I was kicking up points on a fairly recent survey the other day with GR-3s. It involved several (5) corners that were mag-nails on the section line, in the middle of an asphalt road. The points were anywhere from 200 to 700 feet apart and the 1/4c and sec. cor. were both existing monuments.
After I had located all the points I was sitting in the truck with the DC and comparing the points I had shot with a copy of the previous survey. All the points fell very close to their recorded location. I think the 'worst' one was 0.03' long. Not bad. But the bearings in between each on seemed to vary too much; approaching a minute.
Just for the sake of curiousity I had my help set up a TS on the 1/4 corner and take line on the section corner. I was truly aghast with how far off 'line' these nails were. One was nearly 0.2' off line.
The survey was done by a local firm that usually does good work. There really wasn't anything "wrong" or inaccurate with the survey, the points on the section line were just sloppy. I'm almost positive that the nails were set with a base/ rover set-up, I've seen their crews a lot and that's apparently how they do most of their work. I was just a little disappointed that what was being reported on paper as a 'straight' line fell short by so much. It made me think about how much we rely on our GPS. And while our speed and precision is probably state-of-the-art, the results can be less than adequate.
We had a crew meeting the next day and my new "rule" is to use optical equipment on lines that involve multiple points, at least on major or adjoining properties. One of the crew chiefs laughed and remarked that they need to "keep me indoors" as much as possible.
I can see his point, but I think we need to be a little more careful and precise when it comes to what we set and how we leave it.
Help me understand your statements on your own work: "...All the points fell very close to their recorded location. I think the 'worst' one was 0.03' long....
...I was truly aghast with how far off 'line' these nails were. One was nearly 0.2' off line...."
Sorry for not being more descriptive...
the distances between each point seemed good (0.03' or so)...but they were as much as .2' off of line.
I've been on construction work at times where the "heat waves" would let me sight 800 feet with that good of accuracy. Sometimes you can't wait for the cool breeze and just need to move forward. YUP there is the 1-2 cm slop in GPS. At 100 feet seems like a lot. At 20,000 feet it's on a nats butt!
If they produced line with the Transit they could be very accurate.
If they pulled off of their traverse points (such as in rough, brushy terrain) then things got a little more sloppy. Of course, when a small tree is 3' diameter and a tiny tree is 2' diameter and a microscopic tree is anything less than 1' diameter then a tenth or two is exactly on line.
is the line latitudinal or longitudinal? where was the 0.2' off nail? somewhere's in the middle on a longitudinal line?
:-S
> I was kicking up points on a fairly recent survey the other day with GR-3s. It involved several (5) corners that were mag-nails on the section line, in the middle of an asphalt road. The points were anywhere from 200 to 700 feet apart and the 1/4c and sec. cor. were both existing monuments.
Isn't the secret that the "monuments" were PK nails in ASPHALT PAVEMENT? How stable do you expect a short nail in HMAC on a travelled roadway to be, particularly an Oklahoma pavement that probably wasn't all that well built to begin with? Were they all on the top of the crown in the pavement or were some to one side or the other and is the apparent shift off line away from the crown, which is how pavements tend to shift over time.
Looking at my calendar; nope not April fools day :-/
I think I understand what he is saying, the accuracy of the points layout do not fall within the straight line of the road alignment.
You have to understand "how" these points were layout in the 1st place to understand the "why" results.
If the nails are located on the centerline of the road - 101% that the total station or even the transit was not located on the centerline of the road to begin with. My HSE personnel on my crew would report this to the project manager if he saw me standing with a $20000 topcon robot on the centerline of a road even if there are no traffic.
I would again say 101% that the total station is located on a traverse station on the shoulder of the road & these points were layout using the setting function of the total station. That being the case do not expect every point to be straight as if this was placed using the plunging method (0/180) of the scope.
Also when I layout points I make it a point to have the prism located on the bottom of the rod & not on the top of the rod. If your level bubble is not precise (or even if it is) expect to have a deviation of around 1 cm if the prism is located on top.
