I agree with you both:
1. I agree with Marco: Don't trust SK etc, at least put greater trust in the ref points. Maybe use ref points to check DY,DX values? In most cases the ref points are used to automatically calculate DX,DY in the first place, so this check would be for checking the application (or human) creating the header file.
2. Dave, as you I really think we should use the projection when it is stated in the BSB
header. The equations seem to be in the Proj4 which is already used. (Correct me if I am wrong)
PROJ.4 - Trac
I dont understand all the details, specially how to use the ref points if the projection is off. As I understand it, nowadays the projection is usually very good (near perfect) for new commercial charts
since the charts
are not scanned anymore. The errors are usually induced by the ECDIS.
Anyway, with mercator and transverse mercator equations a large number of charts would be well covered. Maybe we could ask the community which projections are used in the charts we have access to in all our countries?
On top of using the correct projection equations the methods suggested by Dave/Marco seems god.
It really is, as I think Marco stated earlier, the commander of the ship that is responsible. With this comes the responsibility for good electronic charts. Either checking them or acquiring them from a trusted source. Some error checking by OpenCPN
is always good, but in the end it seems logical that the projection in the "paper chart" that is the basis for the BSB
chart should be stated in the kap file and used in the ECDIS software
in this case.
Regarding the issue with the missing SK value in the 9312_gotaalv.kap chart, I have heard that the SK value is not mandatory in kaps since it can be calculated from the ref points. So as Dave and Marco suggest, it should be calculated.
5. I really like the idea someone suggested to put a grayed zone around the vessel indicating the accuracy of the system. We always have measurement errors. Always. I think most of us have read articles about this.
We have the inaccuracy of the chart (includes the inaccuracy based on the date when the chart is from. In may areas well over a hundred years ago with the accuracy in bearing, distance and depth
used then). The inaccuracy of the printing system and digitizing/referencing, assuming scanned charts.
Finally the varying error in positioning using a GPS
To put a grayed circle (that can be turned of) around the boat would be a nice remainder of this.