Longitude to 12 decimal places will get you precision to 0.00001mm (that's right, millimeters). The only reason I could see this being useful would be for blasting microbes from space
However I want precision out to around 12 decimal places or more.
Vincenty's formula is accurate to within 0.5mm, or 0.000015? (!), on the ellipsoid being used. Calculations based on a spherical model, such as the (much simpler) Haversine, are accurate to around 0.3%
will get you precision to 0.00001mm