After reading more than one thread on backsight prism arrangements that don't require a tripod, and are easy to use and level, I decided to make my own. I wanted to keep expense and additional stuff to lug around to a minimum.
My version is nothing more than a low platform, made of HDPE, for a standard tribrach, that I'm already carrying around.
1. The platform is set up roughly over the mark:
2. Tribrach is placed on top; "Sliding Center Point"㢠is dropped into place:
3. Sliding Center Point is used to center tribrach precisely:
4. Level the Tribrach in the standard manner, and replace the Sliding Center Point assembly (along with the round tribrach block if desired), and replace with your favorite prism. You can even use a Leica or Topcon Prism holder with optical plummet--anything that fits into a standard tribrach:
I've contemplated a version that leaves the center point in place (so that you can visually verify that it's still on station), but it doesn't seem worth it to do so. Any "gravity mount" arrangement is going to get out of whack if someone kicks it.
The total cost of parts: $0. Piece of PVC or HDPE, three bolts, fiberglass rod, and a plastic/brass bushing. I've actually tried placing the tribrach right on the pavement...no platform at all, by lifting the tribrach, placing the point, then lowering the tribrach...No platform even needed for hard surfaces. In the woods, the platform can be pressed into the soil, depending on how hard it is (just like you would the feet on a tripod).
Essential to proper fabrication though, is a lathe, to assure concentricity.
Humbly present an idea...the sliding white pointing rod stays permanently within the clear acrylic tube, with the tube extended up a bit, and with the mirror mounted to the center of the acrylic tube. The pointing rod will remain in contact with the actual point at all times.
Is that a North arrow on your patent application?
I like the photos.......epic dilemma
What came first, the placement of the nail or the cracks?
"Tools that you build yourself are the best tools"
:good:
dmyhill, post: 364188, member: 1137 wrote: Humbly present an idea...the sliding white pointing rod stays permanently within the clear acrylic tube, with the tube extended up a bit, and with the mirror mounted to the center of the acrylic tube. The pointing rod will remain in contact with the actual point at all times.
Very cool. Thank you! I thought of doing so but didn't want to dedicate a specific prism for the application. Your inclusion of the vertical scale though, has convinced me it's a superior design. I designed this unit to be as close to H.R.=1.0'. If I add some adjustability to the prism height relative to the sliding point it can be adjusted to exactly 1.0. I'd put a detent on it for use on hard surfaces, like concrete.
I like this concept.
I'd also suggest having points on the legs rather than bolt heads.
Then on hard surfaces (concrete) you could drill tiny indent in surface at each leg and it sits more firmly, particularly when windy and on smooth concrete.
On say a timber post you could also push the tips into the timber.
I always drill small indents for my tripod legs when setting up on concrete footpaths. Small enough they don't bother anyone and yet enough to give grip.
Warren Smith, post: 364192, member: 9900 wrote: Is that a North arrow on your patent application?
Little comic relief...I figured the first thing someone would comment was..."Where's the North Arrow?"
rfc, post: 364245, member: 8882 wrote: Very cool. Thank you! I thought of doing so but didn't want to dedicate a specific prism for the application. Your inclusion of the vertical scale though, has convinced me it's a superior design. I designed this unit to be as close to H.R.=1.0'. If I add some adjustability to the prism height relative to the sliding point it can be adjusted to exactly 1.0. I'd put a detent on it for use on hard surfaces, like concrete.
I dont know how much milling you can do, and it would decrease the flexibility (limit brand of tribrachs), but if you milled out an indentation, even a very slight lip for the tribrach, it would stay in place until kicked by a passerby. And that is as good as a tripod.
Edit...stupid me, why get complicated? Just a few self tapping screws would keep the tribrach from sliding...
My question is whether that defeats an important design element (allowing the tribrach to slide on the plate). I am not sure if sliding the whole assembly around is as effective.
Richard, post: 364246, member: 833 wrote: I always drill small indents for my tripod legs when setting up on concrete footpaths. Small enough they don't bother anyone and yet enough to give grip.
I sometimes do the same thing, but my "drill" is usually a hammer. I do try to make the spots discrete, though. All you really need is a little tooth to the concrete -- just enough to take the glaze off -- so the tripod points have something to grab.
dmyhill, post: 364251, member: 1137 wrote: I dont know how much milling you can do, and it would decrease the flexibility (limit brand of tribrachs), but if you milled out an indentation, even a very slight lip for the tribrach, it would stay in place until kicked by a passerby. And that is as good as a tripod.
Edit...stupid me, why get complicated? Just a few self tapping screws would keep the tribrach from sliding...
My question is whether that defeats an important design element (allowing the tribrach to slide on the plate). I am not sure if sliding the whole assembly around is as effective.
I purposely made the platform of something slippery and chose to not have the tribrach locked in position so that the tribrach could center on the point. I use this in the woods, out in the open and on the gravel driveway where I'd prefer to stick the platform legs into the ground (I've made a set with points). The clearance of the sliding rod and tube is only a few thousandths, so it's pretty easy to "scootch" the tribrach around until the point is dead nuts on the point. I use mag nails or spikes with dimples put in them with a center punch or lathe.
dmyhill, post: 364188, member: 1137 wrote: Humbly present an idea...the sliding white pointing rod stays permanently within the clear acrylic tube, with the tube extended up a bit, and with the mirror mounted to the center of the acrylic tube. The pointing rod will remain in contact with the actual point at all times.
Here's dmyhill's Mark II integrated pointing rod backsight system.
I can make the pointing rod stay within the tube (one less thing to get lost);
But unfortunately, without figuring out a way to bore and polish the ID of the acrylic tube, It's somewhat hard to calibrate and mark it perfectly.
Also, if the markings are on the rear of the acrylic behind the slide tube, parallax might be a problem.
I think I might be back to just using a rule to measure HR, but keeping the pointer within the tube seems workable.:-)
I would be tempted to take a skill saw, and cut a window lengthwise (only to the depth of the rod, of course, and only for about half the length), and graduate the rod, making the reference mark on the window.
Or, just drill an observation window...bottom of hole is the reading.
I'd be tempted to wrap that arcrylic tube with reflective tape. Not hard to come up with the "offset". This would eliminate the possible movement of the whole thing when you're putting the large reflector on. Even non-reflectorless guns have EDMs capable of measuring to good reflective tape.
