[Qgis-user] georeferencing maps

[Nicolas Cadieux]

Hi,

Start with finding a background map or a project CRS that is similar to 
your old maps.  That takes a bit of guessing.  Then select the Helmert 
method in the transformation method.  All you need is 2 GCP (ground 
control points). If you add more, you will see how you map is warped and 
how points drift.  Once you are happy with the results, then change the 
method to Thin Plate Spline and you can start adding GCP and testing the 
results.  That will and can really warp the map so be careful with this 
method.

Nicolas

On 2021-08-25 1:26 p.m., Firstname Lastname wrote:
>
> hello
> i am trying to do some basic georeferencing.  i have been using the 
> georeferencing tool and i have been following the workflow suggested 
> on youtube files using the georeferencing tool
> i have tried three different maps of new and old vintage. i have tried 
> to keep it simple with 4 points control points and have used as many 
> as 10 points.  when i apply the georeferencing, my maps are not 
> calibrating.  even the control points end up 400-600 m. of off the 
> true coordinates.  Any suggestions that might help fix this.
> for reference, i used the epsg 3857 CRS
> -- 
> Byron Veilleux, MSc. P.Geo
> Conjugate Geologic Services Limited
> Calgary, Alberta Canada
> byron at conjugategeo.com <mailto:byron at conjugategeo.com>
> Cell:4037108414
>
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-- 
Nicolas Cadieux
https://gitlab.com/njacadieux

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