It's my opinion that a mini prism should only be used for close-up work and not for 200' to 300' topo shots. An argument made at work for the later favors the mini prism due to its smaller cross section, which means less pointing error. Distance aside, a mini prism stinks when vegetation is dangling everywhere. We use a Topcon ES-105 reflectorless total station. I don't know if it uses a laser or an infrared beam. Does the type of beam affect precision?
I also think that a tilting mini prism should be tilted for greater precision. Why not use the darn thing if it's right there? I get tired of people's attitudes. I was asked if I trust trig leveling and I said only if I trust the equipment. Some of these young field survey techs question everything so they can do as little work as possible. It would blow their minds if I showed them how much effort we took to set a property corner thirty years ago. Hope no one minds me venting a little bit here.
LOL I vent all the time. If you were setting corners 30 years ago we have a totally different concept than what the young folks have. I think about this. You can probably visualize a right triangle from say staking offsets along a line using opposite or adjacent and angles left and right as well as setting property corners without the need for a coordinate. You could simply take a couple plat bearings figure the angle between them set on one corner sight another and simply turn the plat angle and hog the distance. We were exposed to things that made us understand the math from a true application such as house topping around a tree as we so we stayed on the original azimuth to keep going. Most crew chiefs that I run into hardly ever look through a gun/scope on robot total station etc. I ask many if they zero on the backsights or set the azimuth. Deer in the headlights. As far as mini prisms. Go depends on the manufacturer and what the specs are for how far you can keep an accurate or precise distance they usually have those in paperwork also depends on the edm itself. I remember never shooting a mini under 150 feet with certain total stations because the manufacturer showed the wavelengths not as good at those distances. As far as sighting goes the smaller the object the better. I remember sighting plumb bob strings. But that sighting has little to do with the distance. Again depends. Also tilting is great especially in vertical component. Now that can me different if one has a nodal prism. Technology has really increased a lot. On one hand I would say if your folks are asking questions that’s not a bad thing use it as a way to teach. But if it’s simply to get out of work that’s another. Hardly any of them have had to go back a redo a job because they accidentally released the lower motion and didn’t catch it. Maybe not even had to compute and write down every angle and distance for every shot they took in a day. Perform all the comps to reduce slope distance to horizontal etc. I mean the data collectors today are so powerful that I believe if we could go back in time we could do more with them than any computer could do back then. Sit those young folks down and teach them how it use to be so they have an appreciation for how good they have it. But beware once you start teaching you will quickly find out who is there for just a pay check and who cares about the quality of the work being done. It’s a catch 22 unfortunately. I was trying to teach a fellow some survey math and apply that to what he learned in middle school and also college. It took me a few minutes to set up the formulas from memory. I can solve all of it but it is so muscle memory and I can see the triangles and directions in my head that showing him how to draw it out I had a few blunders lol. But it came back. As far as reflector-less measurements go the manufacturer shops how the precision’s on that along side the edm specs. Also have some warnings about beam width and color of object you are shooting to.
It's my opinion that a mini prism should only be used for close-up work and not for 200' to 300' topo shots.
Do as you like, but I use a GMP101 for pretty much everything. I rarely carry topo shots more than 500' -- there's too much elevation error in long shots -- but the mini works well for me up to that distance.
The only time I'll mount a full-size round prism is when I'm shooting half a mile or more, and those are rare these days due to GNSS availability.
If you were setting corners 30 years ago we have a totally different concept than what the young folks have.
What always comes to mind is the way my party chief, who started surveying in the early 1960s, set corners. This was somewhere between 1991 and 1993. Say the corner you had to set was 120' from the gun (Wild T1A with Topcon EDM on top). We would chain about 3' further and set a hub & tack. We would double an angle to the tack. If it was good we would hold the line. We would then use a box tape to pull the exact distance from the tack and set an iron. Finally, we would use a prism pole as a final check. I thought that was a lot of work to set a corner, but that's the way we did it.
Moving ahead to about 1999, at another company, we had to set what was called an 'A' point every time we set a property corner. After the corner was set, you set a hub & tack within 3', but not on the same line, of the corner and measured a distance to the corner. It was a check distance to the corner.
Do as you like, but I use a GMP101 for pretty much everything.
Thanks for chiming in. Okay, I stand corrected.
