I think you have to know the projection used for the printed map, and
then set that layer's CRS to match.
Then you can convert ("warp") it to your preferred (project) CRS.
The problem is that different projections distort the map in different
ways, for example conical vs mercator projections.
In such cases if you simply import a conical projected map, say of
Massachusetts, as a raster into your transverse mercator project, say
for New Hampshire, the result will be doubly distorted...
Garth Fletcher
On 4/20/25 3:00 PM, qgis-user-requ...@lists.osgeo.org wrote:
Message: 5 Date: Sun, 20 Apr 2025 14:25:22 -0400 From: Greg Troxel
<g...@lexort.com> To: "John W. Blue via QGIS-User"
<qgis-user@lists.osgeo.org> Subject: Re: [Qgis-user] georeference
intersecting lines of highway map Message-ID:
<rmi34e2ofl9....@s1.lexort.com> Content-Type: text/plain "John W. Blue
via QGIS-User" <qgis-user@lists.osgeo.org> writes:
I have downloaded historical highway maps in .png format that I would
like to georeference.
Very cool; consider publishing the georeferenced versions.
Around the perimeter of the map are a few lines indicating lat and
long making it easy to be able to add a layer to draw straight lines
from left to right and from top to bottom. Which I was able to do.
From there I was able to get QGIS to find the intersections.
And now I am stuck because I cannot save the intersections with
lat/long coordinates it only allows x,y.
I am sure I am most likely doing something wrong out of the gate but
this seems like the logical direction to go in.
The old maps have "lat and long", but what datum? Depends on where and
how old. Basically pre-80/90s coordinates were different enough from
modern ITRF that it is likely to be noticeable.
I would just use the georeferencer in qgis against either imagery or
openstreetmap and find a bunch of intersections. That throws away the
original lat/long information but avoids "it is in NAD27" or "it is in
the New England Datum" or however that is for where you are looking at.
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