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Comment by kbutler | Hacker News Reader

The author also ignores the fact that the distance associated with a degree of longitude changes dramatically as you go from the equator (69m/111km) to the poles (0) - roughly as a cosine. So the number of decimal places of longitude should shrink as you get further from the equator.

https://www.thoughtco.com/degree-of-latitude-and-longitude-d...

"At 40 degrees north or south, the distance between a degree of longitude is 53 miles (85 kilometers)."

So while it's appropriate for Melbourne (37.8S) to use (very slightly) lower precision than Sydney (33.9S), they're both using too many digits.

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Yes, this is what makes it tricky. The units are very strange here. Its not a simple unit like meters or liters. Its not even one of the stranger units like Celsius (which is odd since zero C does not correspond to none of the quantity it measures, which is thermal motion). Its a relative unit, so its hard to judge how many decimals you need to be accurate.

Its almost like if I was to record where an astronomical object was using only time. Assuming you have full knowledge of its past trajectory, depending on how fast that object moves, you would need different precision times for objects traveling at different velocities, to get the same precision in space.

This is why you just give up on the problem and report to a precision that always works, instead of using the standard scientific procedure of significant figures. I guess if GPS measurement devices took all this into account they could report their measurements with the correct number of digits (taking into account measurement error, and the absolute position on Earth). I guess 15 digits is always too many, but whatever.

Err, no, that fact is in the article.

Went back and re-read, still not seeing it there.

It is described in the linked wikipedia article, though: https://en.wikipedia.org/wiki/Decimal_degrees, so it isn't completely ignored by the author.

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