I'm working on my bachelor thesis right now and I want to find a catchment area for different bus stops. Is there a function in QGIS where I could use a point layer as input, use ways as distances and I want to determine the distance (so e.g. a busstop has a catchment area that contains 500m). I think in ArcGIS it should work with the Network Analyst Tool. I'm looking for a similar tool on QGIS.
The easiest way (at least for me) was to use GRASS for this. In QGIS you load the road network and the points (bus stops). Open the tool box (Str+Alt+T) next and search for v.clean and v.net.iso. You need to clean the road network first with the v.clean. After that you can use the v.net.iso to calculate the catchment area on the basis of the road network. This does not create isochrones. I found the following tutorial very helpful for that: http://www.northrivergeographic.com/wp-content/uploads/2016/06/Generating-Drive-Times-in-QGIS-2.14-and-GRASS-7.0.4.pdf
Another approach would be to use the Osm route plugin. A short introduction can be viewed here: https://www.youtube.com/watch?v=KMJVltELU4A. With this plugin you can get polygons however you need to do every calculation for every point manually (or write yourself a script).