Georeferencing old maps unknown datum projection

clivepooleclivepoole Global Mapper UserPosts: 5
Hi

I have a few maps from around the turn of the last century (circa 1890-1910). They have been scanned and I now want to rectify these to Geodetic GDA 94.

I am guessing the maps (one Australian, one German) were produced in some form of conic projection. Lines of longitude converge towards the south.

I previously had a job where the maps were in a known Lambert Conformal Conic projection with known central meridans etc. To do this I used a 2 step process

1. Add this projection to the projections DB (ER Mapper at the time) and warp the map to this projection.
2. Reproject the maps from Lambert Conformal to GDA94.

My question is:

Is this the best approach and should I try something similar with the current maps with an unkown projection. If so can I just guess at and use the Lambert Conformal Conic projection I used previously?

All comments appreciated

Clive

BTW - Great software for the price

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2010
    Clive,

    Yes, I would suggest making a reasonable guess at the projection, then just provide enough control points to get a get match to known data. Once you have it positioned in some projection you can easily reproject to another.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • clivepooleclivepoole Global Mapper User Posts: 5
    edited November 2010
    Hi Mike

    Do you ever sleep. I took this approach and got pretty good results.

    ie went to Lambert Conformal and then reprojected to GDA 94. The only area of concern was around the edges of the map. The grids on the scanned map still converge a bit towards the pole at the edges where I could not select any control points.

    I used a polynomial warp originally to get to Lambert Conformal and then with this new image rectified against a Geodetic grid using triangulation. Does this sound ok?

    Cheers
    Clive
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2010
    Clive,

    That approach sounds ok, but if you can identify some control points around the edges (maybe use the built-in online sources like OpenStreetMap.org or Landsat) you should be able to improve the exterior rectification.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • clivepooleclivepoole Global Mapper User Posts: 5
    edited November 2010
    Hi Mike

    Thanks for your comments.

    Just to let you know that given the state of the old maps I endeed up with a pretty acceptable result (albeit accuracy is probably +-25Km). The maps had quite few visible folds. I actually used a 3 pass system and for the final pass used a modern coastline and also identified a grid around the edges of the map that asissted with the edge distortiion.

    All in all I am very happy with how Global Mapper works. I worked for ER Mapper for 10 years and I found your rectification interface to be superior in many ways.

    Cheers
    Clive
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