Question about these posts regarding such high accuracy test

Jimmy,

I have the same curiosity. I wonder if they also consider error propagation of their entire measurement system.

Same here.
I don't chase millimeters either.

About half of the projects I do are deformation surveys. Every millimeter counts when you are trying to get 3 mm overall accuracy on large structures. One of the 39 dams we monitor has a wall 1 mile long, sitting out in the river...equipment matters, geometry matters, procedures matter...

Every individual monolith has a pin on it that we survey, and each dam pier as well. 200+ points total on the structure. Land wall, middle wall, river wall, and dam.

I too do not sweat the small stuff. I recently had a 17 mile long (east to west) transmission line project. The project was done primarily with OPUS control and RTK between control. It was very interesting to check my control with my optical equipment and push it to the edge of its envelope. I didn't really have to however. And I left the .6' or so horizontal error right where it was at. What a sloppy surveyor I am. :pinch:

BUT...

It's good to know where the limits and precision are of any instrument. Some folks find it interesting enough to exercise it.

I look at it this way:

I have a garage full of motorcycles. All, save one, will do well over 100 mph. Do I ever approach those speeds? Rarely, if at all (it's been a year or so). But I like to keep each and every machine properly tuned...with the knowledge of the envelope's location for that machine.

Some folks are just made that way.

> I respectfully ask what type of work are you doing that requires such accuracy, and chasing millimeters?

Well, part of all professional land surveying work involves estimating the uncertainties of things positioned by survey measurements. For a surveyor who is using a total station to do that positioning, one critical piece of information to be able to make those uncertainty estimates is the uncertainty of angles measured with the intrument, including the effects of errors such as the centering of the target on the marks from which the instrument is oriented and the marks positioned, as well as the fundamental uncertainty of the direction displayed by the instrument.

There are different ways to estimate the fundamental uncertainties of a direction. The standard method is to characterize the uncertainty as a random error from a known scatter (distribution) that is described by its standard deviation or standard error.

Once can fairly easily determine that standard error by testing. I usually do it outdoors over distances of 200 ft. or more. What Conrad has shown is that the testing can be done indoors over very short distances, which is very convenient. Most of the complications of the discussion dealing with centering errors are because:

- the Leica instrument he has doesn't allow the circle to be rotated independently of the alidada, and

- the alternative method of rotating the circle is to physically rotate the tribrach, which changes the centering of the instrument, which shift has to be very carefully measured to correct the directions taken to targets at very chose ranges, to reduce them to the same center.

One interesting thing that Conrad's test shows for his Leica instrument is that there appear to be systematic errors in the readings it gives that are probably corrected by software in the instrument in some of the higher precision models in the line.

For me, it is not a matter of chasing millimeters. It is the idea that I intimately know my equipment and know what I am capable of achieving with that equipment in perfect calibration. If something was to change, I would have that benchmark to later compare results.

Awhile back there was a discussion here when the question was raised "How close do you set your points?" I was floored that even such a question has to be asked and even more floored when certain surveyors stated that a tenth or two was close enough to set points. In other words, they didn't keep taking shots to get to 0.00', nor did they care. Yet, the plat shows distances that the monuments were set to the nearest hundredth of a foot.

I have to ask, why are you even in this profession if you are not doing the absolute best that you can and taking the time to do it right? This certainly involves keeping your equipment adjusted and transporting it so that it stays in adjustment. It is no wonder that we have pincushions when surveyors are just slapping in points for no other reason than laziness or to increase the profit margin by cutting corners.

All depends on the mission. I don't have the luxury of time on most of my projects given the volume and nature of the work I do. If I spent that time focusing on the millimeter errors and missed the meters bust, my new name is Joe Mud. I don't build particle accelerators. I leave that to the John Hamilton's who are better qualified than myself. In the words of one of my mentors, 'remember the mission'. When I've got a half dozen hydroaxes revving their engines waiting for me to stake their clearing route, I'm not about to lose sleep over a few millimeters.

I should add that the other half of my projects are typically photo control and Lidar control. Some are for 7.5 cm (3") imagery (need 1 cm control), other are for 1 m imagery (±30 cm control is good for that). If I don't need high accuracy, I try to survey to the appropriate accuracy. The key is to know what accuracy is needed, and how to achieve it. If I surveyed every project to cm or mm, my clients would look for someone else who can do a project more efficiently. I have done control for entire states where it needed to be ±1 m, that would be way more difficult to do at say 10 cm.

Some of this is driven by the type of work, some by the type of Surveyor. In most cases it's a bit of both...
I've 'refined' my personal definition of 'good enough' a lot over the years. In general I run on the tighter side. Doing things the same way long enough makes it automatic. I scratch my head when I watch people spend time inventing ways to do lower quality work rather than just getting it done. I still recognize our profession has a lot of room for variations in technique.
As for equipment testing, it sounds like you have a decent routine for what you do. You may consider trying a few of the tests you see here. If nothing else you can confirm that your way is just fine. I suspect you (by that I mean us) would learn something from each exercise...

