Something that I have had to address lately. The importance of understanding what a prism constant is. When and how to use this when mixing different manufacturers total stations or robots and even different prisms. We are pretty much a Trimble shop. So running the S 5 with the MT1000 and the Trimble Traverse kits of -35mm. We have crews in different states that do not have these and have an assortment of topcon, seco and other brands of prisms. The standard is for them to set all prisms at the 0mm offset for traversing. In most casesI have no issues but my crews don’t have that option. It causes issues when we go to a different state and things like true height vs S knotch and the -35mm pops up for someone else to QA/QC. I remember running the old stack prisms triples singles and wild prisms seco etc. We had sokkia total stations wild and topcon. A hodgepodge that we all had to understand where to place the prism constant for each piece of equipment. Sometimes that meant in the total station itself sometimes in the data collector. Sometimes we had to do math like -34.4mm for a leica was set to 0.
How does everyone else operate these days. Whats your preferred set up for traverse kits. I can remember taking a BS to a plumb bob string then stacking a natural (range pole, stake with a tack. Shiner in a tree etc.) then I would haul rump to FS while i held a pole and single prism while I man turned sets rounds. Yes BS was angles only except first distance back. Times have changed equipment has and now I can just choose the pre programmed target prism type and all the thinking is done for me. But when something changes or someone does something a little different the issue arises.
Thought this would be a place to share some wisdom for the younger folks to have for both in the field and in office . Standards are great but often can lead overtime to issues because it’s always been that way
I don't have to tell you that having a dog's breakfast of equipment and accessories is a recipe for confusion, inefficiency, and training nightmares. The answer is constant training and attentiveness on the part of both field and office. Perhaps things can be sorted so that you know which options each office or crew has available to them.
Recording shots on known control at each setup would help identify various problems, including prism constant busts.
In Trimble I have the prism constants quick selections all used up because I have a normal and then Leica constant for the 0mm round prism, a peanut prism and the grz4 prism, plus the mt1000 and DR mode. This is the one thing I dislike about Leica is the goofy prism constant compared to all the other manufacturers.
I now save the Leica basically just for if I really need to shoot a bunch of reflector less shots. The Trimble just can't cut it for that.
We use Sokkia's 62mm AP01 nodal prism with -40mm offset, we've got a heap of them including an AP12 rod mount close to the ground and an AP11 at the top of the rod, so we can hold the rod on an angle if we need to. We've got tribrach mounts (set up at 196mm trunnion axis height) some magnet mounts and mini tripod stands. All use the same prism offset. We don't use 360 prisms, just haven't had the need with the Trimble SX12. The tribrach mounts are for locations we need to occupy with the TS.
Prisms generally have the constant physically printed on them, so I'm not sure why this comes up as often as it does. I think only the Leica offsets are not technically correct, but that's a simple fix by just subtracting 34.4mm from the printed value.
I'd work up a cheat sheet that crews can carry with them or stick in the front of each fieldbook, but I would also teach crews to create multiple default targets in Access labelled "Topcon Round" or "Seco Mini" with the appropriate offsets. Since one can have up to nine custom targets in Access, they should be able to keep the "reference" offsets and use them to create new ones.
I teach our crews to write down what prism they are using and the constant that they entered (if applicable) in the field book.
I'm not a fan of switching all prisms to a zero offset, because that almost always means that they are no longer nodal and there's another likely source of error that is really, really hard to track down in post-processing. Especially if there is any variation in grade across the traverse.
We use two types of prisms. Trimble Monitoring at -40 and seco mini at -29. Almost always the monitoring prisms are on tripods (HI>1.0 m), and the seco's are on stakeouts (HI of 0.100 m, 0.290 m, or 0.445 m) . Until I bought the monitoring prisms we always used the mini's for everything. We know the difference between the bottom notch on the S6 and SX10 to these prisms, so once we measure the HI (slope to the bottom notch) we know the HI for the prism as well. We also use Trimble rotating plummets, which is the same as the S6 lower notch. We also put GNSS on top of either one of these, usually the monitoring type.
The S6 and SX10 are both 0.196 meters above the tribrach plate, the monitoring prism is 0.114 m above the plate, and GNSS ARP (or R10 adapter) is 0.100 m above that. As long as one can do math in their head, it is pretty easy to measure one HI at the initial setup and compute the others. HI measurements are one thing that is always written in the fieldbook to be checked later in the office.
The mini prisms with modern total stations have plenty of range; no need for bigger diameter prisms for most work. Just make the small investment to get a set of the same high quality mini prisms for all crews. Use them for all traverse and important shots. Use 360 only for topo and mapping, so it doesn’t matter if you fail to change the prism constant for topo.
The Leica GMP 101 mini prism has a nodal offset, matches the total station height when snapped onto a proper tribrach adaptor, matches the rod height when snapped onto a Leica rod, and is precise and solidly made. What more could you want? Well.. the bubble is no good, so for setting out bolts I have to rely on a shorty 0.66 ft rod with a bubble rather than the included short point.
Yes yes and yes to all. The problem i ran into was not crew chief issues. It was office personnel that have always qa/qced 0 offset prisms from I assume topcon tds days. Well when my crews worked on one of there job’s running the Trimble traverse kit -35 mm measurements to s notch. Things went sideways. Of course my crew chief called me and gave me the heads up. I intervened but they now want us to all use 0 offset. I said this is all we have. Mt1000 and Trimble traverse kits. Its not that hard. Lol. For me or my crews. Its change. Something is different. Change causes issues. I don’t get it. I guess i look at things differently. If my crew’s so so different out of the norm. I usually say hey tell me why unless I figure it out on my own. Even if it causes a blunder. It tells me they are thinking and not afraid to try something new. To me this is positive most of the time. Not what others seem to think is what I am learning as i bite my tongue.
Nodal prisms. Man heads would probably explode if I went into that. Just like when staking on a projection and having 0 or -9999999999 as a default elevation on an occupy vs leaving no elevation and using the correct project height.