Some crews get real nervous when you approach that location. I think it brings back some bad memories.
:good:
Maybe it's a good thing if you check your equipment over and over and never find it to be off....that's not a reason to quit checking it.
What if you have a dispute (lets say a building layout and not even a boundary) and the contractor's lawyer tries to make a case that perhaps your fancy electronic equipment is off. What are you going to say? I have never heard of this kind of equipment being off...or would it be a better argument to show data where you checked your equipment before and after the project in question?
Another scenario....what if you never ever check your equipment against a reliable source, and one day you find out it is way out of adjustment....do you think you might wonder how long it has been out?
I don't know how often people have found their edm's to be off, but another thing it will catch is tribrachs out of adjustment, and prism offset problems. I have no doubt that has happened.
I go once a year to a baseline. I had been going to one at an airport in NW PA, but now there is a new baseline in Ohio, about the same distance away, that has PILLARS instead of mons in the ground (see my post of 9/24/2013). Very nice.
Anyway, I use the results of the calibration when I am doing high accuracy surveys. We shoot all of our prisms (we have several different kinds, although we almost always use seco mini's). The gun gets a determination of the scale and offset (it is a 1 mm + 1 ppm gun), and each prism has an offset associated with it. Usually it is within 1 mm or so of the stated offset, but I do have a pair of larger prisms that were sold to me as -34 mm offset and are actually -30 mm. I can then put this info into my processing and adjustments when I am doing deformation surveys.
In August, 1984, I, along with Pablo and some other volunteers from the South Central Chapter of PLSW, helped on the CBL established about 12 miles Southwest of Casper, Wyoming. The CBL was in the Right of Way of Wyoming Highway 220. The Wyoming Highway Department drilled to bedrock the holes, 1½’ in diameter I think, for the monuments and placed the reinforced concrete and caps. I think there were four in a more or less straight line along the right of way, one at about 10 meters and the others at intermediate distances and the last one about 4800’ if I remember correctly. Also, one was placed at an approximate right angle at the initial point, at 100’ for the checking of steel tapes. NGS sent two people to do the critical procedure and they assigned certain tasks to the volunteers to perform. The first point at 10 meters ± was taped as well as measured with their EDM. It was quite the procedure and learning experience of how this was done and all the steps taken to do the setup and measuring.
Norman there is a baseline on Ft. Sill at Lawton,,,
I have used it several times, each time I had to clear brush around the points. It is located on the west side of I44 north of town on the base property. 🙂
JB has hit on why it has become semi-obsolete.
Tom Adams has it why to keep doing it, if you cannot check your gear and procedure against other means.
In a way, if you are using least squares and comparing EDM distances to GPS distances, you are checking against a baseline on every job. Good enough for practical purposes, and in the absence of a maintained baseline, probably the next best defensible thing to prove your equipment and procedures in court.
Any systematic error, like a bad prism offset, or a bad tribrach, will show up, as Tom says above.
It's on my RSE list to try Dennis Drumm's Primacode Calibrate on some data from our local baseline. I learned more about calibrated baselines from that manual than from anything yet.
I've never seen the modern EDM out of spec when compared in least squares with GPS baselines in a network of redundant observations. Still, good to check. Even better to document.
Norman there is a baseline on Ft. Sill at Lawton,,,
That is good to know. The Sooner Surveyor reported that the Tulsa line was the only one left in Oklahoma.
From Tulsa I think I could be in Little Rock before I could get to Lawton.
Calibration against a baseline is part of our company QA/QC program and in the department handbook. A record is kept for each instrument each time. We have the luxury of being situated a mile or two from the Fort Worth line and the crew was there within the last month. I have also visited the Midland line and the Lawton, OK line. All three seemed to be in decent condition except that the Lawton line is on Fort Sill and I don't remember how hard it was to get on base but I think DeralP. had the connection for that trip.
Norman there is a baseline on Ft. Sill at Lawton,,,
The baseline in Fayetteville would be a lot closer.
Interesting that there is a CBL using pillars. Unfortunately I have not seen much interest in alternate baseline designs at NGS. The requirement for a >1 km site is a problem in some areas. The Australians have some alternate designs that are quite interesting.
While some have replied to this post by stating that they always get good checks, I wonder whether they made a sufficient set if measurements to get a statistically valid result. I attended a surveyor society meeting at a CBL where attendees were encouraged to use the site. Most were happy to measure one or two segments. If the result agreed within their comfort level, all was considered okay.
If there is no need for the CBL program why not terminate it and use the money elsewhere?
One last comment. As everyone knows there is more to a measurement than the EDM and prism. If your setup is not correct you will not get an accurate measurement.
