A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Bob Goethe
Date: 2016 Aug 13, 08:43 -0700
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).
I noticed on NV-Charts' web site that they make a point of saying the *waypoints* they have marked on their charts are based on WGS 84 coordinates. But they give no indication as to the datum used in the construction of the chart itself. I sent an enquiry to NV-Charts regarding this, and here was the reply:
While the WGS84 positions can be plotted onto the charts it should be noted that the source documentation for most of the charting in the Virgin Islands is fairly old and therefore the GPS positions may be more accurate than the charted detail.
That being said all of the waypoints are absolute and all have been checked and are safe relative to the charted positions.
In no instances should one use a zoomed in chart plotter to establish waypoints as this will result in corrupt data.
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. Use that knowledge to get a feel for the practical limits of accuracy I can expect from my chartplotter. It is never wise to cut too close to a navigational hazard. But based on the chart offset, I may want to further increase the leeway I give to such hazards.