I agree the "best" way to do travel scanning is to use a GPS device. The question as I understood it, was how to lock one "one" tower. With GPS, if multiple towers are in range, then all of them will be on, versus the "one".
Also, noting that each scanner listener's preferences may be different, I'm the kind who likes to listen to as many sites as I'm capable of receiving. That's because some traffic may be on a distant site (though in reception range), but not on the closest site.