Remanada and the Song of Pelinal equate these two locations. TES4 puts Sancre Tor on the border of County Chorrol and Falkreath Hold. TES5 puts Hrol'dan in the southeastern part of the Reach.
At the Battle of Sancre Tor and/or Hrol'dan, Falkreath (which was at that time considered part of Colovia) defended against an invading alliance of bretons and nords. That makes a lot more sense if the site of the battle is on the north side rather the south side of Falkreath. Aldudagga puts Hrol'dan in the vicinity of Saarthal but also implies there was more than one Hrol'dan. Can this be sorted out? Also, does anyone else think Hjalti was the breton turncoat?

Just to throw a wrench in the works... There's also a Hrol'dan in High Rock.

Hrol'dan was a mirage. A place that appeared only to the Dragonborn, forcing him to right a wrong of his last incarnation.

Sancre Tor was also the name of a large valley situated near the borders of Skyrim, Hammerfell, and Cyrodiil. The blank space that is TESV Cyrodiil turns out to have such a valley, and as a PC console hacker noted on a screenshot of said place: 'the way it looks would make it a perfect tactical position on any scale, explains why there were so.many battles there.'

http://www.gamesas.com/topic/1337654-a-very-private-breton-gentleman/page__p__20147152__hl__breton%20gentleman__fromsearch__1#entry20147152

ME GUSTA.