There are basically two types available on the market: digitized charts (electronically scanned paper charts), and digital "vector" charts (essentially a large database of info used to "draw" a chart). The digitized charts are usually sold on CD-ROM media (e.g., Maptech) and are generally used with PC-based route planning software. They are faithful reproductions of the official nautical charts you buy from the Gov't. The commercially available products of both types generally (a) derive their data from the primary Gov't source(s), and (b) come in a variety of non-interchangable formats (the way the data are stored & organized). Both methodologies are primitive when it comes to posting the USCG LNM Chart updates. MapTech does have a "Professional" (equals expensive) update service for their digitized charts; CMap offers annual updates for their modules. The digital charts are available on various media (e.g., CD-ROM, CMap-NT plug-in modules, etc.) and are often used with on-board chartplotters (e.g., hand-held GPS, fixed radar plotters, etc.) So, bottom line: 1. Pick the application you want to perform (PC-based route planning or on-board chart plotting of GPS/radar). 2. Pick the equipment you prefer using. 3. Then be stuck with the mapping data type/format/media/source that comes as baggage. PS: The sailing magazines did surveys of these products over the past few years. I recall that 'Ocean Navigator' (Portland, ME) had a particularly good article. PPS: I noticed that DefenderUS had a good discount on these items.
