11 ms·
The radius of the earth + 30,000 ft is about 0.14% more than just the radius of the earth. Basically ignore it, unless you are programming guidance control sys
by TheLoneAdmin 8y ago
The radius of the earth + 30,000 ft is about 0.14% more than just the radius of the earth. Basically ignore it, unless you are programming guidance control systems.
- ummonk 8y ago0.14% is enough to throw the number off by 1mph so it isn’t irrelevant.
- loco5niner 8y agolol, ok. Make that 800 MPH then.
- dogma1138 8y agoIt’s likely less than the reading error in most speedometers including those which are used for autonomous vehicles unless you are writing a guidance system for bombs or attempting to land on another planet that’s pretty negligible.
- ummonk 8y agoGiven that the ground speed was likely drawn from gps data, which should have velocity readings more accurate than 1mph (particularly for an airborne gps unit), the choice of calculation method for ground speed does make a difference.
- microcolonel 8y agoThat's less error than slopes and oblateness introduce, so I think it is acceptable.
- ummonk 8y agoThat is also an interesting question. If they’re measuring speed as if the airplane were at the surface rather than at 30,000 feet, is it speed along the WSG84 earth model?
- Stratoscope 8y agoThis reminds me of a math puzzle I read many years ago. It went something like this: "Assume the Earth is a perfect sphere, and its circumference is 25,000 miles. You have a 25,000 mile long rope looped all the way around the Equator, so it is tight against the surface. Now you want to raise the rope one foot in the air, all the way around. How much more rope do you need?" I thought about this and said, "You need a lot more rope! I don't know how much, but it will be a lot. It has to reach all the way around the Earth, only higher!" Of course the real answer is "You only need about three more feet. To be specific, you need another 3.14159... feet." That's all you need to raise a rope a foot in the air, all the way around the Earth.
- Ono-Sendai 8y agocircumference = 2 pi r, so increasing the radius by 1 foot will increase the circumference by 2 pi feet, not pi feet.
- Stratoscope 8y agoThank you! I had a feeling something must be off in my 50-year-old memory of this puzzle. I'd better buy an extra 3-4 feet of rope before I try this experiment.
- staticautomatic 8y agoI had a science test in high school with a question that involved something to do with calculating the air pressure inside an airplane at some high altitude. I went up to to the teacher and said "is this a trick question? Airplanes are pressurized at that altitude" He looked around the room nervously and then told me to assume the airplane wasn't pressurized.
- Stratoscope 8y agoAh yes, the notorious spherical cow! https://en.wikipedia.org/wiki/Spherical_cow https://en.wikipedia.org/wiki/Spherical_cow
- 8y ago