QGIS 3.2 - Poor conversion from high-resolution vector to raster

Question

I have a shapefile with polygons with several sizes, and I want to convert it to a raster with 30mx30m pixel resolution.

Although I was able to obtain the raster with the pretended resolution (using QGIS 3.2,Raster > Conversion > Rasterize), the file I obtained ignores a lot of several small polygons; that is, the polygons are not converted in one or more pixels.

Additionally, there are small polygons converted in a pixel but other larger polygons are not converted. See this example below:

In this case, the pixel is 30x30m, A is ~111 and B is ~1642 square meters.

Questions:

1) Does anyone know how this conversion works, i.e., what are the criteria for converting a polygon in a raster? Using the example in the picture, why was A converted in a pixel and B was not?

2) Is there a way to calibrate/choose the size of the polygons to be considered for conversion?

Answer 1

I believe this is very much related to how the pixel is constructed. Imagine that QGIS or ArcMAP, or any other software creates a virtual grid on top of the polygons with the extent of these. Then it takes the "centroids" to create the pixels.

I did the same with small and medium polygons, of all sort of figures, and those polygons that are thin (like a sausage) do not get converted in the majority of the times.

If you increase the resolution, for instance, 10x10 you may get not only two pixels, but even more.

Answer 2

I would start by testing to see if you have errors in the polygon. Perhaps that is the problem.