I have a new Topcon ATP1 360 degree prism. It says the prism constant is 7mm. Is that a -7mm or +7mm?
It is a negative 7 mm, meaning that 7 mm must be deducted from the measured distance to account for the travel of the signal inside the prism itself.
What field software are you using?
To be at a ZERO offset, the prism has to be shifted toward the inst.
Because the light goes faster through the glass, than through the air.
So, the offset is the amount added. Not all prisms are the same. I like to check them individually.
N
> What field software are you using?
SurvCE 3.03
Get you a box tape, set the gun up, and shoot the prism 10' away. Play with your offsets. There is a prism offset, in the gun, and in the data collector, and there is an inst offset. All of these must be accounted for.
If it ain't right, it ain't right!
N
The Topcon AT-P1 is the same as the Sokkia 360 that Carlson shows in the instrument settings.
If you select this prism type, the program should correctly apply the prism offset for you. If you use this feature, it will change the prism offset on the instrument rather than doing the math in the data collector.
This should allow you to define and use a -30 mm prism for a backsight prism.
Bow Tie,
Words of Bill93:
"For determining the offset of the prism, AC-(AB+BC) is the way to go."
Rather than guess, wouldn't it be good to measure it yourself?
Dave
IMHO..
No consent is ever constant; and on very rare occasions the combination of equipment and prism can yield a positive (+) constant (combined correction factor).
" Because the light goes faster through the glass, than through the air."
Things must be really different in Arkansas.
In most of the world the light beam slows down going through the glass.
Lisdexic moment. Yes, slower through glass.
N
One of my Prism constants is +34mm on an old Geodimeter Prism.
Easiest way is ti physically check. Take a shot to a known prism, take a show to your new one on the same mark. You should be able to note wether it's +/- 7mm
While measuring the offset is probably a very good idea because it is always a good idea to know how your tools work, the offset on the AT-P1 is definitely -7 mm.
Says so right on it and Carlson will default to the correct offset if you select that style prism in your setup.
Is 0mm or -30mm a better prism offset?
Before, all of my glass was 0mm, but now that I have to mix glass, Is it better to run my backsite prism at 0mm or -30mm (I have the switchable type)?
Is 0mm or -30mm a better prism offset?
Mr. Bow Tie
Might be worth your time to visit Seco Website
and look up nodal prisms. Lots of good info there.
Leica's site also has excellent info with white papers
to read.
Leica user?
The old Geodimeter prisms I used a decade ago were zero offset which is +34.4mm in Leica math.
Is 0mm or -30mm a better prism offset?
If you are shooting square on to the prism, it doesn't matter which position you set if for 0, -30, or whatever, so long as the instrument or data collector (not both) knows what to subtract.
If you plan on taking any shots that aren't square to the prism, then you will get more accurate results with the -30 or whatever is appropriate for the prism. This position puts it where the effective distance does not change with incidence angle.
Is 0mm or -30mm a better prism offset?
Here's a random pic I took. Notice the 2 bright spots in the prism
from the flash on camera. I guess I'll never know what the gun "sees"
but makes me a little nervous as to which bright spot the leica is
looking at. So I mostly use a nodal 17.5mm for traverses and critical
shots and leave the 360 for topo.
You Arizona guys are kinda smart. Except it's always hot where you are Dave...lol
What is the old carpenters motto.. measure twice, cut once. Then there was that one guy missing a thumb, "I measured it and had to cut twice... dang thing was still too short".
My motto is that anything ending with "...ism" is a lie. Too easy to check out and maybe even find that missing 0.04' that's floating around in some beer leggers beer glass...;-)
Is 0mm or -30mm a better prism offset?
Well, no wonder that prism was confused -- see that white stuff on the ground? That's an indication that it's way too cold to be working outside.
I think the Leica software looks for the strongest return in the neighborhood and turns to it. But I don't use my 360 for much, either -- not even hard-surface topo. I much prefer the Leica mini, both for accuracy and size/weight.
Yeah, Leica Math.
