Full text: Remote sensing for resources development and environmental management (Volume 1)

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
Spectral signatures of soils and terrain conditions 
using lasers and spectrometers 
Department of Soil Science, University of British Columbia, Vancouver, B.C., Canada 
ABSTRACT: A pulsed infrared laser system was flown over a number of test sites and the results showed that 
high resolution data on terrain and vegetation height, and surface reflection can be obtained simultaneously. 
With this combined data set it is possible to examine the effect of surface roughness on spectral reflection 
measurements and this greatly facilitates terrain and vegetation assessments. To explore the development 
of multispectral lasers ground based reflection measurements were made with a spectrometer using three 
different sets of soil samples. The results showed that % carbon, % iron and % sand content were significantly 
correlated with spectral reflection measurements at 550, 630 and 1600 nm wavelengths respectively. However, 
these relationships are not universal and are only applicable on a site specific basis. Organic carbon could 
only be predicted with confidence from soils originating from a field where the organic carbon content was 
highly variable and greater than 2.0%. In a different sample set where organic carbon content was below 2% 
the % sand variability could be predicted. Finally, a third set of samples, originating from mine tailings 
having no carbon present and high iron variability, showed significant relationships with reflection at 630 nm 
wavelength. These results suggest that if we are interested in quantifying soil fertility conditions the 
development and use of a multispectral laser might provide a new dimension to remote sensing since it would 
provide both essential reflection and surface roughness assessments at the same time. 
Surface roughness has a significant influence on de 
tailed spectral reflection measurements and until 
recently this subject has been largely ignored 
because of the difficulties in measuring roughness 
by remote means. With the introduction of airborne 
lasers it is now possible to not only measure the 
terrain surface reflection but also to quantify 
surface roughness or height variations. As shown by 
Schreier et al (1984), Krabill et al (1984) and 
Schreier et al (1985) terrain height variation can be 
determined with a pulsed airborne laser reaching 
vertical accuracies of better than 20 cm and a newer 
model laser as indicated by Jepsky (1986) has shown 
even greater accuracy. In this way tree height 
measurements and tracings of the canopy of individual 
trees is readily possible. The laser generates an 
active energy source and laser amplitude or reflec 
tion measurements can be carried out simultaneously 
with the height measurements and this provides a new 
dimension in detailed terrain assessments by remote 
sensing. The surface roughness or height component 
is of particular interest in vegetation studies which 
involve biomass determinations, species identifica 
tions, nutrient deficiency assessment through foliage 
analysis, and assessment of toxicity or plant stress 
for geochemical prospecting. Another application 
where the assessment of roughness is of importance is 
in the analysis of soil fertility where the surface 
structure and cultivation pattern at the soil surface 
has a profound influence on reflection. 
Current lasers operate at single narrow wavelength 
bands and in order to fully exploit this technology 
multispectral laser capabilities need to be explored. 
It is the aim of this paper to first document the 
results of airborne laser test flights so as to 
emphasize the laser capabilities at a single wave 
length band. The second aim is to provide background 
information for spectral properties which are essen 
tial for the development, application and use of 
multispectral airborne lasers. Examples from the 
airborne laser test flights focus on vegetation 
applications, while the quantification of soil types 
for fertilizer assessments is emphasized in the 
second aim. 
2.1 Description of laser system 
A gallium arsenide laser built by Associated Controls 
and Communication Inc. (ACCI) was used in this study 
The system operates at 904 nm wavelength, has a 
pulse rate of 2000 pps and a peak power output of 
80 watts. In order to provide optimum ground 
coverage and to facilitate data verification an 
inertial navigation system, a photogrammetric camera, 
and an airborne data acquisition system were inter 
linked with the laser. The footprint of the laser 
covered an area of 50 x 50 cm on the ground. 
2.2 Terrain assessment with the airborne laser 
The system was tested for height and reflection 
accuracy over the National Research Council photo 
grammetric test site in Sudbury and the research 
forest at the Petawawa National Forestry Institute. 
As reported elsewhere (Schreier et al 1984) average 
height accuracies between 10 and 24 cm were obtained 
by the airborne laser. A wide range of vegetation 
could be differentiated on the basis of reflection 
measurements alone and this in spite of the limita 
tion of using a single wavelength frequency. 
An example of the dual capability of height and 
reflection measurement is provided in Figure 1 which 
shows a height and reflection profile of a mixed 
forest transect at the Petawawa research station. 
As indicated in Figure 1 grass, broadleaf, and 
coniferous trees could be separated by reflection 
measurements alone but the height profile clearly 
facilitates the interpretations since some broadleaf 
trees reflect the near infrared energy at a rate 
similar to some of the understory vegetation. An 
additional component which helps in the interpreta 
tion is the fact that the laser penetrates the coni 
fer tree canopy much more frequently than the broad 
leaf tree canopy. This gives a more spiky tree 
profile for spruce and pine trees and a more rounded 
profile for maple and poplar trees which in fact 
reflects the tree structure. Using a very high pulse

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