I had been given some PDF maps with outlines to digitize, but with enough land features in the map to be able to geo-reference them. I screen-grabbed the maps from the PDFs and saved the screenshots as TIFF. Then, opened QGIS, added the Open Street Maps base layer and zoomed into my area of interest. From there, I used the QGIS georeferencer tool on the first image: https://docs.qgis.org/3.40/en/docs/user_manual/managing_data_source/georeferencer.html
Once I had the image georeferenced onto the map as a layer, I increased the transparency to be able to see through to see how well it lined up with the features on the base map. I then selected “New GeoPackage Layer…” and entered names for the file, layer, field(s) and defined the type (polygon). From there, I traced the boundaries I needed to create polygons that corresponded with the PDF map. In my case, I was going to need to clean up the outer edges with waterway and state boundaries in the end, so I made the edges of my polygons wide past where they would eventually be differenced out.
As I referenced more images, some did not line up great on the first try, so given that this exercise is a bit “interpretive” anyway with hand digitizing (and can be edited) I tried my best to trace a line where it should be. Each time I did that, I was glad my next image lined up with it, or I adjusted the boundary between the zones as needed. Thus, I had a polygon layer with edge boundaries that needed to be clipped down.
For my purpose, I needed…
- the Delaware River: https://www.nj.gov/drbc/basin/map/GIS.html
- the Delaware land boundary: https://www.dgs.udel.edu/datasets/delaware-state-and-county-boundaries
- the NJ counties (to get the land only where I needed it): https://nj.gov/njgin/edata/boundaries/index.html
I used those layers to either clip (DE river) or difference (DE, NJ land shapefiles) to keep only the areas of interest. From there, I had to do some cleanup…
- delete holes from a feature I merged
- split polygons and delete parts to do some finer manual work on getting rid of areas not in the map
- vertex tool to change the shape of a polygon to include a water body that was clipped out