[Proj] Transverse Mercator algorithm with good accuracy/speed trade-off?

[support.mn at elisanet.fi]

> The tmerc of Proj4 is efficient. But it has millimeter accuracy only out to 6° of longitude from the central meridian...


1) The most easy way is to make sure never be futher than 6° away from the central median.

or

2) You could implement a floating central median (let's call it "ftmerc"), which would automatically shift as your longitude shifts. Let's say in steps of 1 degree or less. Not at all difficult to implement. And most of "tmerc" routines can be reused.

or

3) Are we talking about UTM...

Regards: Janne