In a past life I have recovered a lot of those points on the Ohio and Kanawha rivers. We used them as basis for control, plenty tight enough for the civil work we were doing. The company I worked for wanted to get into doing the dam monitoring, they should be glad they didn't. They didn't know what they didn't know...I believe I read in some of your past posts that you achieve this mostly with the distance component of the measurement in Geolab (dual observations near 90)? What is the maximum movement you have seen on lock and dams?

Hello Jimmy,

As a staff member of a semi-rural practice I do any kind of work that is put on my desk, which is usually cadastral survey of rural or urban parcels. I also do/have done monitoring of landslip sites, dams and roads, and engineering layout including, as an example, setout and measuring of components in a power station for turbine placement. It has been pretty varied but that's appealing to me.

When I've posted in a thread or started one I've tried to explain what I've done and why I'm doing it; my intent should be clear. I've set out to quantify for myself some of the 'errors' in the equipment and procedures I have, and that should help me understand what I can and can't achieve when promising and delivering work. I do sort of enjoy the testing too, and because of that I'd still probably test a 0.5" instrument and see what it does, though probably not as intensely.

Cheers.

Well, we try to shoot every point at least twice, and ideally from points that meet at 90°. Of course reality is different than ideal. A few points may get shot only once due to obstructions or location. Sometimes there is nowhere to shoot at right angles, and other problems. But, usually all of the points have error ellipses below 3 mm, sometimes a little higher due to geometry.

We use a high accuracy S6, and each station therefore has at least two additional obs (distance and angle). Plus we run precise levels to each point.

On navigation locks, the way these were setup is that the initial installation (in the 70's) tried to put the punchmarks in the pins on a reference line formed by disks on gate monoliths (those are down to bedrock). Then the initial readings were how much off line they were. They used a T-2 and a glass target with a micrometer to measure the offsets before 2005. There are a number of the walls where the lines are now a couple of centimeters out.

At one of the ones we did this year the lockmaster told us a particular monolith had been hit by a barge. We found NO horizontal movement, but 8 mm of vertical movement (up) on that particular monolith.

Here is an interesting one...it is a flood control dam, and it has data plotted from 2000 to 2014. Note that before 2005, they did it with a T-2 and micrometer, in this case using pedestals that define the line. When we do it now, we set on the same pedestals and shoot each point from both ends. On these structures, it is usually not possible to shoot them from off the line due to walls, cranes, etc.

In 2008 they had me go there during a record high pool (heavy summer rains). You can see that in general our readings agree well with the old way of doing things, and the 2008 readings show a couple of extra mm of movement downstream due to the high water level against the dam. M1 and M2 are pedestals assumed to be stable. That gets proven every year against other pedestals located off the alignment line.

A few examples.

Monitoring
Site Control Networks
Engineering setouts

One upcoming project will be the asbuilt survey of the brackets holding a catenary lighting network (400m long). From the asbuilt model, the net will be designed. If you think about the engineering involved, you can see that the accurate location of the brackets is essential.

Like John Hamilton I do a lot of monitoring type work, but ust a couple more examples of when you NEED to know just what the limits are on the instrument.

Alignments of plant. For lots of installed plant precision is essential. Something like a large centrifuge needs to be set to hundredths of a millimetre. The guys who install these can get that accuracy with large engineers plate levels, thin wire and a micrometer, but it can take them several weeks. I can go in and get everything in place to within half a millimetre or so in a day, leaving them just a couple of days work on final adjustment - cost saving of over a week.

Swimming pools. Competition pools have a tolerance of plus 10mm, minus zero. Not very difficult you would think, bu the minus zero is absolute - shorter than that and the tiling comes off, render hacked back and all redone (possibly draining the pool as well if it has already been filled). If I'm going to tell the contractor to do that then I have to be absolutely sure I'm right before I commit him to spending $20-50,000 on remedial works. For this type of work we carry out a site calibration procedure under the pools atmospheric conditions, so that we are certain that the calibration constant is within 0.2mm. of truth.

I find it fascinating that you have detected that much motion. What do you find your largest sources of possible error to be on this type of work?

I have never done work that was this high-tolerance with sight legs that were this long - this makes the high precision model EDM a requirement (and the digital level).

It is important to have the best possible centering, and to have accurate Temperature and pressure to correct the distances, although that could be taken care of somewhat by using the method of ratios proposed by Richardson of the USACE many years ago. Basically he was saying that if you have good stable fixed points that you could use the ratio of a measured distance between those points to get the index of refraction (i.e. slowing of the light wave), and you could apply that to other distances as a ratio. However, I do not use that method but rather use weather station data to compute the corrections.

For centering I was using a wild ZNL at first, but now we use other rotating plummets because the typical HI is only 1.5 meters. None of our tribrachs have plummets, so we have several rotating plummets as well as the rotating one in the S6. We try to keep them in good adjustment, but if they get a little bit out it is easily verified and corrected in the field.