Awhile back I posted about not really enjoying the Leica GRZ-4 360° prism. It's a nice piece of glass but it weighs a ton, and I'm interested in keeping the rod light. I bought a GMP-101 mini-prism that work great, but for general topo keeping the mini pointed at the gun is a challenge, and I was losing lock a fair amount. The GRZ-101 mini-360° looks nice, but it's a bit spendy at around $650.
Then I saw a GAD102 adapter on eBay for 25 bucks, and that gave me an idea. I took a Geodimeter/Trimble 360° prism I had sitting around, reworked the head screw from an old tripod, and voila:
The prism has a +36 mm Leica offset (that's +2 mm to every other gun manufacturer I've encountered), which is non-standard in the Leica world, but it's easily accommodated in my data collector. I sized the brass screw to make the prism height the same as a standard Leica prism.
I've only tested it out to about 500 feet, but the gun tracked it there without incident. It's *much* lighter than the GRZ-4 (about the same as the mini), and my arm appreciates the difference!
Yes the GRZ-101 is a little pricey, but we have tracked it to at least 1000' from the gun. We have had great success when using it.
Jim, don't forget to box tape check it. If the offset is right, well, it will show up at 10-15' from the gun.
I used to use glass, and I tried all our glass that way. They were all off a few mm's. So, I averaged them, and let her rip. It has been yrs since I shot a retro prism. I made my own, with reflector tape, that I use now.
I think you could do the same, with that system.
Get a ball, and cover it with strips of reflector tape. It would be MUCH lighter. A bit fragile, but, so what? put some round things to keep it off the ground. Bolt through the middle, and now you have done it!
N
Looks like a good set up.
I just wish the bipod was lighter. Man I love it when I can shed it off the pole after shooting my traverse and corners. It's got to be 2/3 of the weight on the rig and my shoulders can't take that type of stress anymore and keep tracking.
BTW: Mine is the Topcon 360° FWIW.
Carl
Nate-
I bet you could rig up some tripods with lath, flagging and duct tape, would save a lot of money! Who doesn't want to save money?
I'm just prodding you for fun.
😉
-Dave
For a little bit more money you could pick up a non- Leica type threaded top rod and dispense with the Rube Goldberg setup. In fact, you may already have one you use with your RTK setup.
The Trimble prism has an offset of 2mm.
> For a little bit more money you could pick up a non- Leica type threaded top rod and dispense with the Rube Goldberg setup.
I want to be able to swap the 360° with a mini or a GRZ-4 when needed, which is why I went out of my way to retain the Leica quick-release. It's a feature, not a bug!
Rube Goldberg was known for elaborate and fragile contraptions that served inane purposes. This prism arrangement is pretty straightforward, as robust as the Leica mini, and has a very practical benefit for me (weight savings). I reject the Rube Goldberg characterization!
> The Trimble prism has an offset of 2mm.
That depends on where you measure from. Most of the world measures from the vertical centerline of the mount, and the Trimble 360° does indeed have a 2 mm offset from that point. Leica, however, measures from the effective reflective point of their standard GPR-1 prism, which the rest of the world would call -34 mm. Thus in Leica-speak, the Trimble 360° has an offset of +36 mm. It's a Swiss thing.
Note to Nate: I didn't check it with a tape, but I did check it against a Leica mini. I hit it dead on for distance and within 1/2 mm for height. Good enough for me!
> I just wish the bipod was lighter.
I own a regular bipod, but I never did like it. I found it slow to plumb, heavy and unwieldy. I've always preferred a brace stick, since I can carry it in my other hand and use it to whack weeds and small branches, dip drain inlets and the like.
For years I used a 4-foot lath, then moved up to a 4-foot piece of an old Philly rod that had a tripod shoe installed on the zero end. It worked pretty well, but the rod graduations, being set below the surface of the rod, made it hard to accurately measure with, and it was too thick to get in between most DI grate bars.
Thanks to the recession -- 2010 was a slow year for me, so I had plenty of time to attend to equipment -- I now carry a custom-made aluminum version. It's basically a 4' length of 1/4"x1-1/2" aluminum bar tapered at one end, which is fitted with a plumb bob point. (A sharp point makes it much easier to get a bite on paved surfaces for bracing purposes.) The other end has a vinyl boot for a comfortable grip. It's graduated in feet, tenths and hundredths throughout, so for many topo purposes I never have to pull out a tape. Think of a big ruler.
I used the Leica 360° for a while with my TCRA1105, but didn't like that the offset was unique to all other prisms including Leica. I switched to a Topcon 360° that has a 0mm offset, which I can match the rest of my prisms to. The TCRA1105 has a preset onboard for "Topcon" prisms along with others, which makes switching easy. I lost a little tracking distance with the Topcon prism, but that doesn't happen unil around 750' and doesn't limit anything I do.
I agree with the prism offset of always being zero. I never have to worry if I changed my prism constants with the geodimeter zero (2mm/0.0065') prisms and cheap seco back site prisms set on the zero side. Just make sure your back site is directly facing your robot if you are letting it turn your angles with a zero prism, or you can get a lot of sighting error. Prism constants are not as much of a problem now with the Trimble S6 and assigned prisms in the data collector, you can mix and match prisms easy, but there is piece of mind when you know all your prisms are zero offset or very close to zero. Now getting your back site heights right is the next question. Jp