SEAMLESS GEOLOGIC COMPILATION MAPS AT MULTIPLE SCALES IN VERMONT: APPLIED USE AND THE QUANDARY OF MINIMUM MAPPING UNIT SIZE

The Vermont Geological Survey (VGS) and academic partners from Norwich University and University of Vermont have been collaborating on STATEMAP-funded geologic mapping projects for over 20 years. Following the completion of the 1:100,000-scale Bedrock Geologic Map of Vermont in 2011, the VGS has focused efforts on 1:24,000-scale (or better) surficial mapping to compile a seamless surficial geologic map that will eventually supersede the current statewide Surficial Geologic Map of Vermont, released in 1970 at 1:250,000-scale. Current mapping is produced on 0.7-meter (quality level two; 2 points/m² horizontal resolution) Lidar basemaps, but new mapping will be performed on quality level one (8 points/m² horizontal resolution) Lidar basemaps anticipated in 2023.

The VGS, embedded within the Vermont Agency of Natural Resources, Dept. of Environmental Conservation, has statutory requirements to provide geologic information, expertise, and advice to assist towns, regulatory agencies, and other “clients” in making complex environmental, natural resource, and hazard mitigation decisions. The upcoming increase in Lidar quality will allow geologic mappers to resolve even more features on the landscape surface that are potentially relevant to these decisions; however, it will also introduce new challenges to the map-making process including determining and consistently applying a minimum mapping unit size for use in both applied geologic problems at large scale (< 1:10,000) and seamless, national map compilations at small scale (> 1:100,000). Although digital technology, such as web-based mapping applications or desktop GIS, allows compilers to customize visibility limitations and users to display maps at a wide range of scales, excessive detail in the form of many small polygons slows down the entire geologic mapping process from field mapping to final map review and introduces problems when attempting to edge match map boundaries and portray the geology within a seamless geologic map compilation.

We have recently applied a minimum size of 2,000 m² (~0.5 acres) or greater for artificial fill polygons and are considering a minimum size limit for other surficial materials despite the availability of extremely detailed Lidar-derived basemaps and the temptation to map at the finer resolution that they enable.