Reconnaissance terrain stability mapping (RTSM)
The primary objective of reconnaissance terrain stability mapping (RTSM) is to identify all unstable or potentially unstable land areas. Reconnaissance terrain stability maps are useful for identifying land areas where more detailed mapping is required. For instance, if a reconnaissance terrain stability map shows limited, widely scattered problem areas, then additional detailed mapping of only those is warranted. On the other hand, if extensive areas with potential stability problems are identified, then detailed mapping over entire watersheds would help to identify the areas where a terrain stability field assessment is required before timber harvesting or road construction can be approved.
Since RTSM is slope hazard mapping, knowledge of specific forest development proposals or downslope/downstream elements at risk is not needed.
A map polygon interpreted as unstable typically shows evidence of natural instability. Unstable areas are expected to have a high likelihood of landslides following timber harvesting or road construction. A map polygon interpreted as potentially unstable may show no overt signs of instability under natural conditions, but have characteristics similar to unstable areas nearby. Potentially unstable areas have a moderate likelihood of landslides following timber harvesting or road construction. A map polygon interpreted as stable delineates an area that is considered to have a negligible or low likelihood of landslides following timber harvesting or road construction (Table 2).
RTSM is usually conducted at TSIL D. Mapping at this level relies primarily on stereoscopic air-photo interpretation, supplemented with limited ground-checking and helicopter reconnaissance. Field-checking should concentrate on those terrain polygons that will be classified as unstable or potentially unstable.
Logistical difficulties will often restrict ground access to many unstable or potentially unstable units. Low-level helicopter verification of mapped unstable areas where tree cover is limited can be quite effective. In addition, frequent landings are recommended so that distinctive terrain features (e.g., steep rock, talus slopes, organic terrain and landslide scars) can be visually verified from stationary viewpoints with the use of field glasses. Helicopters have minimal or no utility for visual verification of terrain units in heavily forested areas. While helicopter and long-distance visual inspections are necessary to compensate for lack of ground access, representative ground checks must be made to calibrate slope gradient estimates and confirm terrain conditions in the map area.