> Depending on where you are you might want to use an NGS calibration baseline. It will provide an accurate value for comparison. If you make observations of all the points you can also calculate and/or verify your instrument's performance. There is a nice little DOS program, CALIBRAT, on the NGS to perform the reductions.

>

> Identifying a wrong prism constant would seem straightforward given a standard for comparison.

>

> NGS site for calibration baselines is: http://geodesy.noaa.gov/CBLINES/calibration.html

That's cool. There's one about 30 miles from me. I'm going to give it a spin. Thanks.

**Yes, Paul is indeed correct**

And if I remember correctly this is also the same reason the original Wild/Leica prisms were designed to have a constant of -34.4mm - because this directly offset the EDM constant.

Thus all of the ensuing confusion over the years because Leica calls -34.4 a 0mm offset.

Sort of the exact same thinking behind their tilting prism concept (the one where the prism was mounted above the horizontal tilting axis. Because the EDM was mounted the same way. Really, both are beautiful in their simplicity.

With newer EDM technology and better distance measurement I wouldn't think Leica would still strive to construct a total station with the EDM at a 34.4 offset internally, but rather compensate with a derived constant (the same as the other manufacturers).

Messing with the EDM constant requires more consideration than prism constant. There have been several discussions about verifying prism constant rather quickly and absolutely on this board. Investigating whether or not your EDM constant is correct should include a visit to a CBL, taking an atmospheric gauge and thermometer with you, and truthfully considering how accurately you can center over a point. At least this is how I would do it. If you're not chasing that last half millimeter for critical projects, you probably don't need to worry about your EDM constant at all - especially if it just came back from the shop.

RPLS NE Texas

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A reasonably robust method to determine the combined constants for instrument and prism is to set up and measure among four points, each about 15 or 20 meters apart, on a straight line and as closely as possible at the same elevation. The objective is to minimize the differences in height between prism centers and instrument horizontal axes.

Given four points

... A_____B_____C_____D,

measure [AB], [AC] and [AD]

... the square brackets [ ] indicate the [displayed] distance values

then measure

... [BA], [BC] and [BD]

... [CA], [CB] and [CD]

... [DA], [DB] and [DC]

note that each [displayed] value includes, but does not disclose, the combined constant "k" so:

... [AB] = (AB + k), [AC] = (AC + k) and [AD] = (AD + k)

where AB, AC and AD are the respective actual distances and, in theory, AB = BA,

AC = CA, AD = DA, etc.

Calculations, once all distances have been measured are:

AB = (AB + k)

BC = (AC + k) - (AB + k) = [AC] - [AB] ....... I

CD = (AD + k) - (AC + k) = [AD] - [AC] ...... II

BD = (AD + k) - (AB + k) = [AD] - [AB] ..... III

BA = (BA + k)

BC = (BC + k)

AC = (BA + k) + (BC + k) = BA + BC + 2k ...... IV

BD = (BD + k)

AD = (BA + k) + (BD + k) = BA + BD + 2k ....... V

CD = (BD + k) - (BC + k) = [BD] - [BC] ....... VI

... repeat for measurements from C and D and note that where there are differences, the actual distances between points are the differences in displayed distances. Conversely, where the distances are additive, the differences are 2k. That is, when distances are differences, the "k's" cancel out and when additive, the "k's" are doubled.

The process yields 12 total equations that allow solutions for "k." Not all values will be equal but, the average should be reasonably and sufficiently valid. For those so inclined, the equations may be solved using least squares that in turn result in a single value for "k" along with residuals and standard deviation, etc. For more redundancy, the method may include five or more points instead of just four.

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.

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.. Do 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.

RPLS NE Texas

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