Originally Posted by autumnbreeze27
What do I do with the North American 1927 datum charts? I only have WGS84 and WGS72 available in imgcalkap
If you don't have known-good charts of the area with a similar scale to compare to these "NAD27" ones, I would get an up-to-date lightlist of the area and make some "wpts" from it.This will give you an idea of the correction required...IF it is required. It may not be, as I have reason to know on my old charts...the stated datum may not
be how the charts were calibrated
....it is really easy to see this if the lightlist agrees with the chart using brick and mortar lighthouses though even they get rebuilt if the chart is really old...
OpenCPN opens NAD27 with NO correction, as though they are wgs84 so, if these charts have lines of latitude and longitude "inked" on them, and
if a waypoint-mark edited to be where they cross (ie" 20N 20W") DOES match the inked lines it is indeed calibrated to NAD27. If the waypoint mark DOES NOT sit on the crossing, the charts have probably been been calibrated to wgs84 despite what it says about NAD27...until someone like you! disproves it, I believe that they had gps and WGS84 before they converted these old paper charts to digital charts so why would they calibrate
them to NAD27? The NAD27 is simply to explain why the inked lat-long doesn't match the gps...
whatever, If they are in fact calibrated as NAD27, rebuilding the chart with corrected calibration is the best way to go. NAD27 corrections are very specific to the area.