Prism offset of unknown prism

Set two points A and B about one hundred metres apart on level ground. Set up on A and measure to B using your unknown prism.

Then set up point C on line about half way between A and B. Measure AC and CB both with the unknown prism. The difference between AB and AC+CB will be the prIsm constant.

All other error sources being neglible (I hope).

I think you demonstrated they have the same (0 or other) offset. Try AC- (AB+BC) method (I label point B in the middle) on each of your prisms to get a real value.

Most brands can be mounted two different ways in the holder to get either 0 or -30 mm (0.1 ft) correction, and you don't know which you have if things aren't labeled and you don't understand the physics of it.

Leica uses a slightly different offset, which off the top of my head I recall as -34.5 mm but I could be wrong. To further confuse things, their instruments are built to expect that offset and the menu calls it "0" for their purposes.

The 360 degree prisms used with robotic stations, and perhaps some peanut prisms may have some other offset, I'm not sure.

The reason for the dual 0 or -30 mountings is to give you a choice of whether or not to compensate for a small error that can result from not having the prism pointed squarely at the instrument. Light travels slower in the glass than in air, so to the instrument it looks like the glass is a lot deeper than it physically is - the apparent reflection point is outside the actual glass and mount.

If you use the 0 offset mounting, you don't have to fuss with a correction. However, when you shoot the prism from off to one side the instrument thinks it is a slightly different place than it really is because the physical point of the prism is not over the pivot and the ground point.

If you use the -30 mounting, and set the correction in the instrument, then when you shoot from off center you get the same distance reading as when square on because the physical point of the prism is over the pivot and the ground point.

[msg=251432]A prior thread on topic[/msg] has an illustration and here's my sketch:
https://surveyorconnect.com/images/uploaded/2012122420511650d8c04450bac.jp g" alt="" />

The old round(2-1/2") prisms were -30mm offset and the apex of the prism was 30mm back from the center of the bracket/threads/prism pole.

The 0mm offset prisms moved the apex forward to be in line with the threads/prism pole.

There were(in the old days), some oddball prism offsets too.

I think ... generally speaking, you should expect -30mm offset if the rough middle of the prism housing lines up with the rod. If the rough middle of the prism housing is forward of the rod, you should expect a 0mm offset.

I re-read my post and think the last two paragraphs should read:

If you use the 0 offset mounting, you don't have to fuss with a correction. However, when you shoot the prism from off to one side the instrument thinks it is a slightly different place than it really is because the physical point of the prism is not over the pivot and the ground point, so the effective reflection point is out in the air behind the prism in line with the instrument and that point is no longer over the pivot.

If you use the -30 mounting, and set the correction in the instrument, then when you shoot from off center you get the same distance reading as when square on because the physical point of the prism is over the pivot and the ground point. The effective reflection point is out in the air 30 mm on the other side of the pivot, and always on line with the instrument, so the correction makes it insensitive to the angle.

> The 0mm offset prisms moved the apex forward to be in line with the threads/prism pole.
>

If, by "0mm offset", you mean a Topcon Prism that Topcon TS's can be set to 0mm, then these are as you describe: The apex at the rear of the prism is in line with the prism pole. The unknown one (which I'm increasingly believing to be a Topcon as well), has the prism in a circular holder with a thumb screw which can be removed, allowing the prism to slide rearwards about 30mm (when used with a non Topcon TS?).

One thing I can confirm is that in both cases, the reflectors, with the instrument set at 0mm, measure the distance within .005' compared with a steel tape, temperature corrected and properly tensioned.

If you are an all Leica total station shop, the prism constants of various Leica prisms are no issues. Very simple.

>The apex at the rear of the prism is in line with the prism pole

If the physical back of the prism is in line with the pole, it is in the -30 mm position. See my sketch.

And the AC- (AB+BC) method is better than comparing to a tape, given all the problems of accurate taping.

Surely if you have a prism with a known constant then all you have to after measuring and recording a distance is swap the prisms in the tribach and measure the distance to the second prism with unknown constant. So that any difference in distance can be added/ subtracted to the initial prism's constant to determine its constant.

RADU

Bill,

Good sketch and info. Thanks.

Dave

Proceed with great care. The number we call a “prism constant” is the combined correction that should be applied to the combination of your measuring device (instrument), prism and prism holder. It takes into account not only the offset that is discussed above but many other factors. Also, take great care to understand the internal settings of your instrument and the combination of instrument corrections and additional corrections that may be applied by your data collector and processing software. Sometimes the corrections are incorrectly applied multiple times by our helpful devices. It never hurts to occasionally check our combined collected electronic data by physically measuring the distance using the old fashion method of “chaining”.

What is This Thing Called a "Tape Measure"?

Steve,

Are you suggesting we use an old-fashioned tape measure to check distances? What strange land did you come from?

Dave

> If you are an all Leica total station shop, the prism constants of various Leica prisms are no issues. Very simple.

Just make sure you pick the correct prism type on the instrument. Standard circular, mini circular, mini 360 and 360 all have different offsets.

What did leica get rid of -30.4mm equals zero logic. It was confusing as heck when not using their prisms a few years ago. My 2 cents, Jp

It still amazes me when a greenhorns trust the TS, rod , and prism over a 25' pull of a tape on flat ground. Jp

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.

For determining the distance between two close points, sure, use the tape and expect better accuracy and faster.

For determining the offset of the prism, AC-(AB+BC) is the way to go.

rfc,

May I suggest that you post a picture of the prism you have? I wouldn't be surprised if someone could identify the manufacturer and know the prism constant just by looking at it.

Al

You and RADU are correct I think. I would run both through itsmagic's exercise as well. You should know and measure your equipment's offset.