[Mapping and Assessing Terrain Stability Guidebook Table of Contents]

Detailed terrain and terrain stability mapping (DTSM)

Purpose

Detailed terrain mapping is carried out to collect and present information about the physical characteristics and properties of the land surface and its geologic materials, and to provide detailed interpretive data on terrain stability conditions and soil erosion potential.

A detailed terrain map should form the basis for the preparation of a detailed terrain stability map. Detailed terrain stability maps can be used to identify specific areas that require terrain stability field assessments. They are commonly used in conjunction with other resource information to guide forest development planning. In particular, detailed terrain stability maps help forest planners anticipate and avoid those areas where road construction, trail construction or timber harvesting could cause landslides.

Detailed terrain stability maps are not to be used for making site-specific prescriptions in lieu of a terrain stability field assessment. Nor are they to be used to pre-judge or overrule the conclusions or management recommendations of a qualified registered professional who has made a terrain stability field assessment of a potential problem area. Since DTSM is slope hazard mapping, knowledge of specific proposed forest development or downslope/downstream elements at risk is not needed (unless specific additional interpretations are made).

Recommended specifications

Terrain survey intensity levels for detailed mapping

Detailed terrain stability interpretations will normally be based on terrain mapping conducted at TSIL C, in accordance with the specifications in Table 1. In some circumstances, it may be appropriate to map at TSIL B (e.g., in watersheds or portions of watersheds with very complicated or very hazardous soil and terrain conditions). Mapping costs for TSIL B surveys will be significantly higher than for TSIL C surveys. The move from a TSIL C to a TSIL B survey is a management decision that should be based on, among other factors, consultation with experienced terrain mappers. Levels D or E may be specified for large contiguous inoperable or alpine areas within a DTSM area.

Field-checking should be concentrated in areas with complex terrain and areas of potential instability or high consequence, especially near streams. Notwithstanding this emphasis, the mapper must ensure that a representative sample of all terrain conditions present in the map area has been verified in the field.

Field-checking of detailed terrain mapping usually involves ground-checking 20-50% of the polygons on the conventionally accessible timber base, with a lower intensity of ground checks required in areas of inoperable timber. Field-checking at this intensity typically corresponds to a mapping rate (total map area per field day) of 500-1200 ha per day, depending on the access, the ratio of accessible to inaccessible timber, and the complexity of the terrain. These rates are suitable for experienced mappers. Mappers with less experience will need to spend significantly more time in the field (see Table 1).

The use of helicopters and stationary viewpoints outlined for RTSM is also applicable to DTSM. Experienced mappers often conduct local terrain verification flights at the beginning or end of each field day or will leave aerial verification until they are familiar with the geography of the map area.

The increased use of helicopters to harvest in previously inoperable timber may necessitate a higher intensity of field-checking in these areas than has occurred in the past.

Map scale

All DTSM should be done on 1:15 000 to 1:20 000 scale air-photos. The air-photos should be the most recent available unless older air-photos have better resolution.

The most common map scale for presenting DTSM information for forest management planning is 1:20 000. Terrain and terrain stability maps should be presented at scales comparable to the photos used for the mapping (e.g., 1:20 000 scale maps for 1:15 000 to 1:20 000 scale photos). The scale of the air-photos used for the mapping must be documented on each map.

Occasionally, maps are produced at a significantly larger scale than the air-photos used for the mapping (e.g., polygon boundaries on 1:20 000 scale air-photos are plotted on 1:5 000 scale maps). This practice is not recommended. In these situations, a disclaimer outlining the limitations of the mapping must be prominently displayed on every map.

Topographic base maps should be used for map presentation when available (e.g., 1:20 000 scale TRIM maps with 20-m contour intervals, or privately produced maps at 1:20 000 or larger scale). Enlarged 1:50 000 scale contour maps should not be used.

Minimum map polygon size should be 1 cm2 (4 ha at 1:20 000 scale) except for unusual or critical terrain features (e.g., gullies).


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