We have a 25-mile transmission line corridor survey mapped circa 1930 which is referenced to True North. The corridor runs generally north-northeast with some jogs to avoid mountains, towns, etc. The alignment shows a starting azimuth with deflection angles and stationing in feet. At various intervals, the alignment is broken in the middle of a sheet with a 40-100 feet gap station equality (i.e. Equation Sta 210+21 = Sta 210+81) along a tangent line.
As I have limited experience surveying along long corridors like this, I was thinking that these segments may mean the 1930 surveyor was taking astronomic observations at certain intervals to stay on true, or was he indicating adjusted distances?
Anyone with more experience with old corridor surveys and True North have some ideas?
Thanks and regards,
Mike
The equations were to fix mistakes and errors in distances only, so that everything did not have to be remarked in the field after a screwup was discovered. That was common on older route surveys.
My guess is he did in fact use astrononomic north as his basis of bearing, either by taking a sun shot... But most likely a star shot.
I wouldn't assume he was trying to keep the alignment of the corridor to true north.
Just like when we take a magnetic back bearing on the first backsight when starting a traverse, he just found astronomic north, set his circle to zero, and then turned to his first backsight.
The older transmission route notes that I've seen do seem to cling to their astronomic bearings as if it were a design consideration, and indeed it most likely was. In days before Google Earth and affordable aerial mapping how did we get from A to B?
From what I've seen it appears as though the latitude and longitude of each end of the route was determined with either astro or solar. Given those two values a geodetic bearing could be calculated and then it was up to the surveyor to stay on that line.
The oldest notes and plans I've seen are from the late thirties. It was an approximately 30 mile route. Their were two lines shown on the plans; "P-line" and "C-line". The "P-line" was definitely a line of sight "Preliminary" Line with noted P.O.T.s, etc. Probably their original traverse line.
The "C-line" probably meant "Construction" line, as it was the centerline of the as-built line. The two lines rarely diverged much, except for natural features like water.
Interestingly enough their was only one major (about 4 degrees) P.I. in the line. That occurred on a hill about a mile and a half shy of the line's terminus. I'm betting they stayed on their calc'd bearing until they could actually see where they were going to wind up...and then put a little bend in it. Makes sense.
