Rocks, Soils, And Man-Made Materials; Experimental Design; Time Of Day / Year; Sampling Strategy - ASD FieldSpec 3 User Manual

www.asdi.com
7.8

Rocks, Soils, and Man-made Materials

7.9

Experimental Design

7.9.1

Time of Day / Year

7.9.2

Sampling Strategy

ASD.Document 600540 Rev. C
The shape of the spectral signature of rocks and soil tends to be invariant with
varying viewing geometry, unlike vegetation. The overall brightness of the
observed spectrum does change with illumination and viewing geometry due
to changes in the amount of shadow in the field-of-view of the spectrometer.
Absorption features in the spectra of rocks and minerals are due to the
presence of specific molecular groups and are often diagnostic of the minerals
present in the sample (Abrams et al., 1877; Hunt, 1980).
The dependence of the optical properties of man-made objects on the viewing
and illumination geometry will fall somewhere between that for vegetation
and rocks/soils. This will depend on the amount of light transmission through
elements of the target and on the surface texture of the object.
The reflectance spectra of objects with translucent elements and highly
textured surfaces have an angular dependence more like vegetation, while
those with opaque elements have an angular dependence more like spectra of
rocks and soils.
The timing of the field data collection is important if field spectra are to be
used to calibrate or interpret a hyperspectral image. Reflectance spectra used
to interpret a hyperspectral image should be collected under illumination
conditions similar to those when the image was acquired.
Time of day and date are the major controlling factors of direct illumination
geometry, while atmospheric conditions relating to scattering and clouds are
the major factors controlling the geometry and relative intensity of indirect
illumination (Curtiss and Ustin, 1988).
If field spectra are to be used to convert a hyperspectral sensor image to
reflectance, spectra should be acquired simultaneously with image
acquisition. If this is not possible, a water vapor correction should be made to
the image prior to its conversion to reflectance (Gao and Goetz, 1990).
The selection of a particular sampling strategy should be based upon the
objectives of the study. If the objective is to determine the detectability of
either a target material or of a process effecting the spectral signature of a
62
Chapter 7 Field Measurements
FieldSpec® 3 User Manual