Georeferencing a Lambert conformal conic

Andy S

I'm trying to georeference a map (a non-referenced tiff) which is (I'm pretty sure) in a Lambert conformal conic projection. Digitizing four control points at the four corners produces a map which is correct along the N-S longitude edges but which "sags" along the lines of latitude.

What method/workflow should I use to do this correctly? Should I just digitize lots of points along lines of latitude and do a polynomial rectification or is there a simpler way?

global_mapper

What are you specifying as the rectification projection when doing the georeference? In theory if you get that right even just 2 points will work, but if you can get close then 3 or 4 points will probably work well. If you don't know, then just adding more control points should eliminate most of the distortion.

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

Andy S

Thanks. I tried using 15 points, a polynomial rectification and a default for the reference projection. That didn't work well.

Then, I tried using my best guess as to the reference projection and two (NW and SE) corner points using the automatic method. That didn't work at all. However, when I added the two other corner points, the rectification on automatic worked fairly well although still not perfectly, but close enough for my purposes.

Thanks again!