[Mapping and Assessing Terrain Stability Guidebook Table of Contents]

Map scale

All RTSM should be done on 1:15 000 to 1:40 000 scale air-photos. The scale of photos used will depend on the availability of photos with the requisite scale, density of the forest cover and the complexity and steepness of the terrain being mapped. For example, 1:15 000 to 1:20 000 scale air-photos are typically used in steep, densely forested, coastal watersheds, but 1:40 000 scale photos may be adequate for lightly forested, dry, interior valleys.

The reconnaissance terrain stability maps themselves should be at a scale of 1:20 000 to 1:50 000. Terrain Resource Information Mapping (TRIM) contour maps at a scale of 1:20 000 with 20-m contour intervals can be used as base maps. The RTSM maps must be clearly labelled as portraying reconnaissance-level information.

Minimum map polygon size should be 1 cm2 irrespective of map scale. Exceptions can be made for critical terrain stability features (e.g., steep escarpments and gullies). Average polygon size will be a function of the natural variability, steepness and complexity of the terrain being mapped.

Classification and mapping conventions

RTSM delineates map polygons for all unstable and potentially unstable terrain. It is not necessary to map the stable portions of the landscape. All areas not designated as unstable or potentially unstable are assumed to be stable. Unlike detailed terrain stability mapping, RTSM does not involve producing a terrain map first.

Map polygons should be as homogeneous as possible. Each map polygon should contain a single slope class or terrain type whenever possible. Each map polygon should be labelled with a single terrain stability class (see Table 2). In those unavoidable cases when map polygons contain more than one stability class, the entire polygon must be placed in the most hazardous category that occupies more than 10% of the unit.

Similarly, when the interpretation for a map unit is in doubt, the more hazardous class should be chosen. Because RTSM is less reliable than detailed terrain and terrain stability mapping, a reconnaissance terrain stability map may show a higher percentage of the land as hazardous than will a detailed terrain stability map.

Table 2. Reconnaissance terrain stability classificationa

Reconnaissance terrain stability class

Interpretation

S
does not need to be mapped

  • Stable. There is a negligible to low likelihood of landslide initiation following timber harvesting or road construction.

P

  • Potentially unstable.
  • Expected to contain areas with a moderate likelihood of landslide initiation following timber harvesting or road construction.

U

  • Unstable. Natural landslide scars present.
  • Expected to contain areas where there is a high likelihood of landslide initiation following timber harvesting or road construction.

a This classification is provided to avoid confusion with older Environmentally Sensitive Areas mapping, the 5-class DTSM system and other terrain stability mapping systems used in the past.

For each polygon identified as unstable or potentially unstable, the following information should be recorded on the reconnaissance terrain stability map or on an attached table or legend:

The terrain and slope information should provide some background for the terrain stability classes assigned to each polygon. This information also allows the interpretive assignments to be revised in light of new knowledge about terrain stability conditions, without the area having to be remapped. Interpretive classes alone do not provide this opportunity.

Reconnaissance terrain stability maps must show the location and type of landslides, unstable gullies and other indicators of unstable terrain that are identifiable on air-photos but too small to be mapped as separate polygons. These features can be shown on the map with appropriate symbols.


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