Since QGIS doesn't yet import GeoPDF's, I'm first loading them into the GIMP (at around 300 dpi from the PDF), clipping the collar out, and saving them to a LZW-compressed TIFF. Then I'm importing the TIFFs into QGIS and using the Georeferencing plugin to mark the grid points on the map. I've read that the old maps were based on the Clarke 1866 ellipsoid, so in QGIS I'm setting the source projection to NAD27, which is based on Clarke 1866. Yes, I know that technically this isn't fully correct in the cartographic sense, but then again georeferencing old paper maps like this also won't exactly put out a highly accurate GIS product either :P I'm outputting them to WGS84 from the georeferencing plugin. Times on my older Dell E1705 are around 5 minutes doing a polynomial interpolation with Lanczos interpolation. Then again, I remember back in the mid to late 1990's when DRG production at the USGS on the old Data Generals would take a whole lot longer, so I wasn't going to complain :)
The output isn't so bad really. Here's a sample of the output draped on top of Yahoo Street Maps (Google Maps and reprojection on the fly doesn't seem to work so well in QGIS right now).
Setting the Fredericksburg 1889 map to 50 percent transparency in QGIS and zooming in to old town, you can see that the map fits to a modern map surprisingly well.
I'll probably play around with this some more and maybe upload the georeferenced maps to archive.org or something along those lines.
Hi!
ReplyDeleteThank you for the decription!
I've started to import 100k maps to QGIS. I could find the maps of Utah in GeoTIFF files (DRGs), with tfw-file, so the georeferencing was automatic - but the georeferencing is based on meters (http://gis.utah.gov/usgs-topographic-maps-drgs/download-usgs-scanned-topographic-maps).
However I couldn't find similar maps, without collar from Wyoming (only with collar) and Colorado. Do you know where can I find them? Or could you write some detail about the georeferencing?
To make .tfw files (or to import maps) I need the coordinates of the top-left corner of the un-collared map - Do you know where can I find a coordinate converter, to convert the degrees into meters?
Thanks a lot!
If you're talking about the GeoPDF's, you can now use the geopdf2gtiff.pl script from Xastir's latest CVS/SVN code. If you have GDAL installed, you can run it against the GeoPDF and it will clip it to the neatline and output a GeoTIFF.
ReplyDeleteDRG's have georeferencing information embedded inside them so you shouldn't need the .tfw that comes with them. If you want to convert coordinates you can use a website like http://www.earthpoint.us/Convert.aspx.