Sorry about the double post.  Itchy trigger finger :)

Adam,

Let's go back to original problem.  We want to map a trail.  Let's
assume that we've got a 7.5 minute topo map.  On CD, if that helps.

Can I do inertial navigation, by integrating the output of a bike
speedo and the output of a digital compass using the PIC as a data
logger, and post processing?  How about a special pickup using the
bike wheel as a distance measurement wheel, and the compass?

How about Loran, which is not intentionally crippled?

How about just good old fashioned orienteering / surveying methods by
triangulation to known fixes?  Is there an advantage to having a
smart data collection system to do calculation on the fly?