312
rate allows the laser to produce many individual
measurements per single tree and it is thus possible
to get a representative average height and reflection
measurement per tree or plant species.
Figure 1. Laser height and reflection profile of a
forested transect.
The separation of vegetation and terrain surfaces
based on reflection alone is thus very restricted
and this can be seen in Figure 2 where mean reflec
tion values for a number of uniform vegetation sur
faces are portrayed. It is obvious that the poten
tial of laser remote sensing can greatly be enhanced
if the analysis can be extended to include multi-
spectral capabilities.
Figure 2. Vegetation differentiation with laser
reflection measurements at 904 nm wavelength.
2.3 Developing a multispectral airborne laser
Having shown the potential of single wavelength air
borne laser observation it is a logical extension to
suggest that the use of multispectral lasers be con
sidered. Unfortunately no tunable airborne lasers
have yet been developed, but the logical alternative
is to use several lasers simultaneously, where each
type operates at a different wavelength. This raises
the question as to the choice of wavelength to be
used. This is not a simple question in view of the
different requirements needed for the separation of
vegetation type, for differentiations amongst rock
types, and for identifying different soil conditions.
Each user is likely to require reflection measure
ments at different wavelength ranges. For the separ
ation of vegetation types, vegetation stress, and
vegetation vigour the chlorophyll sensitive red/near
infrared reflection bands are likely to be most
appropriate (Tucker 1979, Curran and Milton 1983,
Horler et al 1983, Elridge and Lyon 1984). For
geological applications bands at 600, 1200, 1600 and
2200 nm wavelengths appear to be most desirable
(Siegrist and Schnetzler 1980, Buckingham and Sommer
1983, Gladwell et al 1983, and Elvidge and Lyon
1984). For soil assessments the most useful spectral
bands are also in the visible and near infrared wave
length range (Stoner and Baumgardner 1981) but these
might differ from the geological band depending on
soil conditions.
At the current state of technology the testing of
dual or triple lasers for specific applications is
likely to be most profitable and the potential of
such an approach is illustrated in the following
example which deals with the quantification of soils
to facilitate fertilizer assessment in agricultural
fields.
3 SPECTRAL REFLECTION MEASUREMENTS TO DIFFERENTIATE
SOIL CONDITIONS
Most agricultural fields have great soil variability
not only because soils have a naturally high chemical
variability but most fields do not follow soil type
boundaries and it is very common at least on the
West Coast of Canada that two or three soil types
are present in most agricultural fields. If the
soils are contrasting management is considerably
more difficult. The use of fertilizers in intensive
agriculture is becoming a major component in the
economics of farming and there is an increasing
need to apply fertilizers in a more efficient
manner so as to minimize over and under fertilization
of any part of the field and to overcome production
deficiencies in some parts of the field. It is pro
posed that remote sensing using a multispectral
laser system can aid in this process. To show how
this potential can be realized a number of soil
reflection measurements were made using a multi
channel spectro-radiometer to demonstrate what soil
properties can best be predicted from spectral
reflection measurements, what wavelengths are most
useful in separating soil types, and how this infor
mation will aid in the development of a multispectral
laser.
It is well known that organic matter and soil
provenance has a significant influence on spectral
reflection, and soil mapping with multispectral data
has been attempted by numerous researchers (e.g.
Kristoff et al 1973, Westin and Frazee 1976, Cipra
et al 1980) . The main problem is that vegetation
cover, crop residue, disk pattern, and soil moisture
(Gausman et al 1975 and 1977, Huete et al 1985)
seriously affect the soil reflection spectra. It is
anticipated that the use of a multispectral laser
system will facilitate such analysis since it can
provide an assessment of surface roughness at the
same time the reflection data is obtained.
3.1 Method of analysis
Three experiments were carried out at three different
sites. The first site represented a bare field which
had been plowed and which had a mixture of organic
and marine clay rich soils. Thirty surface soil
samples were collected for laboratory analysis and
the samples were all fine textured and highly
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