anyone know the offset on this style of prism? nothing marked on them
from my research so far it looks like 25.3mm
If it's a Leica GMP104, it'll be one thing; if it's a Chinese knockoff, it might be something else entirely.
> anyone know the offset on this style of prism? nothing marked on them
>
> from my research so far it looks like 25.3mm
25.3 it's not
It's 8.92
Time for AC-(AB+BC)?:-)
> Time for AC-(AB+BC)?:-)
No, because there's no way of setting it on a tripod.
We use these regularly for monitoring operations and I have never seen an offset listed for any brand we use. As surveyors when we tie a specific point we shoot the prism and then apply several offsets, such as prism offset and target height, to derive the true position. In monitoring the prism offset is kind of a red herring, you are measuring to the prism and do not have any other offsets to worry about. When monitoring to a fixed prism you are looking for a change in the vector from the instrument to the prism. This is why a real time monitoring system can shot to targets through single plane glass. Just be consistent with the offsets and it really does not matter the actual number.
> > Time for AC-(AB+BC)?:-)
>
> No, because there's no way of setting it on a tripod.
There's not? I see two bolt holes in the angle bracket, one of which goes right into the prism. It's got to be positioned for real use somehow. However that is, it can be mounted on a tripod in a tribrach. That's all that matters. If the OP has more than one of them, it could be done in 15 minutes. If not, it'd take a bit longer.
John is correct about the Leica GMP104 (Art. No, 641762) in that is is meant to be used as a monitoring prism. There is no published offset for this prism and you would have to perform the prism offset test on each of them. The prism is meant to be used as a permanent mount on monitoring projects where initial measurements are compared to measurements taken at the required monitoring time interval. I would just be sure to use the same offset, obviously, each time the prism is measured.
> John is correct about the Leica GMP104 (Art. No, 641762) in that is is meant to be used as a monitoring prism. There is no published offset for this prism and you would have to perform the prism offset test on each of them. The prism is meant to be used as a permanent mount on monitoring projects where initial measurements are compared to measurements taken at the required monitoring time interval. I would just be sure to use the same offset, obviously, each time the prism is measured.
The way I read it Leica has a published offset of 8.92, why they publish it? I don't know. What kind of prism test are you going to perform this?
Ralph,
I have honestly never seen a published offset for this prism. Can you tell me where you have seen this in a Leica manual or datasheet? I have run into this in the past and could never find the published value. I would like to get a copy of this for my support files if you can find it.
From their website....
> GMP104, Monitoring Mini Prism
> Mini monitoring prism mounted in metal
> holder. Supplied with L-bar for fixed
> installations. The prism offset is
> dependant on the mounting position,
> range 2000 m (7000 ft).
> Order No: 641762
Seems logical to me; offset in relation to what?
L BAR PRISM OFFSET? For Mark
L BAR PRISM OFFSET? For Mark
Here's a more accurate description.
BTW Mark, I had this issue back about 12 years ago when they first came out and Geocomp was using them at the South Ferry. I tried to find some data and couldn't, about 5 years ago I ran into this document and could never find it again. Last year I reached out to you regarding the GPR112 monitoring prism and you sent me this document 🙂
Funny how things usually come around full circle. 🙂
L BAR PRISM OFFSET? For Mark
Man my memory must be going to hell... I did look at my hard dive because I thought I did have this info but I did not see it. Maybe I have moved this older data sheet some where else. Yep.. looked again and I have a folder called Old Prism Data Sheets and it was in there (Prism Overview.pdf). I honestly forgot about this. Sorry for the confusion.
L BAR PRISM OFFSET? For Mark
8.92mm is the Leica prism constant, the absolute constant is different. Only use 8.92mm with Leica instruments. Apply correction for other brands.