SatNav is short for Satelite Navigation and is normally associated with car trips in this type of app, for example most hotels, pubs etc offer "directions" which one then "sends to their mobile phone", which then guides the user visually and/or audibly to their "destination".
However the speed of development of this exciting technology seems to have left the SatNav Walks Applications in its wake, generally with the excuse that "the coverage is not reliable in this area (so we left SatNav out)". Now firstly that issue has been addressed recently by Google Maps Offline and secondly we see just as much benefit from SatNav at the Trip Planning stage (see Slide Shows above) where everyone should have good internet access.
Without intending to join in any pedantry we do however mention the "warning" from Google that some features are at the "Beta Trial" stage and are "in a state of flux". So we offer SatNav in the spirit of "it is very exciting and can only get better".
Indeed the only "difficulty" with SatNav for walking tracks is to convince the Google engine that a particular section of track IS capable of being walked and is recognised by Google Maps as such. That is to say there is no easy way (as at Dec 2017) of simply putting in your end point as if it was a Pub and expecting Google to go there via the official Cotswold Way - it has to be "coaxed" bit-by-bit to follow the correct route.
But the good news is that it seems that once Trekker has spread its blue lines on the map it is much easier to twig the zigs & zags into the correct route. The better news of course is that once that has been DONE (with all the zig-zags recorded) then the user simply uses the features as if it was a simple drive to a Pub.
At Dec 2017 there is only one walk (Walk 10) where we have not been able to record a route that is at least 95% correct, and only five more that are not yet 100% correct. But in all cases we have used the "edit" process to inform Google that a fix is desired.
So the Trail Card use is simply to Select the SatNav button (either direction) to get a pre-programmed SatNav Route that replicates the markers he/she will be able to follow "on the trail" and it is emphasised that the walker should always use these ground markers as the primary directional aid. However the SatNav device (SmartPhone etc) using this Multi-App provides a great amount of informational aid for a walker, eg estimate of arrival time.
The planning stage uses of SatNav are described under Slide Shows above.