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moosesamuel
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Hello all. I am new to this forum and this is my first question. I have been told that when doing a survey with a total station, that as long you can see the glass of the prism, the shot should be accurate. I do not believe this to be true but cannot find the documentation on it. Can anyone show me the proof that you need to see the center of the prism in order for it to be truly accurate?

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John Putnam
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For accurate angles the instrument needs to be pointed to the center of the prism.  If you are using a auto-locking or robotic total station the your pointing needs may vary.  For my Leica robotic total stations, if the instrument can see the prism then it will move itself to the center of the prism.  In some cases what the instrument may not appear to be locked on the center of the prism but it is.

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Bill93
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To get an accurate angle by manually sighting, you need to point at the pivot, not the center of the glass.  Targets are designed to be over the pivot and that is centered on your monument.

Basically, if the instrument is pointed so it can get a reading on the prism (and not some other reflection) the distance (not angle) will be pretty close.  Pointing to somewhere other than the center of, or slightly off the glass should not make a significant difference.  Depending on how the prism is mounted, its pointing at or off the instrument can make a measurable difference. 

A typical ~3 inch prism will give you a distance reading to an effective reflection point behind the rear apex of the glass because light moves slower in the glass than in air.  The EDM gives you a larger reading than the distance to the pivot (lined up with the rear point of the glass) which is what  you center over the monument.  Thus you get a -30 mm offset to apply to the basic reading.  With this mounting, the prism pointing is non-critical, and that is why it is used. 

If the prism is mounted so the effective reflection point is at the pivot when aimed at the instrument, you get zero offset.  The rear point of the glass is not over your monument, so aiming the prism away from the instrument may still get you a reading but it won't be as accurate.  The glass has moved further away from the instrument.

If you use Leica, they have a slightly different offset, and a different way of describing and accounting for it, but that's just a number game.  Don't mix Leica and non-Leica if you don't fully understand the difference.

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moosesamuel
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So am I understanding you correctly that if I am using one with a 30mm offset than It will still be accurate, and if it anything other than that I will be getting some, albeit small, error? Because my reflector is a 0 mm offset. 

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Dave Karoly
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I like to see at least 50% of the glass and the center. It's best to clear the line of sight completely for control points.

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Dave Lindell
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Why not run a test?

Set up your instrument and reflector about 200 feet apart. Point the reflector directly at the instrument and take a measurement.

Then turn the reflector 10° at a time to the right and/or left and see what the differences are.

I suggest you might want a helper for this unless you like to walk a lot.

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