This morning’s short discussion at the office was do we need to reverse the total station scope when doubling angles. A senior surveyor said that was relevant with old longer-barreled theodolites but not with short-barreled total stations. I didn’t voice my opinion, which contradicts that. What do you new wave surveyors have to say about reversing the scope?
Reversing the scope has nothing to do with the length of scope, but everything to do with the encoder or vernier. Reversing the scope helps eliminate systematic error. Besides, it would be kind of hard to double angles with the scope still in face one.
The proof is in the pudding. For several years I was obliged to use a Topcon PS-103. Actually, two different instruments of the same make and model. A modern robotic instrument. Immediately after collimation the splits on a set of angles would be just a few seconds. A few hours later they would be several seconds. And by the next day they would be 20 seconds or more, and stay there. So if I'd accepted a single shot I'd be off the mean by ±10 seconds. Ask your senior surveyor to explain that.
Doubling was a lot more important when the least count of the instrument was measured in minutes, that is true. With modern robotic instruments a double can be turned, hands off, in next to no time. So why not?
It's a different world.
If GPS is used for Control and TS for fill-in which is most likely, then flopping the gun, closing angles becomes unnecessary. Closure is generally less than the famous .04' and adjustments are a push of a button.
I do know a few shops that are still TS only and they reverse the gun, close out angles, as we did prior to our modern equipment. If you're still working that way then you should reverse the instrument, close out angles, make sure the numbers are redundant. 50 to 100 mile traverses like we used to run are way different than traverses that rarely approach 1000 feet.
I will always have my robot turn a set for all backsights and traverse points. Elimination of equipment error.
I agree. The physical plates are still there. I think in Davis & Foote 7th edition it states “Reversing the scope eliminates errors in the trunnion axis.”
I’ve never used a robotic station. Isn’t it common practice that you collimate it daily considering you mentioned how rapidly the collimation deteriorates?
Those very long traverses you mentioned must have been fun. I’m sure you did sun shots along your route as a check.
I wish young surveyors were more aware of equipment errors. They focus too much on the precision and electronic compensators of an instrument and don’t give enough consideration to collimation and angle turning technique.
Thanks to everyone who responded to this post.
As Mr. OK pointed out, mileage may very on the need to put an instrument through it collimation routine. I have my Leica instruments set up to squawk at me once a month as a reminder. I rarely see a variance of more than a second or two in the collimation results. I also run it through the procedure any time it travels by air.
I’ve never used a robotic station. Isn’t it common practice that you collimate it daily considering you mentioned how rapidly the collimation deteriorates?
Ask 100 surveyors about collimation. 75 will not know what you are talking about. Of the 25 remaining, all will agree that it should be done at least weekly. But when asked when they last did theirs 20 will not even be able to remember, nor really be clear on how to do it. Maybe 2 will have done it in the past month. If you have a Topcon you should be collimating daily. Almost nobody does. For the record, in my experience, Trimbles and Leicas hold their collimation almost forever.
Collimation isn't just a robotic thing. It's a total station thing.
Isn’t it common practice that you collimate it daily considering you mentioned how rapidly the collimation deteriorates?
Depends on the gun. The Leica 1102plus that I used for several years, and the very similar GeoMax Zoom80 that I use now, hold adjustment extremely well. The GeoMax reminds me to adjust every 6 months so I generally do, but the adjustment values stay within a narrow band of a second or two. That doesn't mean that I don't get splits in my angles, so I always shoot D and R on control points.
D&R is a win win. One even if your splits are small near 0 it gives you a check always as you survey so when you see them jump it’s like that pesky seat belt chime that drives you nuts time to collimate and adjust the total station or robot. I can remember rotating the bottom plates on long route surveys to get the readings from the different quadrants as well as D&R. I know I am not the only one that was busy turning and burning and grabbed the wrong knob and released the bottom plates then had to tell the old crusty party chief uhmmm we got to shoot all that Topo again sorry.
Always direct and reverse observations. Precision equipment isn't perfect equipment, you should do everything you can to try and cancel out systematic errors.
Always direct and reverse observations. Precision equipment isn't perfect equipment, you should do everything you can to try and cancel out systematic errors.
I'll make a deal with my field staff. If they can prove, by clear and convincing evidence, that the instrument has been baselined within the last 30 days, been collimated and all the rods and tribrachs in use have been adjusted within the last week, and that everything has been set up perfectly during the measurement I'll accept single shots on critical marks. So far, no takers.
You're way to nice. I would never give my crews that option. Hell, these days I'm the only crew & I don't give myself the option. Five sets for control & three for monument ties. It is just too quick with a robotic instrument.
The trick is that the only practical way to prove all these things is to provide doubled angles and distances.....