I've always seen the round trip distance from the Portal trailhead to the summit quoted as 22 miles. However, my GPS app gave me a reading of 9.88 miles one way (or under 20 round trip) - here's the link:
https://www.flickr.com/photos/scorpiotiger/29960473902/in/dateposted/I've used this app reliably before, but perhaps it was just confused by poor GPS readings in the wilderness? Or has anyone else come up with similar tracking numbers?
