Cannot get layers to "line up" in QGIS

Question

I am new to QGIS and I want to overlay an old hand drawn forest map (.png file) over a British OS map (.tif file) of the same forest, so I can digitize the compartments in the old forest map.

I have dragged both files into a new project, and they are in the window, but their respective positions are incredibly far away from each other, they aren't "on top" of one another. I have to click "zoom to layer" to find each layer.

I searched through this website and google, and the solution which always comes up is to make sure they have the same coordinate system. If I right click on the properties of both my layers, they both say "EPSG:7405 - OSGB 1936 / British National Grid - Projected" in the CRS section - so I don't know what else to do. Can anyone help? Thanks in advance.

Answer 1

Try running gdal warp (Processing toolbox > GDAL > Raster projections > Warp). I can't recall exactly, but I think changing the CRS may work in some cases but in others the GDAL warp tool is required.

Another issue you may run into is that your hand drawn map might need to be georeferenced before reprojecting. If it's just an image of a hand drawn map, Q has no points to actually reference where that map exists in your ESPG:7405 projection system. If that is the case, my suggestion is to open up Google Earth, navigate to your study area, and find locations that are easily identifiable (road intersections are ideal). You'll need to create a series of points in QGIS and assign those points the coordinates that you've matched up from Google Earth.

Here's a link to the georeferencing workflow: https://docs.qgis.org/3.10/en/docs/training_manual/forestry/map_georeferencing.html?highlight=georeferencing