Georeferencing approach plates

I’m taking a crack at quick and dirty geo-referencing of NACO approach plates for pilots. They come as PDF images, ungeoreferenced, but they contain points with precise latitude and longitude you can align by hand. The result won’t be accurate, certainly not for navigation, but maybe it’s good enough. Here’s some notes on what I’ve tried.

  1. Download a PDF plate from NACO.
  2. Convert it to PNG via pdftoppm -r 300 | pnmtopng -comp 9. The result file will be 1613×2476, about 700k.
  3. Fire up Quantum GIS (I’m using 1.5.0). Launch Plugins/Georeferencer/Georeferencer.
  4. File /Open Raster to load your PNG.
  5. Zoom in and add some georeference points via Edit / Add Point. I picked a few intermediate GPS fixes on a GPS approach and looked up their coordinates via fltplan.com. Don’t forget that X is longitude, and is negative. How many points? 2 is minimum, I entered 4. It all depends on what projection the NACO plates uses, and I don’t know what that is.
  6. File / Save GCP Points, to save your hand edited work.
  7. Settings / Transformation Settings. Lots of options I don’t understand. Target SRS is EPSG:4326 for WGS84. LZW compression gets the filesize from 12 megs to 2 megs.
  8. Look at the table of points below the view of your map. The last 3 columns (dX, dY, residual) are some sort of measurement of the error / warping requirements.
  9. File / Start georeferencing to generate the georeferenced TIFF output. Takes a few seconds.
  10. gdalinfo on the resulting .tif file to see whether the result looks reasonable.
  11. Close the georeferencer plugin.
  12. Start a new Quantum GIS project. Settings / Project Properties to set the CRS to EPSG:4326.
  13. Layer / Add Raster Layer to load the georeferenced TIFF. It should show up in the window, with coordinates of lat/long and a scale in the vicinity of 1:300,000.
  14. Add more layers to juxtapose other data on top of the georeferenced plate. State boundaries are helpful for getting oriented, I also overlayed a GPS flight track.
  15. The NACO plate background is RGB 236, 236, 236. You can make that transparent with the Transparency properties of the plate layer, adding a band for just that one RGB value with 100% transparent.

One thing I haven’t figured out how to do in Quantum GIS is draw my plates on top of some reasonable map, like Google terrain maps. I can load Google maps via the OpenLayers plugin but the coordinate system those maps use is something screwy with X/Y in the hundreds of thousands. I’m not even sure Quantum GIS will warp raster layers on the fly, nor have I figured out what target SRS I should be emitting to be in the same format.

My real goal is to end up loading this stuff in Javascript, with Polymaps.

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