Global Mapper v24.0

Georeferencing historical maps again

wmiswm
wmiswm Global Mapper UserTrusted User
edited September 2014 in Georeferencing Imagery Discussion
I meet another issue, please advise how to proceed. Thank you very much.

Usually, maps should have grid lines printed, and the edges of the maps are usually trimmed at northing easting of given round up values. However, I have some historical maps that the northing easting value is labelled at the edge of the maps only. Further complicating the situation is that the northing easting of the map boundaries are not given and are not of round up values (cut quite arbitrary at any position the producer feel comfortable with). This means, at any point that the northing is given, I do not have the exact easting value, and vice versa. It will also be very difficult to locate existing GCPs due to changes of landscape with development, which means, I still have to rely on georeference information printed on the maps.

I can imagine that, by using photoshop, I can draw lines joining the northing easting labels from one side of the map to the other edge, then I can produce a grid, and I can then use the grid for georeferencing. The problem is, then, I will have no means to remove the grid after georeferencing, and I will have introduced permanent artifact into the map.

Alternatively, I can also shrink the map to minimal size and use the usual GM georeferencing tool to try to locate the position where the northing and easting labels cross. But then, there are two major problems with this method. The first is that if the maps are shrunken to minimal size, the estimation of northing easting cannot be accurate. And also, the maps are usually distorted due to paper distortion and scanner distortion, the northing and easting labels at the edge will usually be shifted somewhere from the exact vertical and horizontal positions.


I wish to understand, is it possible to georeference using Ground Control Lines (GCL)? I can position both ends of the GCL accurately, meaning that along the lines, I can have either the accurate northing or easing values, but not both at the same time to form a point.

Any comment?

Comments

  • Mykle
    Mykle Global Mapper User Trusted User
    edited June 2014
    You can make a copy of your map file. Draw grid lines on one of the files, without changing the map in any other way. Georeference the gridded map, then save a workspace file (GMW) and close Global Mapper. Change the name of the UNGRIDDED file to the name you used for your gridded file (after renaming the gridded file to something else). Open the workspace file (GMW) and you should have a georeferenced and ungridded file being displayed. In Global Mapper, turn on the grid display and verify that it is georeferenced properly.

    Another way to swap filenames is to edit the GMW file (it is just text) and change the name of the gridded file to the name of the ungridded file. Save and load the workspace file with Global Mapper.

    Again, do not change the gridded copy of your map in any other way (same format and dimensions). Once you have swapped filenames and your ungridded map is loaded in Global Mapper, then you can make any changes that you like.
  • wmiswm
    wmiswm Global Mapper User Trusted User
    edited June 2014
    Thank you very much for the reply. Can drawing the grid lines be done in Global Mapper and output to tiff? Drawing exact line in Photoshop is not that straight forward.
  • wmiswm
    wmiswm Global Mapper User Trusted User
    edited June 2014
    I tried to draw the grid line in GM, but find no way to export the resulting map with grid as tiff; exporting the raster will result in the original raster image without the drawn grid. Is there any trick to do this?
  • Mykle
    Mykle Global Mapper User Trusted User
    edited June 2014
    The trick is on the export GeoTIFF dialog. Check the option to include vector data.

    That assumes that you are drawing your grid as lines. You will want to export two copies of your raster file, one with vector data, and one without. When you georeference your gridded file, you will want to save your workspace GMW file. Then exit Global Mapper, edit your GMW file, and replace the filename of your gridded image with the filename of your ungridded copy. Then open Global Mapper and load the workspace file (you should be able to double-click on the GMW file to open GM and load your ungridded image).
    Mykle
  • wmiswm
    wmiswm Global Mapper User Trusted User
    edited June 2014
    get it, thank you very much.
  • Abbyabi098
    Abbyabi098 Global Mapper User
    edited September 2014
    Drawing lines can be in Mapper, hope this will help you.
    Georeferencing historic maps results in a number of important advantages for libraries..
    Through georeferencing, early maps can be visualised through better or more familiar interfaces. Georeferenced maps can be easily integrated with Google Maps, Google Earth and related web-mapping APIs. This takes advantage of the wide familiarity with these search tools and their user interfaces. It is also possible to allow historical maps to be delivered into other websites through using open standards

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