Finally, take into consideration the accuracy required for the kind of survey done. If for cross-section plans then 1-2 cm won't even matter to the design engineer. You could measure every point to less than 5mm accuracy but it would take you a couple of years to finish the road alignment survey right?
Paden strikes a nerve
What does it say about all of your work when you can't get three or four points to fall on a straight line? Sloppy, sloppy, sloppy.
He is pointing out that many do substandard work without realizing it. Having one point off by 0.2 feet suggests you may have others off by much greater distances and not know it. Say that error occurred in a fairly short sideshot using a total station. You have another sideshot at 10 times that distance. What might that error be, and in what direction? Waving a receiver around like a plate spinning atop a stick is a different situation, but, may produce significant errors as well.
Paden strikes a nerve
Within some of the areas I work anything within 0.2' would be considered "manna from heaven".
A lot of speculation here. You/we don't know how they were set, and/or whether they moved. It seems like pretty random if a GPS machine got them so close for distance, but so far off for line. wouldn't its accuracy be more random where there is just as much slop for line as for distance? Same with setting up a total station on the side and running the computed points with angles and distances.
Maybe Kent's on to something with asphalt shifts, if the asphalt would tend to shift sideways (left and right) on a roadway. Perhaps the passing cars seldom run on the line and the path is to one side or the other "pushing" the asphalt over perpendicular to line.
If you are set up on a line and have a lot of slop on seeing line through the instrument, but good edm distances, that might be a possible solution for this type of error as well.
It's probably good to have some kind of check on your control, whether it's a closed traverse, or locating from different points, or if you set them on a line, setting on the side and shooting them.
Years ago I located a number of pk nails around a block that the company I was working for at the time had previously set. I was scratching my head a while about a potential bust when I finally found the pk nail that fit our old notes. A bad thing about lone pk nails is that they aren't specifically identifiable. Maybe a nail through a washer with a point number on it would help. I like to set points that I can find again, and can identify as mine.
I like the asphalt shift theory, but I would not suspect a car causing the change; I would point to the Big Trucks.
I had a few points to check into that were out. PKs in sidewalks all near trees with roots and cracks.
Those are subsequent monuments, where are the original that the first firm based their work on? Find them, hold them (if they are undisturbed) and walk in the footsteps of the original surveyor.
Why not use your DC to tell you how far off line the points are? Project point to line command?
It's a radial world in surveying. Very few occupy the line. Fewer understand the math when trying to set a point on a line from control that may be far removed from the two points that create the line... Coming from the old school, I do it radial now too! With the robot or rtk but first I check into the two points that create the line before blindly setting a point that is supposed to be on the line and will make adjustments in the field to get it on the line. I don't think this is being taught in a lot of places. We are teaching the Field technician to set and run control in the field, fly tying monuments, office guru does adjustment and figures out what he wants to hold. Technician goes back to field and set "points" based on "Control" without understanding that he is not only setting point "10" but setting point "10" online between point "100" and "200". That all works great if everything is tight and with small errors, but many times control gets lost and reestablished with the error growing until you get the point "10" the right distance from "100" and "200" (because of our 1 second instrument) but it is also the exact distance form control point "X" which pulls it .2' off the line??? I'm just glad they are typically small numbers. The mentors are gone so now all we have are technicians that are taught by office gurus that have little practical field experience. My 2 cents, Jp
Points in asphalt are prone to wander with heavy traffic. I have personally supervised setting points in asphalt to 0.01' and came back a year later to find them off by as much as 0.07' both in line and distance. Sometimes points are only good the moment you walk away from the job site and what happens after that is beyond your control.
Humor would be the correct catagory.
Mr. Penry and McMillimeter hit the p.k nail on the head...:-) I have returned to the point of the crime in 1 and 5 year increments on p.k's set in ASPHALT. They MOVE ....rebars set off road as TBM's for measuring asphalt thicknesses and pay quantities MOVE..
Pablo
I agree, they've got it. Also, I had to set up on asphalt on a hot day once, very undesirable. Can hardly keep it level long enough to get the shot.
BTW, what kind of large critter have you guys bagged there?