Create Section Corner Position Using Only Accessories
We have a 2004 CCR for a section corner that has most likely been destroyed after the road was repaved. The section corner ( a 1/4 corner, I think) is a nail and disc. We're getting a variety of tones with our Schonstedt because the corner is located above a large stormwater pipe (CMP?) running under the road. There's probably a good amount of rebar surrounding it. Anyway, we have 3 perfectly intact accessories (nails and discs) set in two different headwalls. That's 2 on the west side headwall and one on the east side headwall. The west side accessories form what looks like a nice 60°-60°-60° triangle with the likely position of the section corner. The east side accessory appears to bisect the triangle.
The plan is to RTK each accessory and create a virtual section corner position by using the accessory tie distances. Would you use a distance-distance intersection calculation from the 2 west side accessories? We could use the third distance from the east side accessory as a check.
Using the accessories is not a problem, that is what they are there for. I would recommend using an instrument to tie them vs RTK given the short distances usually associated with accessories. Distance is your fried with GNSS.
As for methodology, I would tie all of them and then start off looking at them together holistically. If the notes give good bearings then you could throw them in along with distances. If the best fit of all accessories fits record then you have it. Only if I found one to be significantly off would I withhold it from the solution.
What John said. Of course this presumes you’re happy with the pedigree of the position you’re perpetuating. Be thankful you have 3. It won’t hurt to check a few adjacent lines for bearing and distance while you’re at it…. Also wouldn’t hurt to establish substantial reference mons if the 3 accessories you’re looking at aren’t robust and have a significant expected longevity. Do you need to put something in the road over large pipe?
I agree, this sounds like a job for a total station. Measure all the distances and angles you can between the reference points. Then either use least squares, or else plot a solution for the corner from each pair of ties and see how they agree. If the triangle formed by the three solution points is small, use its center of gravity. If one point is far off the other two, then maybe disregard it.
Might double check to see if the ties were shot in horizontally or were they taped in at a slope distance to the former level at which they measured. That should get you close enough to dig a small hole in the new asphalt to find the prior marker. They probably simply paved right over it. We have found tons of old corners by digging through the new asphalt.
BTW, that is a sure way to get the attention of the appropriate party. You aren't doing anything wrong. They did by obliterating a very important monument. They have a responsibility to protect section corner monuments, etc. Sometimes, they need to have that fact reinforced. Repeatedly. Especially when key personnel leave and someone new comes in.
Once, we dug through a four inch layer of fresh asphalt and then down about ten more inches to get to the existing monument. A county employee witnessed our activities and told us it was placed two days earlier. I then called his boss and chewed on his butt for awhile. We haven't had a significant issue with obliterated corners in pavement in that county again.
I like everything cow said.
Maybe not if they label the ties as slope, but still should be encouraged to do horizontal.
Also wouldn’t hurt to establish substantial reference mons if the 3 accessories you’re looking at aren’t robust and have a significant expected longevity. Do you need to put something in the road over large pipe?
Robust in terms of stability? All 3 were set in concrete and are protected by a guard rail. I assume a nail and disc were set in the road because of the large pipe running underneath the road.
Depends on the depth to the large pipe. Cotton spindles, railroad spikes and 12-inch iron bars can be used in most cases. Leave more references than normal because eventually (decades) that pipe will be replaced.
Slope ties are great. Very quick determination of where the monument SHOULD be found. There are no guarantees in life.
On a route survey I have two sets of ties. One with slope ties and the other with different references using horizontal ties farther from the corner, thus more likely to still be found after construction.
@holy-cow accessories are part of the monument, ranked only below the undisturbed monument of record. The ties should be of a quality to place the monument with certainty if it is disturbed. Slope chaining doesn't cut it.
I hear two arguments on that.
Most common, the GLO didn't survey it thst well, why should I. Because once patented your crappy survey effectively transfers property from ine owner to another.
Next up is, I only need ties to find the monument. My answer to this: read a book. Learn the purpose of accessories or find a new line of work (preferably in a colonial state).
We will never get the public to respect the critical infrastructure of the PLSS unless we respect it ourselves..
My .02, Tom
What is your typical source of solid, long-term, repeatable reference points? In rural areas it is difficult to find much of anything for which shots via GPS make any sense. Driving bars for references introduces more problems than benefits. Besides, a tape measure is more accurate than GPS.
@holy-cow I agree on rebar. We select or set different items depending on the circumstances. One of my favorites is a fence post that does not match the type or color of the existing fence. A T-post driven with the ribs reversed and at a different height stands out well. True reference monuments below the plow line with rare earth magnets work well too. There is nearly always something reasonable if you plan for it.