Investigation Methods and Procedures - Geotechnical Engineering Investigation - page 30

TABLE 2.5
Elements of Imagery Interpretation

Imagery Feature Imagery Type Interpretation

Topography Satellite images; SLAR: Rock masses as formed or subsequently

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stereo-pairs of aerial deformed have characteristic land

photos and topographic forms ( Chapter 6) as do soil formations

maps classified by mode of deposition or occurrence ( Chapter 7),

which in all cases depend strongly on climate

Slope inclinations and heights are related to material
types in terms of strength and structure

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Slope failures, sinkholes, erosion gullies, etc. have
characteristic forms

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Drainage patterns Satellite images; SLAR: Drainage patterns on a regional and

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and stream forms stereo-pairs of aerial local bases reflect rock type and

photos and topographic variations, rock structure, and where

maps soil cover adequately thick, the soil type (see Tables 6.2

and 7.4 for typical patterns)

Stream form is also related to its geologic environment
(see Section 7.4.1)

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Streams, lakes, and swamps are indicators of the ground
water table, which usually follows the surface at
depressed contours (Section 8.3.1)

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Gully Stereo-pairs of aerial Various soil types have characteristic

characteristics photos (large scale) gulley shapes (see Figure 7.2)

Photo tone B&W aerial photos Tone shows relative ground moisture and texture. Some

general relationships are

White — concrete, or free-draining soils above the water
table

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Light gray — primarily coarse soils with some fines; acid
rocks

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Dull gray — slow-draining soils; basic rocks
Dark gray to black — poor draining soils,
organic soils groundwater near the surface

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Vegetation B&W aerial photos Vegetation varies with climate, geologic material, and land

use. Some relationships between vegetation and soil
types are given in Section 7.8.3
Tree lines often delineate floodplain limits and fault traces

Land use B&W aerial photos Most significant are the locations of man-made fills, cut for

roadways, borrow pits, open-pit mines, and other man-
made features. Development is usually related to landform

Color-enhanced Satellite images Filtered through red: color significance

(false-color) Red normally used for Vegetation — the brighter the red the

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imagery near infrared. Can be healthier is the vegetation

presented in various Water bodies — water absorbs sun's

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colors to enhance rays, clear water shows black. Silt

specific features reflects sun's rays, sedimentation shows light blue

Urban areas — bluish-gray hues

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Multispectral photos or Various filters are used to emphasize the

color IR desired feature (vegetation type, water-body pollution,

thickness of snow field, etc.)

Thermal IR Various filters are used to emphasize a particular feature.

Can delineate water gradients to 1°F

Note : In all color-enhanced imagery, ground truth is required to identify the feature related to a specific

color.