Forest Investment Account

Abstract of FIA Project 5030002

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Reliability of SINMAP terrain stability in five watersheds in the North Coast Forest District

Author(s): Butt, Gordon; Madrone Environmental Services Ltd.
Imprint: B.C. : International Forest Products (Interfor), 2003
Subject: Forest Investment Account (FIA), Watershed Management, British Columbia
Series: Forest Investment Account (FIA) - Land Base Investment Program - Innovative


We compared conventional Terrain Stability Mapping (Level C) with SINMAP product in five watersheds in the North Coast Forest District. We conducted the mapping on randomly selected portions of the watersheds, then overlaid our TSM with a provided sheet showing SINMAP polygons. We calculated the total area (and proportion of the map areas) lying within Class IV and V terrain, comparing these numbers between TSM and SINMAP. In general, TSM was more conservative, identifying substantially more unstable terrain than SINMAP in four of the five watersheds. We calculated the Type I error, defined as the areas mapped by TSM as Class IV or V, but as stable by SINMAP. This error was substantial, amounting to, on average, over 50% of the terrain we mapped as Class IV and V. The Type II error, namely the areas mapped by SINMAP as Class IV or V, but not by TSM, were much smaller, except in Pyne Lake. On average, less than 20% of the TSM Class IV and V was delineated in SINMAP as neither IV or V. A total of 663 visual checks done during low-level helicopter passes suggests that roughly 30% of SINMAP Class IV or V are actually stable ground. Two potential sources of error in the SINMAP process may be: 1. the failure to distinguish bedrock slopes that were deemed stable under TSM; and 2. inadequate recognition of terrain processes in SINMAP. Additional iterations of the SINMAP routine, using updated landslide inventory may improve accuracy. Alternatively, reconnaissance level TSM should provide a reasonably cost-effected method of achieving stability mapping objectives over large areas.
[Gordon Butt].

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Updated August 02, 2006 

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