[Qgis-user] georeference intersecting lines of highway map

[Greg Troxel]

"John W. Blue via QGIS-User" <qgis-user at 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.