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Computing a coordinate from road as-builts…
I have to scan a 1/4 mile section of toll road, and was given as-built plans. Part of the scope is to recover existing control. The plans show copperweld pins set at some of the PC, PI, PT, and at some random POT’s. There is a table with coordinates (NAD83 (1986) I believe) of the PC’s, PI’s, PT’s, TS’s, SC’s, CS’s, and ST’s. So I need to compute coordinates of the reference pins to try and recover some. The sketch shows the survey & construction baseline (centerline of the highway) with angle and distance to pins. The one that is in the section I am working on is at a PI. But, it is referenced off of the PI station on the curve, not the PI on the tangents. So how does one backsight along a curve?
In a separate note, it says that the stationing on the BM is the same as the PI station. So it is on the line from the radius point through the PI. But I can’t figure out where they get an angle of 263?ø00’00”. By definition it is 270?ø from the curve. If I use the PC back (which is how it is drawn), I get an angle of 265?ø17’17”, if I use the PT ahead, I get an angle of 92?ø00’00” (i.e. 268?ø00’00” left angle). Curve radius is 11459.16 feet (0?ø30’00” degree of curve), no spiral.
What am I missing?
As-builts are stamped by an engineer, not a surveyor…maybe that is my answer.
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