Sinkhole volume metric based 3m digital data. TN topo is about 60 sq miles. The heatmap is based on subdividing each topo quad into a 3x3 grid, so each cell is about 6.7 sq miles. Metric is sum of sinkhole volumes in cell. Sutherland's density map based on 3m+ deep sinkholes weighted by volume.

See TN sinkholes