Re: BVI chart/nav app
Getting back to GPS tracks overlaid on paper charts: You need to know how position is indexed on your paper charts (the "datum"). All of the GPS units I've used are indexed by default to WGS 84 datum, although I had at least one device that would allow you to reset to use other datums.
My Imray-Iolare Virgin Islands chart was "corrected" as of 2000, but it was not on WGS 84. Rather, if you read the "fine print" on the chart, you needed to adjust your GPS position by 0.12 minutes North and 0.01 minutes West. Basically you need to know that your GPS latitude offset is 246 ft, whereas the longitude offset is small enough to ignore. Actually, there aren't many areas where the 246 ft offset matters--maybe on the approach to the Salt Pond Bay moorings on St. John.