People are familiar with the units they use, less so with the ones they don’t.
Americans are not very good with units they don’t use either.
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From a very practical point of view, the measurement system barely matters these days. I manufacture a diesel emissions control system used on very large yachts. The parts I purchase mostly come from Europe or Asia and are metric. Even my American suppliers sell some parts by the inch that are domestically produced in metric dimensions. For instance I use American made heat sinks that are 150mm wide but are described and sold as 6 inches.
I design on a CAD/CAM program that allows me to use either standard and will automatically convert in the background whenever I choose. My CNC mill is happy to work with either though it is dumb enough crash a spacecraft if I tell it to. When dimensions are measured in microns it gets a bit ridiculous to use fractions of inches. Watching die hard inch guys trying to measure and cut 150mm diameter parts with a stick marked off in 64ths is amusing. Why anyone would prefer to use bizarre fractions or 5 digit decimal inches is beyond me.
My CAD/CAM program will let me design in metric and select a machine tool that suits the task even if that tool is measured in inches. A quarter inch end mill will happily cut stock to a few microns.
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The polar diameter of the planet is suppose to be the base for measurement for cellestial bodys. Not long after the French effort to determine a quadrant of the earths diameter it was found to be off by a substantial amount rendering the meter as based on nothing whatsoever! Measurements/formulas in the Great pyramid of Cheops are so close satellite observation can not differentiate even today. besides, a 5 base system is much more versatile.
A thing with old aircraft plans is “decimalizing” them, i.e. a spar 1-1/8th thick becomes 1.125 inches thick. Makes working with them 1,000 times easier 
Almost all technical documentation and drawings for mechanical equipment is decimalized and has been for longer than any of us have been around. It arose with the industrial revolution when tolerances began to matter and fractions were considered vulgar.
The aviation industry in America stubbornly stuck with inches for fabrication even while aeronautical engineers used decimal or metric because it was more precise. The workshop trades fought hard (and still do) to hang on to ridiculous fractions when all modern measurement tools are either metric or decimal inch or both. Electronic devices don’t like vulgar fractions much and besides maybe a few mates I don’t know anyone who doesn’t know intuitively that point one two five is 1/8 of something. Given the loose tolerances of older designs of essentially hand built aircraft before the jet age, fractions were good enough for sheet metal work but engineering was decimalized. No doubt your spar was carved out of a spruce log in a sawmill and finished with carpentry tools.
I actually do not own a plane with wood spars, but I have flown them. Wood is actually a pretty good building material if you have a hangar, otherwise not so much.
Sign seen at Oshkosh at the Vans booth (seller of metal airplanes): Wood - a transition state between dirt and dirt
Seriously, I worked once on a project involved WW II era plans for all kinds of stuff and people must have going nuts adding 2-3/4s+1-3/64s and so on 
Source: metre (m) - NPL
Originally defined as: one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris.
Since they didn’t have a long enough tape measure to reach from the pole to equator they had a clever way of ensuring that everybody had the same “meter”. They made a “Standard Meter” that was kept in Paris:
yes, they’ve tried the half life of thorium and other things too.
If you are a alien visiting our planet with absolutley zero knowledge of our life forms the default base measurement will undoubtably be our planets polar diameter.
In addition to the meter having no basis in anything but measured now on flashy stuff since the advent of modern science… still, based on nothing.
since the advent of the wright bros. Steamer, tenths of a inch is the standard ‘‘engineers’’ tape measure.
Ultimately we are witnessing the re-writing of history, monuments torn down, heros shamed, authors chastised, etc. etc. etc.
it’s all part of the ‘‘reset’’ … understood in here and in the hereafter!
Diameter of the earth at the geographical poles = 0
I thought the metric distance was based on 100mm, = 1ltr of water in a square at a specific gravity and temp?
That was the definition of a kilogram.
FYI; 1 Kg =1000 gram, or 0.001 metric tonnes. (or 1/1000 if you prefer fractions)
so the measurement came from where?
True to some extent but, even Boeing used fractional inches more often than not until at least WW2. Many of the blueprints predominately use fractions but have decimal inches seemingly thrown in randomly, apparently when fractions annoy the draftsman. I say randomly but tubing and sheet metal has always been in decimal as far as I know.
Postwar Cessna and others use fractions unless the part has critical dimensions like this part for a 1946 Cessna 195.
plate and wire was/is a gauge I thought?
Gauge thickness depends on the material.
Plate thickness usually begins at 1/4 (.250) with thinner stuff spec’d as gauge. I have never seen aircraft tubing (4130) wall thickness in anything except decimal while the OD is fractional inches.
In the Metric System a Kilogram was defined as:
1 kg = 1 liter of distilled water (S p.g. 1.0) at temp. od 4C and at 45 degr. North Lat.
PS> 1 kg. = 1000 grams.
In the SI System the Kg is defined as:
Source: - kilogram - BIPM.
In the SI System a liter is defined as;:
This is volume of a container with dimension 100 x 100 x 100 mm, (10 x 10 x 10 cm), which is eqv to 0.001 Cu.m. or 1 Cu,dm.
Source: https://www.nist.gov/pml/owm/si-units-volume
yes I got that
Its was distance I was asking about or did that come from the water cube?
Here is answer to ALL your questions in respect of the SI system:
https://www.bipm.org/en/measurement-units/si-base-units
I machines lotsa airplane parts in seattle, I don’t think i ever saw a fraction used, that was back in the 70’s-80’s.
The standards for the meter were kept at 80 below zero for a time, not too convienent! american std. at 68F (the inherit temp of where most live.}
polar dia. would be straight line N pole to S pole.