So how about a little quiz. 1.?ÿ Using NGS data sheets and a calculator or a blank computer spreadsheet, compute the grid azimuth from FY2147 to EZ060...

@Norman Oklahoma?ÿ Right you are on the geoid height being zero, The ellipsoid and orthometric heights are the same because the geoid height is zero. ...

@MightyMoe. Since the NGVD29 elevations are orthometric heights, using them with a geoid to approximate nearby NAD83 ellipsoid heights ought not cause...

There is another disturbing issue here. NAD27 assumes that the ellipsoid and the geoid are the same surface; ellipsoid height is everywhere zero.?ÿ Ge...

Garmin have a geoid model. Mine are old, so they have EGM 96. I assume the new ones have EGM 2008.

Speaking of old methods and straight lines, pages 113 and 114 of this report from 1898 or so give a good description of 10 proxies for straight lines ...

Yep. The math is just a model, not the earth itself. Like in the loan business; you never see an amortization schedule with a default or a payoff plan...

What constant bearing did you assume for the rhumb line?

Locations are random, huh? So buy this and 500 tennis balls. Pick a spot or several spots and launch them 4 at a time. Dig where there's a tennis ball...

Hire Ike to do it. ?ÿ

This stuff is way over my head, but I think the answer is that they don't use a constant correction for altitude. A description of the various adjustm...

Two websites that might help: ?ÿ

The calibration referenced above in Wisconsin appears to be in Kenosha County, which shares a reference system with Milwalkee, Ozaukee, and Racine Cou...