[kbolino]

> People are convinced you have to use doubles because of the inaccuracy of floats

With single-precision floats, considering the worst case, i.e. longitudes close to the equator but far away from the prime meridian, one ulp can translate into as much as 1.68 m (5.5 ft) of distance on the surface (*). That's good enough for some uses, but not for dGPS or any other serious geometric computation. Whereas, with double precision, one ulp in this worst case scenario corresponds to about 3 nanometers. It's overkill, for sure, but if these are the only two types you have, you pick the latter.

* = To represent the integer 179, you need 8 bits, leaving only 16 for the fraction. Since 1 degree of longitude near the equator is about 110 km, you have 1/2^16 degrees * 110 km/degree giving 0.0016784... km.

[taeric]

Right. My point is that I wish more people would reach instead for a fixed point lat/lng library. It would almost certainly be easier to reason with for most people. Similar for most people doing financial math should probably use something other than IEEE floats.

And again, IEEE numbers are amazingly well done for what they are. Which is largely the digital version of scientific notation that is using base-2.

[kbolino]

It's quite difficult to find fixed-point number systems that include trigonometry, never mind fancier tools like elliptic integrals. The problem is not just one of static number representations, but also geodetic/geometric computations. [ed: I see this is where the other thread is heading already]