I am new in using GIS and I'm having some problems in properly dissolving some polygons. In particular, I have a single layer with about 3,000 polygons (protected areas), some of which overlap (the same geographical site can have multiple designations). I would like to calculate the areal coverage of protected areas, avoiding the overestimation due to multiple designations for the same site. According to what I read on the net, the solution should be to dissolve the polygons in a single one.

I tried with QGIS but it takes a very long time (1 hour for only 1%), so I switched to SAGA GIS: Geoprocessing -> Polygon Dissolve, without selecting attributes (because I want to dissolve all polygons). The process was much quicker but the result is not correct. In fact, in the resulting polygon there are some gaps where protected areas should be.

The error is visible in the picture: on the left the original layer, on the right the result of the operation, with the gaps marked with a red x

Does anyone know how can I solve?

enter image description here

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