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You must live farther north than I though, always considered you a southern guy. ????
- Posted by: @mightymoe
SPC convergence can be that large at about any latitude if you are far from the standard line of the projection.
. @bill93 MM is correct. I was referring to convergence of the meridians. This causes large apparent misclosures on PLSS plats. It gets worse at higher latitudes.
Projection mapping angles have other variables and wouldn’t be an issue on (unprojected) GLO plats.
- Posted by: @thebionicman
Nate said
In this area, theta is close to 55″ per mile. (EW)
which is what got me thinking about SPC, as I think that is a common term there.
. The angular expeession can be similar as both are computed (nominally) as sin of lat x delta long. The difference is in how the angle is applied. One is between the points, the other between the point and central meridian.
Early surveyors used the standard field tables to obtain various offsets for correcting in the field. I have found them surprisingly accurate compared to proper geographic calculations. So much so I keep a redbook in the truck.
Care needs to be taken to sometimes “get on” the early ways of calculating convergence and constant bearings.
It is simple to mean LATs and LONGs to do prorations and put points on a constant bearing line. I’ve found them to be really close to older methods.
I did obtain notes and calculation sheets from a private surveyor who probably started his surveying with the GLO (1930’s era). These notes were interesting, he calculated lat long then converted it all to northings and westings. Made it really difficult to imput it into autocad.
@flga-2-2 Nice slam. ????
Or you ccould use a program like Caldson or Traverse PC, that can handle true mean bearings.
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