Digitizing historical Brazilian map in QGIS3.4

Question

I am trying to overlay a map of a historical boundary (in .tif format) onto a Brazilian shapefile with subnational administrative boundaries. My general question is: what is the most direct (and reliable) way to transform my figure into a polygon onto my shapefile, based on the geographic features?

Answer 1

I assume you want to generate a polygon representing the Quilombo dos Palmares location but your source map is not very clear about the exact location. The legend tell us the shadowed area is Serra instead. Maybe the author intention is to show the location based in the minimap in the lower left side. Anyway, your raster contains only scale and I imagine it is not georeferenced.

The first step to generate a vector map based on the raster is to georeference it to match your brazilian boundaries shapefile. In this sample I use a shapefile from IBGE as basis (obtained here).

To georeference a raster in QGIS (2.18) you need a plugin named GDAL georeferencing. The plugin comes with the QGIS instalation but I had to activate it:

Then you need to use the plugin:

It is needed to define projection/datum, I'm using lat-long WGS84 (EPSG:4326) to match IBGE's shapefile:

Then you need to add the control points, using the map on the screen (your raster have low resolution, so keep in mind the work scale):

The coordinates are showed in the fields:

Keep adding points distributed in the raster, try to cover the extremes and add a central point as the features allows:

Define transformation type, projection and output raster name:

Hit the 'play' button to run the georeferencing. It takes some time, in the 2.18 version there is no bar to watch, wait for a brief warn message in the top (it disappears so fast as appears). The error of each point appears in the table at the bottom (in degrees in this sample):

I organized the layers with the new raster at the bottom and added transparency in the polygon vector layer clicking 2 times at the symbol. The result was this:

You must decide what you want to vectorize from here, if you prefer to vectorize the shadowed areas, I recommend to create a shapefile and do it manually like HarrisonCarto suggested.

I used the automatic raster to vector tool to show the results:

And it is awful:

I recommend to see this documentation if you will create vector data.

Answer 2

Assuming that image is what you want to transform into a polygon, I would import it to your project as a raster, georeference the raster, then trace over it using a polygon creation tool. You can also try using a raster-to-polygon conversion tool (Arc has one), although this is a simple enough shape that tracing might end up being faster and/or easier. The process for doing this will differ depending on what software you're using.