Angular accuracy importance (2" vs 3" vs 5") ?

Do you have 5" targets?
I always have a (kind-hearted) chuckle when guys tell me they bought a 1" second gun and use range poles for traversing. Even with first order targets, your side shots are more then likely with a rod.
Is there even such a thing as a 1" range pole?

> Looking at some used instruments, upgrading to a robotic. Looking specifically at the TCRP1203 or 1205
>
> When doing large traverse loops (talking several miles) should we stay away from 5" instruments?

I'd say that the key pieces of information are:

- How long are typical traverse legs?

- What method is used for centering targets?

- What does the standard error of a direction actual test as being?

- Is least squares survey adjustment software such as Star*Net to be used that will allow the combined adjustment of GPS vectors and conventional measurements?

- Do you use GPS to survey control points at intermediate points along the traverse?

Explain Your Techniques For Doubles At 15"

I use a 5" single axis compensated instrument and do not see that angular spread on most single D&R sets. If it is a larger project and my 2D&2R spread is large because not every backsight-foresight combination is ideal, I just add another set until I am comfortable. With much larger projects I have worked with a 1" gun with 2D&2R minimum.

I would have no problems regularly using a 2" or 3" gun 2D&2R with appropriate redundancies, such as sometimes adding a set or on possible problematic configuration such as short backsight/long foresight re-observing with BS/FS reversed. In any case that large project requires BS D&R distances and remote azimuth marks.

No matter what the instrument I carry traverse point elevations as a data check. In about 1 in 4 projects the elevation comes in as tight or tighter than the horizontal coordinates. The second reason is that when I choose static GPS as a part of my project quality I want to match XYZs all around.

I know some who discard and reshoot their large spreads, and antagonize themselves making a loose position better. If you leave those large spreads as they are and put a larger tolerance for those points, most times LS shows no big adjustment because that large spread encompasses valid observations that still mean out well. If there is a large adjust you are better off reshooting on a different day under different conditions.

Paul in PA

> > Looking at some used instruments, upgrading to a robotic. Looking specifically at the TCRP1203 or 1205
> >
> > When doing large traverse loops (talking several miles) should we stay away from 5" instruments?
>
> I'd say that the key pieces of information are:
>
> - How long are typical traverse legs?
>
> - What method is used for centering targets?
>
> - What does the standard error of a direction actual test as being?
>
> - Is least squares survey adjustment software such as Star*Net to be used that will allow the combined adjustment of GPS vectors and conventional measurements?
>
> - Do you use GPS to survey control points at intermediate points along the traverse?

If a person wanted to get all sciency and all, and had Star*Net, he could use its preplanning feature to compare what the results of the same traverse (same series of control points connected by measurements) would produce in the way of uncertainties at various points using total stations with different standard errors for horizontal angles.

Those +/- 5" ADDS up

Had a similar situation several years ago when we did a road alignment survey that stretched for ~80km.
We were using a Topcon GTS 227 or 237 unit then that had a 5" settings. Since the length of the survey work was quite long, we were downloading the data on a weekly basis & sending it back to the office for processing. At that time the GPS units of our office were being used in another project so what our CoP did was to occupy several government control points that were located along or near the road alignment. These control points sometimes are located 5-10 km apart if they were still existing.

Back at the office, the guy processing the survey data would start & close from those control points based on published coordinates. Upon processing a segment of the alignment, he called the CoP & asked if he could check on the calibration of the total station by doing several direct/reverse observations. The difference in the distances were in the mm level while the difference in the angular were in the 5-15 seconds. Not a big deal right?

Apparently the coordinates that were computed were off by 10-20 meters when compared with the published coordinates. This is for segments that were ~10 km long depending on availability of govt control points. We had no way to verify govt control positions but these were quite reliable and would not be off by 10-30 cm.

We had to redo a 5 km segment just to check if our present readings were in agreement with the original readings. Well deviations were in the 5-15-20 seconds per station. Nothing much except that when you add these up depending on which side of the mark the surveyor would judge as target center errors at the end of the segment became quite big.

Eventually we had to call in the GPS units & had readings done at every 1 km mark. These GPS positions were then used to close the traverses to make the entire 80 km line agree to tolerance.

Sometimes I would wonder how those early guys close a traverse using only tape & transits.:pinch:

Those +/- 5" ADDS up

An instrument with 5" settings should not be used beyond a few acres. My understanding is consideration of a 5" instrument than can report to 1". While my 5"(1") instrument was under repair I used a straight 5" units on two sides of a 20 acre woodland survey and noticed no difference in precision. On D&Rs the plus or minus spread was no greater than my 1" readings. 2D&2R is quite effective with a straight 5" gun.

A 5" reading is not a large survey control instrument.

Paul in PA

We use 3 sets of legs for traversing and I do my best to keep the tribrachs in check. All our critical sideshots are done either with a dist-a-line or a peanut prism.

I use a 20" gun. It's not ideal, perhaps; but, it's what I've got. I turn 3 sets of doubles on traverse points. My closure ratios have ranged from 1:20,000 to 1:100,000(typically between 20,000 and 30,000). Since I work by myself, I keep my sideshots short.

I initially thought this gun would slow me down. 3 sets of double is time consuming. However, most of the time spent on a survey is doing other things besides turning angles.

Find a gun you like. Don't worry so much about angle precision. If you use the right procedures; you should be fine.

You don't need anything tighter than a 5" gun for boundary surveys.

I purchased a 3" TS after considering two items-

First, I can recall an exercise where determining the TS standard deviation and applying it to real-world conventional traversing, made the 3" instrument the best compromise from a cost/benefit standpoint. Simply put, my need to generate a 1:100000 conventional traverse by observing beaucoup angles D&R is very rare nowadays, but not zero. It wasn't worth the extra cost for a 1" or 2" gun, and a 5" instrument was out of the question because of the dramatically increased number of angular observations required.

Second, I have always worked with either a 2" or 3" TS so yes, there may well be some personal bias on my behalf but a 3" gun is the minimum I am comfortable with (robotic or manual) for both general and reasonably high-precision work.

Just my two cents' worth.

I think I'd be putting my economic hat on here. There is a big difference in going from 5" to 1" or even 2" or 3". Including all the associated equipment, personal, training, weather conditions, and project requirements. Especially project requirements, as you ask yourself "close enough"?

I've got a 5603 robot (3" allegedly) that gives me tighter level loops than my old level. Just balance your shots, that I try and keep under 600' to 700' max, but have stretched them out at times, depending. Runs great traverses too, but I'd rather check into somebody elses monument and hit close than goof around getting closures that mean nothing in the big picture.

Ok, construction and certain control work can justify it. Certainly not boundary surveys.

Hey Gentlemen,

Nowadays we are using state of the art guns, EDM and double compensated, very accurate instruments. In a 500 m circle (roughly 1500') , an error of 1 second is equal to 1.21mm, 5 secs aprox. 6mm. Good enough for everybody. On top of that any time you set up your instrument you should check the tolerances are into the range, which means, in my opinion, 5" TS are suitable for a lot of works.

Ivan B.