The best open source alternative to QGIS is GRASS GIS, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to QGIS and ten of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to QGIS are Mapserver (Free, Open Source), SAGA GIS (Free, Open Source), MapWindow GIS (Free, Open Source) and uDig (Free, Open Source).
GRASS GIS (Geographic Resources Analysis Support System) is an open source, free software (FOSS) Geographical Information System (GIS) with raster, topological, vector, image processing, and visualization functionality.
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GRASS GIS vs QGIS opinions
pros, cons and recent comments
Equaly or even more powerful. The only downside is that it's native GUI has a learning curve. But it handles the processes way better.
MapWindow is a fully free and open source geographic information system project. It includes an open source programming component, MapWinGIS ActiveX, and an extensible desktop application that supports plug-ins written in .NET (C# and VB.NET).
uDig is a GIS software program produced by a community led by Canadian-based consulting company Refractions Research. It is based on the Eclipse platform and features full layered open source GIS. It is written in Java and released under GNU Lesser General Public License.
Mapnik is a Free Toolkit for developing mapping applications. Above all Mapnik is about making beautiful maps. It is easily extensible and suitable for both desktop and web development. - C++/Python GIS toolkit