Workflow for Georectifying Large Map Images

The other day, I was trying to figure out what I would do about georectifying images of historic (C19 CE) maps from an Ottoman-era atlas held by the Library of Congress. The TIFF files were too big to upload to MapWarper, my preferred space, and the JPEGs were too low-res, even the large ones. It occurred to me that I have used libtiff‘s tiffcp in the past to reduce TIFF filesize, and a workflow was born. (Though I don’t use Homebrew for package management on my machines, I do have it installed on my home laptop and it made getting libtiff set up easier.)

  • Download 254MB TIFF from https://www.loc.gov/resource/g3200m.gct00235, page 61
  • Install libtiff from Homebrew and process the TIFF using tiffcp. Parameter explanation:
    • -r sets the strips per row. Don’t ask me what this really means, but I got away from an error by setting this.
    • -c sets the kind of compression. Similarly, I’m not real deep on compression algos, but I knew I wanted a JPEG output
    • :75 sets the compression quality. If you’re used to working in a graphics editor GUI
  • brew install libtiff
  • tiffcp -r 16 -c jpeg:75 {/path/to/your/download/file}.tif {/path/to/your/output/file}.jpg

Upload the file to mapwarper.net or your preferred georectification service and rectify, producing the conformed map.

From there, I can use http://mapwarper.net/maps/tile/23528/{z}/{x}/{y}.png in a Leaflet map overlay (which is what I’m going to do).