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Where to sight?... and Why?> NATE
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Years ago, a gentelman walked into my shop in Oregon and promply made a statement about prisms "you and your industry are doing prisms all wrong" I was taken back by his statement and asked him why he thinks -30mm offset was wrong. It turns out he was an optical engineer and said he had a design for a nodal prism with a -40mm offset. At the time I told him the whole industry centered around 0 and -30mm prisms and it would be difficult at best to convince the surveyors otherwise. We got together and I hooked him up with SECO and that is where their "ECLIPSE" prism came from. On page 33 of SECO's 2010 catalog they cover prism basics and Nodal / pivot points on various prism offsets.
Test your prisms visually.
Take a 0 off set prism put it on a rotating adaptor and center on the lines, rotate the prism it will shift up to .10' off center in both directions, do the same with a -30mm and there will be a slight shift perhaps .03' either way. If you had a -40mm prism there is no shift when rotating the prism.
Best method I have found is to site the pole or center of the tribrach prism setup for horziontal and vertical at the convergence of the prism lines (unless of course it is tilted) then use the target.
PSS man
> I wonder more about the 360° prisms for robots. The robot will find one of the prisms but unless you're really careful, it is not going to be pointed at the gun and therefore the center of the prism will not be over the center of the rod.
Yep...I rotate the prism(s) and the gun will track back and forth...
That is with a very old zero offset prism with 6 mirrors all in an assembly, centered around the attachment for the pole.
-David
Dan, that setup brings back.....
some memorable times packing a tripod, tribrac, yoke and 3 of those triple prisms thru Arkansas.
They were designed to keep adding on units and basically build a wall of reflective glass.
Pacific Survey Supply Man ?
I would like to correspond with that optical expert. If you could forward some contact information, I would appreciate it.
Paul in PA
Light travels slower through the glass.
It is really simple to adjust a prism pole bubble, why not do that to begin with?
The pole adjuster costs approximately $200 so I am making one myself. I will post it for comments once it has been done. Back to the topic, it's about the prism on the tripod that was not placed looking straight at the gun, both horizontally and vertically because sighting at the center of the prism did not give the same reading as sighting at the marker on the back of the prism itself.
you don't need a $200 pole adjuster.
If you have a prism pole tripod it is really easy. If you only have a prism pole bipod it is a little harder but still possible.
You can use the insider corner of a desk (or a tripod) to hold it more or less vertical. Center the bubble, rotate the pole 180° then adjust out half the error. It takes a couple of times to get it but with practice you can do this in under 10 minutes.
The other way is to use your total station vertical cross hair.
Dave
I think the issue is that you can plumb the rod so you can look at the point at the ground and look up the rod and it's plumb, but then you look at the intersection of the lines in the prism and it's off a little. Is that just bad equipment or what? I go with the angle to the point of the rod if possible but back to my previous concern, what about robotic where the gun is picking the prism and is not smart enough to look down at the base of the rod?
Dave
He said in the original post he had a problem with an out-of-plumb rod.
either the bubble is off or some object (such as a fence) is preventing plumb-ness.
Another technique is to lean the rod over and shoot the prism. Then without disturbing the rod, run the prism all the way up and shoot it again (keeping track of the two target heights. Then later in the office produce the 3D line between the two shots down to ground level. Sokkia used to make a rod with two prisms and a routine in the DC that did this automatically.
Mr. Harris
Up and down and around SW Arkansas...Wild T2 and a DI-10.
DDSM
I gotta ask...
can you see far enough anywhere in Arkansas to need a triple?
seco makes a pole adjuster that works great for around $30. Much easier than making your own.
Here is the url: http://www.surveying.com/products/details.asp?prodID=5195-01
I just use the prism pole tripod I already have.
I have a magnail in my driveway which makes it easy.
center the bubble.
lock the nut on the third leg.
loosen the tripod clamp.
turn the pole 180°.
adjust out half the error.
lather rinse repeat until you get the bubble to stay however much you spin the rod.
The other way is to set up the total station 15' away and line up the pole with the vertical cross hair then adjust the bubble to center perpendicular to the line of sight. Then turn the pole 90° and do it again. This should be a more accurate way to do it.
Another hint on these poles is the bubbles tend to come from the factory too loose so they go out of adjustment a lot. I turn down the screws a few turns then adjust and the bubbles seems to stay in adjustment a lot better.