spatial subsampling

https://arxiv.org/pdf/math/0503671

https://pro.arcgis.com/en/pro-app/help/analysis/geostatistical-analyst/an-introduction-to-sampling-monitoring-networks.htm

https://gis.stackexchange.com/questions/52365/creating-a-subset-of-points-shapefile-with-minimum-distance-between-points-in-ar

https://gis.stackexchange.com/questions/21616/select-maximum-number-of-points-more-than-x-meters-apart

https://desktop.arcgis.com/en/arcmap/10.3/tools/analysis-toolbox/near.htm

http://www.spatialecology.com/gme/

https://cran.r-project.org/web/packages/spatialEco/spatialEco.pdf