Robust method is fine, but bear in mind that if the question is asked about what a prism constant is, the questioner is probably now totally lost.
Simple answer to the question: when an instrument measures a distance this is from a "fixed" point inside the instrument. It may not be at the "centre". Many manufacturers make their prisms "off centre" in the other direction, so the two offsets cancel out. Therefore the distance displayed is the distance between centre of instrument and the mounting point of the prism.
Fos some uses the prism can't be made with the "centre" in the right place, so it is marked with a correction value - use that when the distance is measured with the manufacturers instrument.
End of simple answer.
NOW: if you are in doubt do a simple version of the test outlined previously. Use three pegs a,b,c. Then the equations produce just one answer, no need to worry about least squares solutions. The resulting correction k is applied to all your readings - the instrument will probably let you set an "offset" within the menu.
One of the first things to do with a new instrument is to test all the offsets. This should be done with each prism and all the various programs in the data collector. You don't want the program to check into the prism on the backsite tripod and be set the same for the 360 prism or the tiny layout prism. Test, test, test, it's simple to do.?ÿ
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Hello. Thanks for all the answers so far.
I know and understand what prism constant is and how to apply it.
What I don't know and is confusing for me is the instrument offset written inside and if I have to apply it. For example if I use a Leica prism that has the real constant -34.4mm on a Pentax total station...I have to apply it in the instrument. Ok so far. But...
Do I ad or not the 20.2 mm ? In the Pentax software menu there's not an option for something like this, only for prism constant.
So..?ÿDo I ad or not the 20.2 mm ?
This in the question ???
Set two points in your yard, the distance apart does not matter.
Pull out a steel tape and measure and/or setup an older TS and measure a distance.
Set you new gun on the instrument tripod and shoot the same and apply the resulting difference into your instrument until when you take a measurement it will be the same as your other TS or taped distance.
Bottom line - if you use Leica, use Leica prims. Anything else, I just set to 0, in instrument, DC, and prisms, and no worries.
Al Harris is exactly correct, test the actual distances, it will become obvious what you need to do. This isn't really complicated. Measure it!!! All you need is a tape measure.