this kind of error and more all the time but I'd expect this gun to do a little better.

    I'll try this check and adjust program. Thanks!
     
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  8. Conrad

    Conrad 5-Year Member

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    I should have expanded a bit more on my first answer. The height is measured up to the centre of the glass, no matter which face of the prism is being shot to. But the height reading graduations on a leica prism pole corresponds to the reflective centre of the side without the yellow arrow facing the instrument. So if you are standing behind the prism pole, facing the instrument, then the height reading will be correct, and the horizontal pointing error is minimised, when the yellow arrow exactly faces you.

    My testing suggests that the recent leica ATR sensors, under good conditions, are able to resolve a radial position difference accurately to about 0.5". So you should have a near undetectable error at 50' due to the system if the ATR and level sensor are working and adjusted properly. But even with a level sensor in poor adjustment you should see the vertical offset error growing with increased distance, and reducing to near nothing near to the instrument.
     
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  9. VA LS 2867

    VA LS 2867 6-Year Member

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    If using the orange glass hexagonal prism, only sight the faces of the prism with the yellow arrows for vertical. There is a different version of the prism which has smaller glass where the vertical differences in the adjacent faces were almost to nearly eliminated. I always got in the habit of making sure I was facing the instrument for every shot so the same prism face was being sighted.
     
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  10. amdomag

    amdomag 6-Year Member

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    I was informed by a very competent Leica person that there is difference between the GRZ122 and the GRZ4.

    The GRZ4 is mainly used for land surveying where elevation is of less importance than horizontal angle. THE GRZ122 is good for machine control where elevation is of more importance than horizontal angle. He added that due to optics limitation, it is impossible to design a 360 degree prism that is good for everything.
     
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  11. John Putnam

    John Putnam 6-Year Member

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    The older GRZ4 is the one with the yellow arrows and no threaded top. Per Leica it has a pointing accuracy of 5.0mm unless pointing at the arrows where the accuracy is 2.0mm. The newer GRZ122 is the one with the threaded top to allow for attachment of a GNSS antenna. Due to newer manufacturing techniques, it has an stated overall pointing accuracy of 2.0mm. This comes at a cost of over twice the GRZ4. I have never heard that the GRZ122 was meant for the machine control crowd. As others have said, neither of these are meant to be used for control. Under certain circumstances I will use them as a back sight on known control.
    As for the original post, the prism center is 86mm above the bottom of the glass mount per the Leica prims dimensions sheet that comes with new glass.
     
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  12. Conrad

    Conrad 5-Year Member

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    hello roger_LS,

    I think i have incorrectly told you the leica prism pole graduations are correct when the yellow arrow faces away from the instrument. this was based on my own tests with some well-used equipment in our office and not based on the leica literature. the leica material states the opposite. so i will defer to what a leica store website states which should be correct for well made equipment:

    "...when aiming directly at one of the three prisms, indicated by yellow arrows, an accuracy of better than 2mm (<1/8") can be achieved."

    however, as i already stated, if you use the other face (or mo matter which prism face you use), you can still get the correct rod height by measuring to the crossing lines in the center of the prism, which is where the signal is being returned from.
    [I][/I]
     
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  13. roger_LS

    roger_LS 1-Year Member

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    I do have the GRZ122 w/ the threaded top, no yellow arrows but when I measure to the center crease in the prism it looks to be right on. Must be still something going on because the 2mm (.006) pointing error wouldn't entirely account for what I'm getting. There doesn't seem to be any problem with horizontal pointing error as you can run around a city block and close practically flat if you're careful. None of this is all too crucial for the jobs I do but would be nice to know what's happening.
     
  14. John Putnam

    John Putnam 6-Year Member

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    Roger,
    If you are trying to determine your rod height you need to measure to the bottom of the prims mount (see attached diagram) and add 86mm. I think it would be near to impossible to measure to the center of the glass with any assured accuracy.
     

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