Tuesday, April 9, 2013

Adjusting SeaClear for Canadian NAD27 charts

As Kristi noted in the prior post, when entering Canada all of a sudden our chart plotter was off.  Even
Hey, I can assure you we were NOT in the land!
having us Plow the Ground while making our way down Tod inlet.

The cause of this is a difference between Datums used to create each chart.  Most modern charts use NAD84, or an equivalent   However, the ECharts we have for Canada are based on NAD27.  (This is easily seen by looking at the notes on the charts, they will always state what Datum was used for drafting the charts).

It seems SeaClear is clever enough to make adjustment for datum sources, providing it gets the correct information    As each electronic chart includes its datum in a machine readable format, most the hard work is done for us.  But still we were doing the Farmer John thing the other day.

Well, turns out there are several NAD27's...   In the file datums2.ini  4 are listed:

    North American 1927=E5,-8,160,176,5,5,6,405
    North American 1927, Alaska=E5,-5,135,172,5,9,5,47
    North American 1927, Bahamas=E5,-4,154,178,5,3,5,11
    North American 1927, Canada=E5,-10,158,187,15,11,6,112



I can not figure out how SeaClear decides which one to use, so what I did was simply make the choice easy - by commenting out the 3 I did not want, placing a semicolon in front of them,  SeaClear was forced to use the Canadian variant of NAD27.

    ;North American 1927=E5,-8,160,176,5,5,6,405
    ;North American 1927, Alaska=E5,-5,135,172,5,9,5,47
    ;North American 1927, Bahamas=E5,-4,154,178,5,3,5,11
    North American 1927, Canada=E5,-10,158,187,15,11,6,112



With this edit, and restarting SeaClear, charts are now displayed correctly.  Yee Haw, will not have to dig out that John Deere Hat after all.


Note:  The file  datums2.ini  is located on my system at:
     C:\Program Files\SeaClear\datums2.ini




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