A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Aug 15, 23:20 -0700
Bob Goethe, you wote:
"My curiosity has been piqued recently as to whether how my NV-Charts products for the British Virgin Islands relate to the GPS coordinate system (i.e. WGS 84). "
Can you provide an example, maybe in the form of a screen capture? The email reply that you received from NV-Charts sounds to me like a failure to communicate. I don't think the person responding to your question really understood the question. A mapping datum has little to do with the age of the survey data that went into producing the chart. Any survey data, of any age, can be re-projected onto any mapping datum. It is true that old survey data should be suspect, of course, but it's not necessarily connected with WGS84 questions.
You also wrote:
"I think I can find an action step for myself here...which is that every time I charter a boat, while I am still tied up to the dock, endeavor to ascertain what, if any, is the offset between the chart features and my GPS-indicated location. "
Yes, I would call that sound practice no matter what. At a known location, compare the GPS position as mapped on the chart with your actual position. I do this even on land. In addition, I recommend checking for the "delta" on that in the sense of rate of change. Confirm that the GPS-charted position is a high quality match for your position, and then make sure it moves as it should. There are cases where position-finding systems "downgrade" to backups, and a useful clue is motion.