I have to do distance measurements from time-to-time and usually wind up renting an EDM. Each time I do that, I make it a point to first go to our local Calibration Baseline in Port Allen, LA. Sooner or later when I'm on the witness stand in Court, some attorney is going to ask me if the instrument's calibration is traceable to NIST. I have to make sure I can answer such a question in the affirmative.
CYA.
The lack of calibration baselines effects Europe as well.
Cliff and others rightly make the point that there are occasions when you MUST be able to state that the measurements are within a certain tolerance and can be traced to a National Standard. High precision work will nearly always need a calibrated instrument (even if the work is done by a firm that doesn't use one!).
For several decades we have kept a calibrated instrument, which is then used to check all the other EDM and GPS units. Proper pillared baselines are in short supply. For a long time there was one in London owned by Thames Water, which we used. This consisted of 8 pillars with built in met. instruments, set on deep piled foundations. It ran parallel to an aqueduct which had a moderating effect on the micro-climate along the line.
Thames Water calibrated the baseline every three years with a Mekometer and undertook commercial work until 2003. Although the line still exists it is no longer available. We now send the calibration instrument (with a dedicated prism and tribrach set) to the baseline at Neubrandenburg University, Germany. Again, this is a pillared baseline with Mekometer calibration.
Fairly locally to our office we have a set of 7 reference points along the back of a dam wall. (In Google Maps put 54.857565,-1.959858 in the search box to see the site.) These were spaced so that the measured distances cover a full spread of metres (0-9) as the residual of the tens and hundreds. This way the line also provides an indication of whether there is a significant cyclic error. Note that cyclic error is largely a characteristic of instruments that measure by phase difference, so it shouldn't appear with time of flight measurement methods.
WE set a tripod on each point (together with a longer range "scale check" point) and then measure these with the calibrated instrument. This set of readings then provides the reference for measurements by all our other instruments. While we never find gross errors, these tests will often highlight an instrument which is starting to "slip" accuracy, so it can go for an early service.
One interesting matter to come out of the tests is that the GPS regularly give a variation of 2 to 3 mm on a certain part of the line - despite what others have said you can't conclude that if several instruments give similar results then they must be in calibration. It might just mean that they are all out of adjustment. (It's not uncommon to find, when investigating a site problem, that the contractors instrument is well out of adjustment. Supplier certificates of conformity aren't calibrations - they are just what they say, the instrument has measured to +/- whatever the spec. is.)
With a calibrated instrument the corrections to be applied are known. With a checked instrument all that is known is that it will be within a stated amount.
The sheet below shows the results for an old Geodimeter 440 which we use for rough work. Distances are in metres: the main baseline length is 594 metres, with the scale check length at 900 metres. After that we ran out of dam wall. The scale check point is actually at the start of the woodland south-west of the dam, if you have looked at the Google location.
The first graph shows the raw corrections of the instrument being tested compared with the calibrated instrument. The second graph shows the residuals after we have determined the corrections (graphically) from the first graph. We do that by trial and error: in this case having both scale and offset corrections at zero gave the neatest solution.
The distribution of residuals shows a very nice cyclic error curve of amplitude 4mm, centred as it should be, on the 5 metre mark.
First off, I'm new to this site, this tread was recently forwarded to me and our NGS state advisor. Great discussion, I have a few points to add.
1) while true that modern total stations typically measure very well, it is also true in some areas that GPS is the primary tool, often leaving ts instruments unused and likely transported for long periods.
2) in many state standards, indeed in other specifications, the need for calibration arises often
3) It is an expensive endeavor to create new baselines (or even to offically reobserve existing baselines). I have been through the training (2003) and have assisted with the creation of two baselines, in both cases - additional personnel were available from local surveying associations.
4) I wrote a recent article on the topic, I would like to invite any of you to read it, and/or republish in your own state newsletters (it would be cool if you sent me a courtesy copy!)
The link below is to the "Benchmarks" magazine of the New Mexico Professional Surveyors. The article on EDM calibration is in the September 2013 edition.
http://www.nmps.org/Benchmarks/NMPSBenchmarks.htm
Regards always,
Kurt B. Wurm, Ph.D., PLS
New Mexico State University
Kurt:
Thanks for the link to your article, I will enjoy reading it.
I did see that you wish to develope a program to help with the calabration process, you might check this program out, written by a friend of mine.
http://www.primacode.com/calibrate.htm
T.W.
Good article. I enjoyed reading it.
Have you reviewed the approach to the problem of EDM calibration in other countries? I am impressed with the Australian approach. See http://www.landgate.wa.gov.au/docvault.nsf/web/PS_SURVEY/$FILE/EDMCalibration.pdf
Cheers,
DMM
