I've mentioned that I've been working on surveying a tract straddling the county line between two Central Texas counties, those of Hays and Travis, and that part of the work is to locate the county line in relation to the tract boundaries.
Well, I got around to actually doing that bit today. Here is Concrete Monument 149 (my designation) marking the county line as run in 1954 by transit and tape methods and fixed by agreement between the adjoining counties.
County Line Monument (Pt. 149)
That monument took some work. It was out of plumb by a couple of inches and fell along the NW line of a county road that was fairly well overgrown by yaupons, gum elastics, and greenbriar. Some work with a chain saw opened up a view of the sky and some work with a shovel and rock bar returned the monument to plumb. I drilled a small, shallow hole in the top at the intersection of the arms of an old "X" for the station mark.
Shortly after tying this monument, I drove over to position one situated to the NW about 2.3 miles distant. I'd previously plumbed it, so it was ready to go.
County Line Monument (Pt. 150)
So how good a line did Marlton O. Metcalfe, the 1954 surveyor, run? In previous work, I'd gotten accurate coordinates of other monuments, including the monument No. A154 (my designation) that he set on the same line for the 18-Mile Post about 15 miles away from Pt. 149 shown above. No. A154 was also where he observed Polaris to get the average astronomic aziumuth of the county line per his resurvey.
My resurvey results are as follows:
149 to 150: 309°00'16.9" (Grid) 12,204.61 ft. (Grid)
149 to A154: 309°00'15.63" (Grid) 77,558.62 ft. (Grid)
I'm thinking that they actually knew how to run a line with a transit. Probably even cheated and ran it by double centering.
Kent,
You're example reminds me of a time when after retracing an older, retired surveyor's work in my home county I had the chance to visit with him and share the results of my work.
The work he'd done was more than forty years old and he used an old Paragon transit and steel tape for his work.
It was after confiding in him that I'd managed to hit most of his bearings within a minute on each line and found his corners to differ by no more than a tenth at the worst that he said to me, "you keep practicin', son, one day you'll get it right".
It appears that some of our predecessors really took pride in their work.
> It appears that some of our predecessors really took pride in their work.
That 1954 county line survey was actually a fairly interesting example of how transit/tape technology was used to run a very straight line for about 36 miles between two endpoints that were fixed by the original statute that created the county boundary. So how did they do that? They ran (and cut) a preliminary line, measured the miss and then reran the entire line. Then, when they had done that, they set substantial concrete monuments for the mile posts and the even more substantial ones shown in my photos at road crossings.
It was worth a chuckle that the actual "County Line" signs and road maintenance boundaries don't match the county line as marked by that substantial marker No. 149 I found. I'm thinking "GIS".
Was he trying to run by grid bearing? Seems unlikely. If not, it would be interesting to know how much the grid convergence changes over the length of the line.
> Was he trying to run by grid bearing? Seems unlikely. If not, it would be interesting to know how much the grid convergence changes over the length of the line.
No, the line was a straight transit line between fixed terminals about 36 miles apart. The astronomic bearing taken at Mile 18 was just for descriptive purposes. The bearing didn't determine the line.
I quoted the actual grid bearings of 149-A154 and 149-150 to show that the monument 77,558.62 ft. from 149 was within about 0°00'01.3" of being on an exact prolongation of the 12,204.61 ft. line 149-150. That is pretty darn straight.
Edit: In figuring the deviation angle, I neglected to calculate the second-term correction to the grid bearing of the longer line 149-A154. Over 77,558.62 ft., it is significant. Allowing for the second-term correction, the actual angular deviation of 149-A154 from being an exact prolongation of 149-150 is 0°00'02.0", which still is hardly shabby.
better shoot it again, something is wrong!!!!
tee hee hee
so is that a curved line or straight??
KENT'S MEASUREMNETS ARE CURVED, BUT HE PLACED THEM ON A GRID?
> so is that a curved line or straight??
Well, a transit line when projected onto the Lambert Grid of our SPCS will actually have a very slight curvature over distances as long as 15 miles, depending upon the azimuth of the line. The other finicky detail is that a transit line is run in the field using an instrument set up normal to the geoid, not the ellipsoid. So the minor undulations in the gravity field introduce very small differences between it and the theoretical curve of alignment on the ellipsoid (where at every point the direction of the line forward differs by exactly 180° from the direction of the line backward).
Well Kent,
I'll bet that Marlton O. Metcalfe & associates could set a 1/16 corner on a line 40 rods long closer than +/- one half a link.;-)
on the grid
On the grid, the line drawn between grid coordinates is a straight line, the geodetic line projected upon the grid will be curved, as you stated....
Thanks for the wonderful posts...
> I'll bet that Marlton O. Metcalfe & associates could set a 1/16 corner on a line 40 rods long closer than +/- one half a link.;-)
Yes, the work that Metcalfe and a few others turned out with just transit and tape was pretty phenomenal. Somewhere in my files, I think I have a copy of his report upon the county line survey that has some interesting technical details. In addition to Travis County Surveyor Metcalfe and the Hays County Surveyor, T.A. Breeze, the field party for one part of the work consisted of eight men, of whom at least four later became licensed as surveyors and opened up their own practices.
