One subject that figures into retracing 19th century surveys is the expected accuracy of distance measurements. I mentioned the other day that in the course of the project I'm working on at the moment, I had found two corners originally established in 1875 by Hays County Surveyor, B.C. Hardin, who reported them as being 950 varas apart on a line apparently actually chained, but my resurvey found the distance to actually be 937.090 varas (Horiz Surface Dist). That's a chaining factor of 937.090/950 = 0.98641
Most of the surveys made by Mr. Hardin in the 1870's that I can recall following were made out on the prairie, not through the rocky hills and cedar forests of the Hill Country in the West part of the county through terrain similar to the present project. There was one survey that Mr. Hardin and his party had run in 1873 in a setting that is somewhat similar to the recent work. The comparison between what he reported for lengths and what was actually on the ground?
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1873 Survey
Record Actual Actual/Record Elev. Diff
817 vrs. 810.02 vrs. 0.99145 3.12 vrs.
1373 vrs. 1365.58 vrs. 0.99460 40.80 vrs.
1875 Survey
Record Actual Actual/Record Elev. Diff
950 vrs. 937.09 vrs. 0.98641 29.04 vrs.
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The two different surveys used entirely different chain carriers and so cannot be exactly comparable as each would be expected to have somewhat individual biases. However, to the extent that both sets of chain carriers were following Mr. Hardin's instructions, they may generally reflect the practices he followed. What strikes me is that on all three lines, shortage was typical and the magnitude of the shortages were not that markedly different, considering the fact that all most likely used a 10-vara chain and chaining pins to keep track of chainage.
> What strikes me is that on all three lines, shortage was typical and the magnitude of the shortages were not that markedly different, considering the fact that all most likely used a 10-vara chain and chaining pins to keep track of chainage.
I would have expected the error to be greater with increased elevation difference, but that seems to be opposite of the results.
Also, is the reason the current measurements are lower than the original due to chain sag?
The usual method of chaining was to chain as level as possible. So a pair of chainmen could accommodate a slope within the length of the chain equal to the height of the person at the low end of the chain. Very seldom was chain slope actually measured - rather level was estimated.
The differences noted here were probably due to the chain not being calibrated correctly, in this case the chain appears to have been short. Or they were always measuring with the chain sloped.
A common rule of thumb that we have found for in-town surveys in platted blocks is that distances determined with a 100-steel tape will measure very close to 99.9 feet with a total station. A common monumented 400-foot block containing eight 50-foot lots will have a total length very close to 399.6 feet. Each lot will be very close to 49.95 feet.
If the 1875 Surveyor was laying out 950 Vargas you would expect him to fall short especially if slope chaining.
> The usual method of chaining was to chain as level as possible. So a pair of chainmen could accommodate a slope within the length of the chain equal to the height of the person at the low end of the chain. Very seldom was chain slope actually measured - rather level was estimated.
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> The differences noted here were probably due to the chain not being calibrated correctly, in this case the chain appears to have been short. Or they were always measuring with the chain sloped.
I think the shortages are most likely the result of the chaining pins not being stuck plumb on steep slopes, but tending to be set leaning downhill. That is, the way a line was chained was that the rear chain carrier held his end of the chain plumbed over the chaining pin in the ground by eye, lining up with the shank of the pin sticking out of the ground, so if the rear pin isn't plumb, the chain carrier would be holding over a point above the ground on that same non-plumb line.
The front chain carrier set the pin by dropping it from the chain and then sticking it in the ground, so the drop would have been nominally plumb.
This is I think why the chaining error ought to be greater on an uphill line than on a downhill line of the same length, which is the case with the lines reported above. The 1373-vara line was run downhill and the 950-vara line uphill.
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Who makes that light weight, apparently adjustable, prism pole tripod?
SECO is one brand, I'm sure that others make similar offerings. I believe they're known as "thumb release tripods", they don't come cheap though.
> SECO is one brand, I'm sure that others make similar offerings. I believe they're known as "thumb release tripods", they don't come cheap though.
This one has lever locks on it. Looks like a CST-Berger 67-4250X. That it?
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> Who makes that light weight, apparently adjustable, prism pole tripod?
That is a model that SECO made. That particular tripod is probably about twenty years old. I have another than it nearly thirty years old and still in regular use.
I have 3 of these (or very similar). Like Kent, my oldest one is close to 30 years old now (still as good as new), the other 2 only about 10-15 years old.
http://www.secomfg.com/products/details.asp?prodID=5215-00-RED
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