[Qgis-user] Question - georeferencing a chart

[Alex Mandel]

On 10/07/2011 09:00 AM, Laura Leite wrote:
> 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

How many points total did you use? What was the RMSE, and which
calculation did you use for the georeference transformation.
For high accuracy professionals use upwards of 20. The key is getting
the error to be low (below 5, preferably closer to 3 or lower - rule of
thumb).

Magnetic declination should have no effect on Map north.

I would not trust a digitized coast from Google Earth. I've seen varying
reports on the accuracy of the photo registration. Let's just say it
varies and is not as accurate as you might hope or at least has not been
in some places in the past (2008 paper Santa Barbara was 20 m off).

What coastline do you need? Maybe a better source can be found.

Thanks,
Alex