[Qgis-user] Question - georeferencing a chart

[Laura Leite]

Hi,

This is not a question directly related to QGIS but I can't get it anywhere
(or in too much places leaving me really confused).

I need to georeference a nautical chart (an admiralty chart) and by the
information in it I know the datum (WGS84) and the projection (mercator).

I used the 4 corners and other points from the intersections of the grid in
the nautical chart as GCP

In the end I get the map well positioned but with a small displacement from
other references (a coastline digitized from google earth) and waypoints
marked using a GPS (with WGS84). I have tried with other charts and the
displacement is always more or less the same

What am I missing?

1) Should I make any lat-long corrections. But if I am using the same datum
and projection I assume that no...


2) Should I consider the magnetic declination? But how? I now that the dm to
2011 is 1º 18' W. Should I do something with this value? Isn't it an angle
to mark azimuts or routes?


3) I can't digitize the coastline directly from Google earth? I have used it
more than once (when I don't have any other resources) and until now....
everything was fine.


p.s. - the admiralty chart I am using is the 773. But I have tried others
and I get the same result

many thanks. Hope hearing from you even if it's not a QGIS question....

Laura
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