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PART 1 Here's why the earth is round, or math wouldn't work. for my job I use state plane coordinate system. surface (arc) or grid (chord) coordinates used for distance. to calculate a scale factor that adjusts each value back and forth i use the ellipsoid radius of the earth
PART 2 To bring a google earth image (ground) in my drawing (grid), I need to scale down to match grid coord. which is just x and y plane. ground (surface) distance is larger bc its an arc (nautical mile 1.15 to 1 mile) if my calc is wrong it doesn't fit, if correct it overlays
PART 3 What I used in TX would not work if I was NY bc it has different coordinate zones. The constant that gives me the correct scale factor is the radius of the earth so it has to be round. If it was flat, a surface distance wouldn't exist and that isn't true, it does.
PART 4 The combined scale factor (0.999915513),
Coordinate Zone name and number (Texas South Central Zone No. 4204) and the horizontal datum (NAD83) must be displayed on every single plat that is recorded with the County Clerk. None of this would be necessary if it was flat
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