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The JTS Topology Suite is a Java library for creating and manipulating vector geometry.
GeoMesa is a suite of tools for working with big geo-spatial data in a distributed fashion.
Geospatial Raster support for Spark DataFrames
GeoWave provides geospatial and temporal indexing on top of Accumulo, HBase, BigTable, Cassandra, and DynamoDB
GeoTrellis is a geographic data processing engine for high performance applications.
Java port of the Proj.4 library for coordinate reprojection
uDig parent project containing all core components. More plugins can be found in community repos: http://github.com/uDig-Community
LocationTech Spatial4j: A Geospatial Library for Java
Geoff project at LocationTech
LocationTech SFCurve is a Scala library for the creation, transformation, and querying of space-filling curves