Prism Offset Diagram

We locate buldings like this all the time, the EDM has to be set for 0mm though. Your error sounds like yours is set for 30mm, 25.4mm = 0.083ft.

With my Sokkia SET530R3, if I check a backsight prism after shooting reflectorless shots, my distance checks are off by 0.10 to 0.12, reminding me to change the EDM setting on the face of the display from "reflectorless" to "prism".

Check the mini prism with your plumb bob. I also shoot buildings all the time with the prism out of the bracket. Do some testing with the plumb bob and you will learn where to place the prism on the corner for the best shot.

Thanks guys. I will have to play around with it.

We were using my peanut prism with my Partner's robot and data collector. We are going to have to get a prism set up in the Smart Target options for that.

Shoot it reflectorless and check it, I find that it is very important that the prism be normal to the line of sight.

Here's a sketch as I interpreted a standard large -30 mm prism. Peanut might be different in some respects? Leica is probably different by a few mm.

Peanut Prism Offsets

I took my peanut prism apart and noted that years ago I had filed a bit off the plumb bob holder nut so that the prism matched my large prisms at 30mm offset. On my plumb bob holder the 30mm string slots align with the front edge of the finger grip knurl, which is 0.01' back from the front of the case. The rear string slots are 0.005' in front of the small point at the back of the prism. I believe they intended the small point at the back of the prism to be the 0.00 offset point. My mini prism plumb bob setup not being made perfectly. From the 30mm to 0mm string slots I measure 0.098' which calcs to 29.87mm, Close enough for hand held work.

My mini prisms are seldom used with a plumb bob, spending most of their life on mini targets on plumb rods. I prefer to use a mini prism except for those 1000' plus shots and difficult visibility areas, when you just need a 6"x6" target.

My EDM has an eas switch from 30mm offset for a prism to prismless at 0mm offset. I have always checked in at 0.00' when I test shoot. Holding the prsim the way you did requires a 0.oomm offset in the instrument or data collector, wherever you set it.

Paul in PA

I have a 0mm robot prism , so I just set all my targets to 0mm giving me one less thing to remember. Most peanut prism brackets have a string slot for -30mm and 0mm.

Peanut Prism Offsets

I use the horseshoe bracket for my peanut prism, it in in my vest pocket except when I am using it, my plumb bob is on my belt and they work as a unit. That horseshoe bracket is made for use with a plumb bob and the offsets are marked, just put the string in the proper slots. For a distance to a corner, just aim the thing at the gun with the proper offset slot perpendicular to the line back to the instrument and the corner and wind the horz. angles to the corner before or after you get the distance. If you are wanting to use the prism alone, a bit of measurement with it in a bracket set for the right offset should get you the + or - for the bare prism, but I don't need that, for the face of something like a wall or beam, I do need the add which I measure directly on the horseshoe bracket. Working without a plumb bob is like walking through gravel bare footed.
jud

I would check everything against a steel tape just to make sure.

I thoroughly recommend that you set up your total station and another tripod, about 10' apart, and try all your prizms, and you will find out what I found out. 3 separate 0.30mm offset prizms can be different by up to 0.02'!

You can measure the "True" distance with a box tape.

I have tweeked the offset setting in my gun, so that I can stay in the real world.

N

Prism Offset Standard

I am very happpy with 30mm all around. When I switch to reflectorless the gun automatically switches to zero.

When I set up I test everything in the house, 3 large tilting targets, 3 large prisms with candy canes, 4 mini tilting prisms with 5/8" holes, and the mini tilting prism with the mini hole that stays in the instrument box with a mini 4 section rod just in case. One mini is on a Seco stake prism, one stays on a 1' stakeout rod, and one on a 1/2 height 3 section rod.

If anything tests more than 0.005' off it gets adjusted or replaced.

My minimum equipment to go out into the field is a 3 section extendable 12' rod on a 3 legged bipod, a 2 section 8 foot extendable on a bipod and the 1/2 height 2 section wd ith a 2' extension between the rod and the tip and one tripod. Default is 5' rod height since I almost always carry elevations. If it is a major traverse I might up it to 3 or 4 tripods, with 4 prism bags, 3 with bases, tribrachs and 1 or 2 prisms. The 4th bag is spare prisms. Heck, I broke a brand new prism once, the wind took it over before I made the first shot with it.

Paul in PA

Just curious...

Why are there 30mm (etc) offsets in the first place?
Why not all 0mm?

Is it a "compactness" issue?

For most topo (ex. a tree) we just used a road reflector glued to a yoyo tape.

The reason for the offset is to reduce the effects of the prism not being pointed directly at the instrument.

The speed of light in the glass is lower than in air, so the effective reflection distance (from time of travel) is beyond the apex of the glass. See the sketch I posted above. If the prism is rotated about its physical apex, the distance readings are nearly constant. If it is rotated about the 0-offset, reading will vary.