LIDAR MAPPING OF SINKHOLES IN WINONA COUNTY, MN

Sinkhole mapping in Winona County, MN has been an ongoing effort for twenty-five years (Dalgleish, 1985; Magdalene, 1995; Gao, 2002). The resulting data is routinely used in permitting and zoning issues. As of early 2009 the effort had identified and inventoried 663 sinkholes in Winona County. Over half of those had either been filled when inventoried or were filled thereafter. The locations of both the filled and unfilled sinkholes had varying, significant uncertainty associated with them. The original locations had been based on field observations recorded on USGS 7½ minute topographic sheets. The majority of the sinkholes were not visible on the topo sheets. One meter LiDAR data for Winona County became available in July 2009 and the shaded relief DEMs, in a GIS environment, allow us to verify the locations of the open inventoried sinkholes and to locate additional sinkholes that were missed or have opened since the original surveys.

232 (35%) of the inventoried sinkholes could be located on the LiDAR DEM images as identifiable depressions. 63 (27%) of the sinkhole locations did not need to be adjusted. 169 (73%) of the sinkhole locations were adjusted to more accurate locations. The GIS environment allows us to add air photo information to the mapping from online sources such as Google Earth and Bing Map. Google Earth shows aerial photos from several different years. Bing Maps has low angle, low altitude pictometric photos from several directions with very high resolution.

422 (64%) of the inventoried sinkholes could not be located on the LiDAR DEM images. Most of these have been filled to return the land to agricultural production. Many of these filled sinkholes are still visible in the aerial photos as areas of contrasting soil moisture content due to a thicker soil profile over the filled sinkholes.

Mapping is not yet complete but hundreds of previously unmapped sinkholes are visible on the LiDAR DEMs. Field checks of as many of these new features as possible will be critical to their identification as sinkholes.

Sinkholes are ephemeral features in this karst landscape. They are routinely filled by the land owners. While filling a sinkhole may render it invisible at the surface, the underlying karst conduit systems remain and present a host of challenges to environmental management.