Obviously if every road in the USA went due north or south this would be a moot point, but since they don't we are led to issues.
and if that isnt bad enough, most roads don't even follow straignt paths from start to end... thus, the optimum solution would be to use a very rough algorithm and download an excessive number of caches, but in a stripped down format such as XML, with only basic details of the cache.
so my suggestion would be to do the follwoing for a multipoint path. Asking for a search radius would be somewhat ridiculous so why not get every cache in a box between the starting and ending point, with say 30 miles padding on either side. (use a few rectangles when going really long distances, creating a staircased approximation of a diagonal)
If all we are downloading is the x/y location of the cache (don't even list caches that arent available) and the type of cache and its capsule descriptiont his should suffice.
That solution isnt pretty, but it is the easiest solution for now. many of us with newer GPS's can hold hundreds, maybe thousands of points, so I see no reason not to :-)
the bigger issue to me once i have every cache programmed into my gps is that, ok fine, I am in nebraska and i know there is a cahe three miles away, but how do I get the details of the cache? I could call a friend with an internet connection but that seems impractical. I could mirror the geocaching site in full before i leave on my trip, but that may cause Jeremy to murder me in cold blood. What options exist for carrying hundereds of cache candidates on the road?
I'm beginning to feel this would be a killer premium service. as a premium user, one might be able to download a zipfile with all the caches along the way as well as XML formatted cache details that can be accessed on the road from a PDA or if space is an issue, thru a laptop. I can't imagine the entire text portion of the geocaching website is over a gigabyte, so certainly the basic info (no logs, or possibly only the latest 2 for each cache) for a thousand caches would fit in a few megs right?
I might be off my rocker but we ought to be able to make a solution here.