Here is the link you were looking for:

http://www.movable-type.co.uk/scripts/latlong-vincenty.html

This is your go-to friend for how to make precise computations with Ellipsoidal Surfaces

You will find several other useful utilities at that site as well.

If you are a master of RTK methods you probably already know this stuff.  If you are learning about RTK and/or vehicle safety using DSRC you probably do not. The above site is maintained by Chris Veness – send him a tip for the time his work just saved you.   This is a how-to for using the Vincenty method.

We have been pointing people to this site for several years now. You will find this link mentioned in published SAE work, in CAMP work,  and several other deployments of DSRC where precise offsets between vehicles requires a localized conversion of the  LLH and Ellipsoidal surfaces into a local 3D orthogonal space. Often this is a key element to building an effective MAP-SPAT processing framework.   If you are dealing with with sub-meter accuracy and precision, you will need to master this topic and this web page is the best place the start.

While this page presents its work in Java code, we have translated this to C/C++ and Qt implementations over the years.  The SCSC DSRC tool kit which was licensed to the US DOT FHWA for open use with SAE J2735 (when DER encoding was used) used this as well, in the provided support utilities.  Our MAP-SPAT engine tools use it, and of course SNIP does as well.  If you wish to have a copy of the converted source code, drop the SNIP support email a line and we will provide it.