Hello fellow surveyors,
As I work on more and more surveys I download a copy of the GLO maps and want to learn more about these maps. The problem is like anything else on the internet, there's a lot of information to sift through. I don't always trust the sites I come across. I wasn't really trained in a lot of the things I believe I should know as a licensed professional. With that said, if I ever get a young surveyor to work under me I intend to educate them in as much about the history and profession as I can.?ÿI admit I don't know as much as I should.?ÿ
When I comes to the GLO maps, I can compute the length of the closing sections with the area calculation and I understand the chain measurements, but there are other details I'd like to learn about. A couple things I see, but have no idea about are, the v. 12?ø 4' and the letters on the left margin. Does anybody have a good site I can visit to learn more about these maps??ÿ
I'd also like to learn about star shots, sun shots, and using the monument from NGS data sheets (like water towers and church steeples)?ÿ to determine my location.
It's a notation for magnetic variation at the time of the survey.
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Suggest you get a copy of A History of the Rectangular Survey System by C. Albert White
https://www.blm.gov/sites/blm.gov/files/histrect.pdf
DDSM
I would suggest obtaining copies and reading the official GLO field notes for the specific plat.?ÿThey're?ÿan essential part of the plat and will detail how the work was done. For me most of them are now available on line and don't require a visit to the BLM public room anymore. The information in them like bearing trees and topo calls can be essential to my?ÿcompleting a survey faithfully. I think of the GLO plat as the bones and the notes the meat.
Just because I'm paranoid, doesn't mean they aren't out to get me.
What I don't understand is why the variation is listed on each mile (on the 1870's plats) - the newer plats just have it listed once by the north arrow for the whole township.
It seems like the least important or useful piece of data on a plat. Most older plats in my neighborhood have it repeated on each section line, even the closing section which will have a returned bearing, but the mag variation stays the same; how can that work?
The GLO/BLM Manuals contain chapters devoted to Plats...
Older GLO plats show the variation on each line to account for local attraction. Newer plats don't show each line because they didn't use a compass and didn't expect retracing surveyors to use one. Most of the even newer ones don't show it all, or only state it in the notes.?ÿ
It was useful when most surveyors used a compass, and is still occasionally useful in helping to figure out what went wrong.?ÿ
If you are willing to spend some time learning I recommend the CFedS program.?ÿ
Older GLO plats show the variation on each line to account for local attraction. Newer plats don't show each line because they didn't use a compass and didn't expect retracing surveyors to use one. Most of the even newer ones don't show it all, or only state it in the notes.?ÿ
It was useful when most surveyors used a compass, and is still occasionally useful in helping to figure out what went wrong.?ÿ
I've never paid much attention to them, I've also never used them for anything, mostly I see them as an anachronism. Often they will be repeated identically through the entire plat on each north line, they also may show slight variations. I?ÿnever could understand how they can have say 17d30' on each line, even the closing lines.
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