QGIS

QGIS is strikingly similar to ArcGIS and is without a doubt the #1 free GIS software package available. From automating map production, processing geospatial data and creating stunning cartographic figures, QGIS is jam-packed with a plethora of powerful tools. QGIS version 3 recently came out and its functionality, stability, and UI make it a powerful alternative to ArcGIS. QGIS is also cross-platform, and can be installed on a Mac.

Read this review of QGIS3 from gisgeography.com

Download QGIS 3 from here

 

PostGIS

PostGIS is a spatial database extender for PostgreSQL object-relational database. It adds support for geographic objects allowing location queries to be run in SQL and is a powerful spatial anayltics tool. Because you are directly accessing your data and its geometry, processing is incredibly fast.  Read some of these data testing results on a “Big Data” dataset comparing processing speeds on several different platforms and see how fast PostGIS can be.

Learn more about PostGIS here

 

Whitebox GAT

Whitebox Geospatial Analysis Toolbox is another open-source GIS and remote-sensing software package packed with over 410 tools to clip, convert, analyze, buffer, manage and extract geospatial information.

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