PLSS Variance information?

PLSS Magnetic Variation

The notes I've read call it "variation" not variance. It is of course now called declination.

In the case I've studied some, a treaty line was straddled by the sections run just a few years later. There isn't nearly as much correspondence between the variation numbers used by the two surveyors as one might hope for. Likewise the variations used in meandering the river are rather different on the two sides of the river for no stated reason.

Be sure to consider the order things were done. The township lines were probably a better indication of the true magnetic variation than the section lines.

When they ran a trial "random" line to close a section it was their best guess as to what would hit the previously set corner considering how recent sideways fallings had been (from whatever causes), and not necessarily the best estimate of the true variation.

On the return "true" line they corrected for the falling from the corner they had hoped to hit, and thus again was not really a determination of the true magnetic variation.

Then heading out to cut out a new section they may have considered the recent chaining fallings and corrected their compass accordingly.

My +/-0.04 (and I hope it isn't proved negative)

Now THIS should get interesting...

> Aside from recon with a compass, does the variance calls on the GLO plat tell us anything?

Essentially, the early practice of calculating the variation that the compass vernier would need to be set for to run true line was a sort of Custom Projection. From reading the early instructions given in the PLSS cookbook, it would appear that the variation was simply calculated from the fallings of the trial line.

For running with a compass with a declination arc, it was operationally better to simply change the variation set off on the declination arc to be able to rerun the line on some cardinal bearing than to estimate the minutes fraction of the bearing from the needle.