GEOMORPHIC MAPPING IN GLACIATED TERRAIN: LIDAR IN THE PUGET LOWLAND, WASHINGTON

High-resolution LIDAR (LIght Distance And Ranging) topography has allowed rapid, accurate remote mapping of surfaces (fluted ground, fossil beach, alluvial flat, landslide, scarp, etc.) on Bainbridge Island, a suburb of Seattle. The high-resolution LIDAR topography:

• Shows 4 types of glacially-striated ground on Bainbridge, only 2 of which were predicted on the basis of prior knowledge
• Allows clear definition of ice-recessional landforms and deposits—and there are very few on Bainbridge
• Shows locally-preserved latest Pleistocene shorelines that may record regional Holocene vertical deformation with sub-meter accuracy
• Permits accurate, consistent inventory of large deep-seated landslides
• Shows Holocene fault scarps beneath timber cover, scarps that are invisible in both 1:24K contours and in aerial photographs (1:12K panchromatic and 1:40K false-color infrared)

This experience suggests several lessons: (1) The Bainbridge LIDAR survey (10⁷ ground points in 71 km², nominal vertical accuracy 15 cm) does not reach the limit of useful topographic detail. Even greater detail would be useful. (2) The LIDAR DEM is more informative than 1:24K color stereo photographs. In part this reflects year-round rainfall and glacial homogenization of surficial deposits that minimize the degree to which vegetation reflects substrate in this terrain. But this also reflects the primacy of topography over reflectance. (3) Topography should be recorded via mass points, not contours. Contours often introduce unwanted artifacts and unrecognized smoothing. Random mass points, and the TIN-derived grid computed from them, contain an internal record of how well a surface has been recorded that is too easily lost with contours. (4) Geomorphic mapping should be inductive, with units arising from efforts to classify and comprehend the landscape of a particular region. Map units deduced from a priori classification of processes and landforms have no place in the scientist’s toolkit.

This mapping is being extended throughout the Lowland as LIDAR data become available.