Estimate area from raster different resolution

Question

I am trying to calculate the area that each pixel in the raster has, in order to know the total area per country. This area refers to those pixels equal to 1 which refers to where protected areas in the world should be. According to the authors, they constructed this raster with a resolution of 20km at the equator decreasing it towards the poles. The raster is in GCS_WGS_1984.

I have tried raster statistics with a shapefile of all the countries in the world. I have now the count of pixels equal to 1 and 0 but I do not have a resolution to multiply with.

It can be found here: https://drive.google.com/drive/folders/1jmqbUj95iYCH_jTfoKfi17-J-rJAgq1f?usp=sharing

Protected areas = 1 (blue), not protected = 0 (black)

Protected areas = 1 (blue), not protected = 0 (black)

Answer 1

Instead of dealing with nominal spatial resolution (which is what's commonly used to find pixel area, and is not the same for the entire image), why not polygonize the pixels and let GIS calculate the geometry area for you?

In QGIS, go to Raster -> Convert -> Raster to Polygon, and choose a field name for your values. Then just select the polygons with value of 1, go to Field Calculator -> Create New Field, and enter $area in the expression. The resulting value is your area in m^2.

You can further refine your calculations by clipping the polygonized layer with your countries layer, so you don't get square country borders.