Here's something to ponder over the weekend:
Several years ago I posted a blurb (old board) on the “theoretical” effect that Local Attraction might have on a section line run with the magnetic needle (which most where in the 19th Century).
Well a couple of weeks ago “we” recovered and tied a series of County Road Stones set in October of 1889 by Francis M. Lyman Jr, County Surveyor. ALL of these stones were well set and in situ, AND marked with a clear 'X' on top. They were also all made from a “granite” that was NOT native to the area, and were no doubt hauled in by wagon along the county road being surveyed. Now I had suspected for some time that these County Road Surveys were based solely on the Magnetic Needle, but this turned out to be the nail in THAT coffin.
It just so happens, that this is an excellent example of what a modest magnetic anomaly CAN do. Not only that, it is also one mile in length, AND has random course (vector) lengths that are topographically controlled for inter-visibility. Now it does NOT run North-South, but we can still get a feel for what would happen on a Section Line using the observed declinations (differences) and distances. For our purposes, “declination” will be defined as the DIFFERENCE between Mr. Lyman's Bearings and Our Bearings (based on a georeferenced LDP). To be fair, I have rounded our data to the nearest arcminute and link as was returned by Mr. Lyman in 1889.
At XXI MP-10 thence,
Record S 38°40' E, 8.85 Chs. To XXII
Inv. S 37°39' E, 8.86 Chs.
Diff. 1°01' R. +0.01 Chs.
Record S 62°52' E, 10.17 Chs. To XXIII
Inv. S 62°22' E, 10.21 Chs.
Diff. 0°30' R. +0.04 Chs.
Record S 50°57” E, 6.98 Chs. To XXIV
Inv. S 50°24' E, 6.91 Chs.
Diff. 0°33' R. -0.07 Chs.
Record S 26°54' E, 6.00 Chs. To XXV
Inv. S 26°19' E, 5.94 Chs.
Diff. 0°35' R. -0.06 Chs.
Record S 45°54' E, 5.30 Chs. To XXVI
Inv. S 47°21' E, 6.29 Chs. ***
Diff. 1°27' L. +0.99 Chs. Rotation now to the Left
Record S 29°41' E, 6.86 Chs. To XXVII
Inv. S 31°05' E, 6.87 Chs.
Diff. 1°24' L. +0.01 Chs.
Record S 22°46' E, 22.03 Chs. To XXVIII
Inv. S 24°11' E, 22.02 Chs.
Diff. 1°25' L. -0.01 Chs.
Record S 10°44' E, 12.14 Chs. To XXIX
Inv. S 12°08' E, 11.11 Chs. **
Diff. 1°24' L. -1.03 Chs.
Record S 34°01' E, 1.67 Chs. To XXX MP-11 (Not recovered yet)
Inv. S 35°28' E, 1.67 Chs.
Diff. 1°27' L. -
*** = picked up a chain
** = dropped a chain
Unfortunately, we have not recovered Mile Post 11 (XXX) yet, so I have extrapolated the last (1.67 Ch.) course based on a measured vector farther South along the Road. For that short a distance, I doubt that it will pollute things too much.
Total distance 80.00 chains (1889), and 79.88 chains measured (2011), including the 1 chain “pickup” between XXVI & XXVII and the 1 chain “drop” between XXVIII & XXIX (which would really complicate things in the PLSS sense). There IS some grade involved, and I'm pretty sure that Mr. Lyman was returning SLOPE distance along the road, but we'll not belabor that point this time around.
I will NOT bore you with the geologic map which indicates that we are “driving” down a narrow (~1000') corridor of Quaternary Alluvium, with a Mountain of Permian/Pennsylvanian (unconfomity) Sandstone, Siltstone, Limestone on the West, and a Middle Eocene Lahar/debris flow on the East, that forms a spur ridge. The lahar contains intermediate-composition volcanic rocks and thin lava flows (and according to some, andesite which I will mention usually contains Magnetite). We pretty much leave the spur ridge (lahar) behind, about 1/3 of the way through the traverse.
Okay...so PRETENDING that our stalwart GLO Deputy RAN his Section Line from North to South (which I HAVE seen incidentally), we get:
“True” Measured T/Lat T/Dep
S 1°01' W, 8.86 Chs. 8.86s 0.16w
S 0°30' W, 10.21 Chs. 19.07s 0.25w
S 0°33' W, 6.91 Chs. 25.98s 0.31w
S 0°35' W, 5.94 Chs. 31.92s 0.37w
S 1°27' E, 6.29 Chs. 38.21s 0.21w
S 1°24' E, 2.70 Chs.* 40.90s 0.15w ¼ Cor.
S 1°24' E, 4.17 Chs.* 45.07s 0.05w
S 1°25' E, 22.02 Chs. 67.09s 0.50e
S 1°24' E, 11.11 Chs. 78.19s 0.77e
S 1°27' E, 1.67 Chs. 79.86s 0.81e Sec. Cor.
* = assumes that the ¼ Corner was “dropped off” based on Mr. Lyman's chain count between XXVI & XXVII.
Soooooo....if'n we wuz to “shoot” the Section Corners on each end of this line, we would get a “true” bearing (inverse) of S 0°35' E, and a distance of 79.87 Chs.
Assuming that the ¼ Corner is LOST, we would be putting that sucker @ 39.93 Chs. South, & 0.41 Chs. East of the North Section Corner, which would be 0.97 Chs. North, & 0.56 Chs. East of the ORIGINAL position (so much for single proportion).
This turns out to be a GREAT example, because it includes BOTH a Magnetic Variation issue, AND compensating chaining errors! In the real world, we won't have these stones at each of the deputy's “compass points,” BUT we could still get a feel for things by ACTUALLY running the line with a compass.
Moral of story....there IS a reason why we “walk in the footsteps” and don't blindly trust the NUMBERS!
Flak vest ON!
PS....I hope I got this assembled correctly...
Loyal
That kind of ties into the scale factor business earlier I believe.
I've scale factors VERY different from one side of mountain to the other.
That was in the elavation differentials from hell when I was in NC.
I had it all explained to me once and I got it at the time.
Now I've forgotten it.
You know what they say, "if you don't use it you'll loose it".
(I hope little Eric Jr. didn't hear that) :-O
That is interesting. There is a place up by Gardnerville that has at least 5° of local attraction. I walked 600' and was 50' off, no wonder I couldn't find it the first time (a 3/4 pipe at a 1/16th corner). This was a little volunteer recon at a summer camp; Emerson Smith surveyed the camp in the 1960s from found stones at 7 of 8 original corners.
I then sighted a baseline of two points observed with GPS using the correct declination and the answer I wrote down in the field book was at least 5° off of the geodetic north I calculated from the GPS processing (static). The compass is a plastic Silva Ranger; I get good results with that thing (don't worry I'm not setting property corners with it). I use it a lot to get an initial bearing to enter into the DC just to get my traverse started (for later processing to a real bearing basis); I'm surprised at how often my Silva Ranger gets me within less than a degree of reality.
Dave
As I have mentioned several times before, I have seen 6 degrees in a mere 200 feet! 4-5 degrees in a ¼ mile is very common in mining districts in the West, and I'm sure that some folks have seen significantly LARGER local variations. It doesn't take all that much to put you in the wrong zip code when the terrain gets rough, and the compass sights get short with that magnitude of variation.
Loyal
Dave
there is a lot of evidence of hydraulic mining up there.
I see that there were over 300 views to this thread over the weekend...Hmmmmm
How many folks plotted the “theoretical section line” in CAD, just to see what it looked like???
🙂
Loyal