@field-dog I am still bit of an over kill. I still refuse to traverse and then do a radial tie one angle to a round propycorner. I always turn a set of angles to it and usually another from a different location. If that’s not possible I will change my bs to the pin of my foresight and turn to it or other known in that sequence. I have done what you have sorta in certain situations. Set a temp nail or hub and tack at a given offset direction not that big of a deal but say 3’ x 3’ or 5’ x 5’ with box tape and measure those as well as a check. I still do that sometimes with GNSS set up a few offsets to the corner and locate them as well as corner and tape all in as a check. Could not get gps on a corner I will yank some offsets 10’ x 10’ 14.14 for the diagonal to make it 90 and locate those so I can comp where the corner is. Those basic old skills still get me out of a pinch or help me not have to walk back in more as I get older I hate toting all that stuff. What’s scary is RTK is so good that almost no need for a total station or robot. I oversaw a project for a larger firm last year and did all the static planning RTK and traverse along with levels. The digital level from 1st and 2nd order BM’s. I had the crews leave all turning points in one it helped me get vertical in all the quadrant that was a time savor. As they rtk a bunch of stuff I had them hit some of the turning points along with observation from robot as they traversed a certain area as well. I was pretty amazed at how tight everything hit the elevation both rtk and the traverse. With correct procedures and having the instrument in calibration as well as a couple rtk 3 minute observations from two different base locations I didn’t have .03 ft vertical anywhere on the whole site. Well the Topo shots probably were but all the control and all was pretty tight. That was doing independent checks before running the network adjustment. The wild T1A was the first instrument I learned to run. We didn’t have a top mount edm for it. We would turn all the angles for a traverse and the. Come back and tape with steel tape. For Topo we used stadia except on curb and gutter. And building corners. Other horizontal locations that needed to be tighter. We always came off our traverse with some points to set up a baseline to work the distance. When the bought us a leitz total station man that was like wow. Along after that a sdr24. I still recorded everything in field book as well as that sdr24. Now days getting a name and a sketch or a north arrow on any field notes is rare. lol. I have seen so many base station hi bust I could scream. Another reason I make them collect raw data and move the base and re locate control and property corners twice. It gives me a check.
Do as you like, but I use a GMP101 for pretty much everything. I rarely carry topo shots more than 500' -- there's too much elevation error in long shots -- but the mini works well for me up to that distance.
I use the GMP101s for higher precision ties. I don't have anything bigger in the truck. I suppose that there is a distance limit to them but I haven't found it yet. Like Jim I, also, limit topo shots to something less that 500 feet. This because of horizontal and zenith angle precision, not any issues with distance or distance precision.
IMO, centering errors trump any issues with prism glass precisions, and most things with instrumental angular errors as well. I use the 360°prism for property ties knowing that, while the 360° is not as precise as the GMP101, the effect is trivial compared to centering errors.
The OP mentions using Topcon. If the highest precision is your number 1 goal you probably shouldn't be using that brand. And no matter what brand you use it will always be the blunders, not the hairsplitting of precision that is of highest concern.
I studied prism reflector pointing errors and concluded that for a mini-prism the worst distance error with any attempt at pointing was on the order of a couple millimeters.
If you try to download the file, ignore all the prompts to log in and look for FILE / DOWNLOAD and in the pop-up find the Download link without using any of the other boxes. Dropbox has gotten obnoxious with their updates.
A mini for topo is fine. We stay under 300 feet for robotic most of the time anyway. Monitoring or grid control it's matched nodal only. Different jobs, different tools...
Like Jim & Mark, the GMP101 is my standard prism for control and boundary. I just finished a project where I shot almost 875 meters to a GMP101 with ATR with on my MS60. The specs put the ATR limits in the 1500 meter range with a MS60. The specs for the GMP101 state a pointing accuracy of 1 mm and a range of 2000 meters. As far as pointing/tilting, 10 degrees is Leica's magic number.
For topo I use the GRZ122 360 degree prims and keep my shots down to the 150 - 180 meter range for verticals sake.
The difference in results using a mini prism vs a standard size prism while doing topo will never be detected by the end user of the data collected. The choice between the 2 styles of prisms for topo is moot for all practical intents and purposes.
For topo I use the GRZ122 360 degree prims
The reason I use the GMP101 all the time is because I usually set control as I go, even when most of my shots are for topo. I have a GRZ4 that I never use, both because of the pointing/distance accuracy and because it weighs a ton (or seems to after a few hours).