The unit is the kilometer and is consistent with what is done in Lucene.

Glad to hear it. From what I see in org.hibernate.search.spatial.impl.Point.getDistanceTo(double, double), the unit used in Lucene is indeed the KM.
So we only have to add this information to the javadoc of org.hibernate.search.engine.ProjectionConstants.SPATIAL_DISTANCE and org.hibernate.search.elasticsearch.ElasticsearchProjectionConstants.SPATIAL_DISTANCE.

The unit is the kilometer and is consistent with what is done in Lucene.

See:

• https://github.com/hibernate/hibernate-search/blob/master/elasticsearch/src/main/java/org/hibernate/search/elasticsearch/impl/ElasticsearchHSQueryImpl.java#L492 : we use arcDistanceInKm

• https://github.com/hibernate/hibernate-search/blob/master/elasticsearch/src/main/java/org/hibernate/search/elasticsearch/client/impl/DistanceSort.java#L46 : we clearly specific the unit of the distance returned in the results by the sort.

HTH