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I'm new in QGIS and recently I have found out how to use its tools for analyzing DEM images. My purpose was to find steep slopes on surface and mark them. Firstly I did Fill Sinks operation. Then used Ruggedness Index to get a slope steepness map. In the end I convert raster image of Ruggedness Index to vector shapefile. An the output file has about 208000 polygons with a coloumn that shows a percantage of slope or a sort of. Finally I wanted to merge these polygons according to percentage to get only three types of slopes (0%-30%; 30%-60%; 60%-99%). But when I tried to use field calculator or dissolve tool it took lots of time to complete the task. What you would recommend me to do? Is there any way to decrease the quantity of polygons or any other tools to do calculations faster?

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Try reclassify your raster into few classes (0-30 etc.) before converting to vector.

Here you can find out how to do that in QGIS How To Perform Raster Reclassification In QGIS

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