For the modern world: 822 MPH = 1322.881 km/h
https://aviation.stackexchange.com/questions/2566/what-is-th...
In my experience, US pilots usually use feet for altitude, but use nautical miles for distance and knots for speed, not standard Imperial units.
"Imperial units" is ambiguous and can refer to many different sets of units all sharing the same name. US Customary units use SI as a baseline for measuring based on physical constants, and standardized conversions from them to units very similar to what was conventional. Eg, since the meter is defined by the distance light travels in a vacuum for the duration of 1/299,792,458 second, and the US defines an inch to be exactly 2.54 cm, any refinement to the speed of light inherently changes the length of an inch. 12 inches equals a foot, a mile equals 5280 feet (exactly 1609.344 meters), and all these things are based on physical constants. Nautical miles are defined as exactly 1852 meters, which is approximately 1.15 miles.
In addition to the other replies you're getting here, the US for instance does not use imperial units, despite what many people on the internet believe. Imperial units are used by the UK (hence the term "imperial"). The US uses "US customary units", which sometimes overlap with UK units, and other times have the same name but are different. An imperial gallon is quite different from a US gallon, for instance, but imperial and US inches are the same.
- 'knots' is faster to say on the radio than 'kilometers per hour'
- the way the world is divided in wgs84, maked it so that 1 degree of latitude equals exactly 60 miles. So that makes quick measurements a bit easier.
What with the Earth being an ellipsoid and not a sphere and all that?
For a sailor in the 16th century, assuming a sphere was perfectly adequate when combined with 1 degree = 60 nautical miles.
The geoid model is the surface of gravitational equipotential and is a "shape of the gravitational field" not a "shape of the physical earth".
If we were discussing 16th century sailors rather than 21st century aviation you might have had a point.
The origin of nautical miles is from 16th century sailors.
And 21st century aviation doesn't need anything more accurate than WGS84. The people who need custom geoid models and local coordinate systems are surveyors.
I believe you're thinking of distance between lines of longitude which converge at the poles and are at maximal seperation at the equator.
I was referring to the arc distance (surface distance) between lines of latitude which don't converge to zero but do get a little smaller closer to the poles (due to the lesser minor radius of the ellipsoid shape to the earth - oblate sphereoid, squashed sphere, not a perfect ball, etc.)
But it really helps you as in you are able to pick any distance you want to fly on the map and by moving your fingers to the Y axis roughly guess the distance within a few seconds. Even in a very small cockpit and with turbulence. So i absolutely see the need for this with relation to navigation.
Sure, if you also want to say water doesn't freeze at 0C or boil at 100C. In practice if an author casually leaves out the variables needed to calculate ground speed from mach or airspeed, it usually means they're doing engineering calculations set at the ISA standard day model (sea level, 15C, no humidity).
I replied above, but knots have a similar problem when flying (indicated air speed).
If you ask a pilot, ground speed only really is important for calculating ETA. Everything relevant to flying is a relative number since atmospheric density is fluid.
I guess the real benefit to km/h is that it allows people to act superior without knowing it's just as dumb as imperial units.
I have a habit of measuring ground distance in meters, for example, because I was in the (US) Army and yes we measured those things in meters/kilometers. But other things are not measured like that.
Measuring systems are only useful if they are, well, easy. And some systems are more easy in certain circumstances than others. Which is to say, it's easy to get used to a meter being a pace (tie a meter long string to your ankles).
Anyone have a rhyme for it?
It's really not. Most of the time, I just convert to grams and use a scale so I can get an actually accurate input quantity. I don't want to think about "heaping" and "leveled" and "packed" and different densities, humidity or uneven distribution within a measuring implement. If I'm going to be putting it on a scale I'm definitely not setting it to ounces.
When was the last time you really looked and said, geez, thank the lord I can divide 1lb 14oz into eights really easily in my head?
On the math thing, probably during Christmas when we were trying to stretch a baked French Toast recipe to feed 9 people in stead of 6. Worked great.
My french toast is ruined! I measured the exact amount you said! Not realizing that the original cook was using the Larry David measurement system. "Ehhh that's close enough".