I'm shopping for a new 360°prism. I've been quoted for both a GRZ122 and a GRZ4. These are both Leica products. The only apparent difference is that the '122 has a 5/8" threaded stud on top. The price difference is over $700. For those who have used both, is this indeed the only real difference?
When the GRZ122 first came out it was high precision in the vertical on all sides where the GRZ4 was only in 3 of the 6. That was what the arrows were for on the top of the GRZ4. You always wanted to have a side with the arrow pointed at the instrument if vertical was a concern. I'm not sure if they've improved that with the GRZ4 over the last 4 or 5 years but I tested it and would consistently see a 0.02' difference between the arrow and non-arrow faces. Your mileage may vary.
As Cameron pointed out, the main difference is pointing accuracy. The GRZ4 has an all-around accuracy of 5.0 mm and 2.0 mm at the yellow arrows while the GZR122 is consistently 2.0 mm. As I recall, the GZR122 is constructed more robustly. Apperntly the GRZ4s delaminated on occasions although I have never seen it happen. That may have just been a bit of marketing 15 years ago. The threaded point does have some uses. First, you can place a GNSS receiver on it allowing you to choose GNSS or TS observations on the fly. While I rarely do use the feature, it is handy sometimes. More importantly in my view is integrating it with the AP20. You can put the point on top and reverse the rod for improved IMU accuracies.
Why do you want a 360° one? I find for topo in the wide open where the tracking may be useful then it's GNSS anyway. I'm only using total station around trees/building where no matter what prism you have you end up driving the instrument to next shot. Boundary even more so.
My 360° just sits in the truck while the GMP101 gets out every time. Cheaper, lighter and more precise, plus I find it's not that hard to keep it pointed to instrument on few occasions when I'm in an open enough space to be able to use tracking.
The price difference is over $700.
I don't know if your agency is cost-sensitive, but I have a genuine Leica GRZ4 that I'd let go for less than that difference. I got it in like-new condition when I bought a TCRA1102plus from a colleague in 2011. I used it for a few hours on a topo and hated the weight so much that I haven't used it since. Like lukenz, I use a GMP101 for everything but the rare half-mile shot, in which case I break out a GPR1.
I really don't do anything in the "wide open". My work is all urban/suburban. A 360 is a necessity. I use the GMP101s for control and like them but wouldn't think of attempting to do topo without a 360.
The comments rendered have convinced me that the '122 is worth the extra money. Thanks all.
A new GRZ4 sells for nearly $1500. You should be able to get well over $700 for it, and pretty quickly, too.
I can get a half mile easy with the GMP101 and an MS60.
Maybe the MS60 has more power than my GeoMax, but on a 100°F day the GMP101 gets lost in the shimmer at half a mile, while the GPR1 will still return enough signal for the GeoMax to get a return. However, these days I almost always default to GNSS for those shots.
@jim-frame Yeah, I would rarely plan on a shot that long. I make it a point to tie between intervisible control stations when ever possible and sometimes it works out that I can see that far. I've done it for really long azimuth pairs. Like I said it is rare, probably once or twice year. What I'm really shocked about is how well the ATR works at that range.