How-to guide: Add georeferencing to a PDF map

[winnph]

Background

I've been amassing a collection of old geological maps showing various areas of interest around Washington, but I've been disappointed that most of them are not georeferenced (i.e. they can't be used in a mapping app and show your current location on the map). While this is only a minor inconvenience, since I can toggle between a topo map and the geological map, I've been searching for a way to add georeferencing to existing maps. And I've found it!*

 

Software needed:

• QGIS (free & open source): https://www.qgis.org/en/site/forusers/download.html

• Any image editing software that can import pages from a PDF, such as GIMP (free & open source): https://www.gimp.org/downloads/

• Avenza Maps or another map app that can import GeoTiff maps. The free version of Avenza allows only 3 imported maps to be active at a time, or a “Plus” subscription lets you import as many as your device storage will allow: https://www.avenzamaps.com/maps/how-it-works.html

 

Step 1: Converting PDF to TIFF

 

Import the PDF into GIMP using File->Open (or similar process in Photoshop/Acrobat Pro/etc). It will prompt you to pick which page(s), even if there is only one, so click on the map page and import it with the default options.

 

Then, use the "Export" function (File->Export As), and select "TIFF image" from the  drop-down and type a filename (e.g., “Everett_quad_24k.tif”), leaving the default options and choosing "None" for compression. You can then close GIMP and open QGIS.

 

Step 2: Opening the map in the Georeferencer

 

The first time you use QGIS you'll need to enable the Georeferencer plugin by clicking Plugins -> Manage and Install Plugins and checking the box next to "Georeferencer GDAL."

 

Once you've done that, launch the Georeferencer by clicking Raster->Georeferencer->Georeferencer, and then in Georeferencer open your TIFF via File->Open Raster. It will ask you what CRS to use, and unless you're using a map with a coordinate system other than latitude & longitude, select WGS 84 (EPSG:4326). This tutorial covers only lat/long.

 

Step 3: The careful clicking step

 

You now need to use the Add Point tool to add at least 4 and preferably 6-10 points with known latitude/longitude. If your map has a lat/long grid printed on it, this is an easy step (remember to use a minus for west longitude, and note that QGIS puts longitude first, then latitude).

 

If your map has no grid lines, you'll need to pull up Google Maps and look at the same location. Zoom in as much as possible on both maps and click on any landmarks you can identify on both, inputting the coordinates (intersections are good, or the points where streams/rivers meet). They should be spread across your map as evenly as you can manage.

 

Step 4: Saving the map
 

To save your map as a GeoTiff, select Settings->Transformation Settings and use these values:

 

• Transformation type: Polynomial 1

• Resampling method: Lanczos

• Target SRS: WGS 84 (EPSG:4326) (even if your map used another system)

• Output raster: choose a filename, must include the .tif ending (e.g, “Everett_quad_24k_geo.tif”)

• Compression: LZW (or None if you have trouble loading it in your map app)

 

Then select File -> Start Georeferencing, and it will process and output your map.

 

Step 5: Troubleshooting

 

Before you close the Georeferencer, look at the values in the “residual” column for the points you created. If any of these are greater than 2 or 3 pixels, either your map isn't very accurately drawn (is it an older one maybe?) or you didn't click carefully enough on the exact right spots in the last step. If only one or two points are out of the range, delete those points only, create replacement points, and try again. If all of them are slightly out of range, your map is probably distorted a bit but it'll still work OK. If many of them are far out of range (6+), then something went wrong. Delete them all and start over with Step 3, zooming in more or clicking more carefully. If the problem persists, then your map isn't accurately drawn, but it will still give you an approximate location despite being pretty warped looking.

Step 6: Using your map

 

First, put the map on your device, such as by using Google Drive or Dropbox. Beware! If you choose "None" for compression in Step 4, these TIFF files might be fairly large (100+ MB), so make sure your device has enough free space.

 

In the Avenza app, click the “+” button in the bottom right and select “Download or import a map,” then select your map. That's it!

 

Tips:

• You can remove the files from your phone storage once they are imported, to save space, but keep a copy somewhere!
• If you have multiple maps from the same or adjoining areas, put them in a “Collection” in Avenza to smoothly move between them by scrolling or zooming.

 

* This is a simplified version of an outdated post I found on another forum from 2015, but I'm not linking to it since it's on another forum, no longer accurate, and I'm not sure what the rules are for that anyhow. I can edit this post to add the link if anyone wants more details/background on the QGIS software.