Thanks to Jbmoto and Mudmaps for your replies. I had found the CT Global site before, but the links on the compass page to get details on the individual compasses seem to be broken. Whoever built their website was not paying attention.
Why just a compass and not a GPS which has a compass feature on it? Simply a matter of battery life. GPS receivers use up a lot of current. I could plumb it into the bike's electrical system, but in principle I do not want to rely on the bike's electrics. Bike electrics - exposed as they are - are notoriously unreliable.
I have a bike where I can sling the battery away and I can still start, run and stop the bike, which I consider a good survival strategy. (Being able to run the bike - not slinging the battery away
). A digital compass should be able to last a whole trip on one set of batteries and so, like a digital watch, be inherently reliable.