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I'm a tabletop roleplaying gamemaster wanting to use QGIS to help organize my notes according to place. GIS addresses my need to store notes and other data that may be anywhere from a few feet from each other to several hundred miles apart, and to store notes that apply to paths and areas. Also, I'm just interested in getting into GIS anyway.

My fictitious planet is a conveniently perfect sphere with a circumference of 25,000 miles. Here's the CRS I cobbled together:

GEODCRS["FictionalPlanet",
    DATUM["Perfect sphere, circumference 25000 miles",
        ELLIPSOID["Perfect sphere, circumference 25000 miles",21008452.5,0,
            LENGTHUNIT["foot",0.3048]]],
    PRIMEM["Prime",0,
        ANGLEUNIT["degree",0.0174532925199433]],
    CS[spherical,3],
        AXIS["geodetic latitude (lat)",north,
            ORDER[1],
            ANGLEUNIT["degree minute second hemisphere",0.0174532925199433]],
        AXIS["geodetic longitude (long)",east,
            ORDER[2],
            ANGLEUNIT["degree minute second hemisphere",0.0174532925199433]],
        AXIS["ellipsoidal height (h)",up,
            ORDER[3],
            LENGTHUNIT["foot",0.3048]]]

My immediate goal is to set up a grid with an interval of 5ft along the equator and the meridians. With a circumference of 132,000,000 feet that works out to 26,400,000 intervals. In turn, that's 73333.33... intervals per degree. So flip that under 1 and I have 0.00001363636 (last two digits repeating).

If I throw that into the properties window for the grid decoration, I get almost what I want, but not exactly. The distance between two grid lines at the equator and along meridians is 5.027 ft.

I'm guessing this is down to precision limits within QGIS, and ultimately something I will have to live with. But is it possible to improve precision further with some settings somewhere? Or by using a different technique?

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