398
step this map was refined (and partly modified) by
the subsequent soil survey. During the soil survey
the agro-ecological land units were subdivided into
soil units. Due to the small scale each soil unit
usually consists of a complex or an association of
different soils.
The mapping was done on toposheets at a scale of
1:50,000, to which the interpretation results and
the agro-ecological boundaries had been transferred.
4. REMOTE SENSING DATARIAL
Aerial photographs and two types of satellite
imagery were used for the survey work. For the nor
thern and central parts of the area recent black &
white panchromatic aerial photographs at a scale of
1:50,000 were available. These photos were taken in
1982 and are of excellent quality.
For the southern part of the survey area old
(1969-1970) aerial photographs of the same type and
scale, but of poor quality, had to be used.
The satellite imagery consists of six Landsat
Multi-Sp>ectral Scanner (MSS) scenes. Four of the
six scenes were recorded by Landsat V (September /
October 1984). The other two scenes were taken by
Landsat III (November 1982).
Two typies of imagery were produced of each scene:
- standard false-colour composite (FCC),
- colour-enhanced imagery (ratio/IHS encoded
data).
The result was displayed on paper prints at a
scale of 1:250,000.
The ratio/IHS encoding procedure has been des
cribed in detail by Haydn et al. (1982). The abbre
viation 'IHS' refers to the three colour components
intensity (I), hue (H) and saturation (S). Single
band values or band ratio values are assigned to
each of these three components. Summarized and sim
plified, the data transformation works as follows:
- band 7 value is coded into intensity,
- the ratio value band 5 / band 4 is assigned to
the hue (as the ratio value declines from high
to low, the hue changes from red to yellow,
green and purple),
- the ratio value band 5 / band 6 is assigned to
the component saturation (high values result in
saturated colours and vice versa).
As compared with the FCC, the IHS imagery has the
following two main advantages:
- better colour discrimination,
- easier colour interpretability.
An additional advantage for soil mapping is that
in dry areas with little vegetation for several
soils the actual soil colour corresponds to the
colour on the IHS image (e.g. red soils appear red,
yellow soils show up yellow).
A disadvantage of IHS is its inferior discrimina
tion of texture differences (e.g. drainage lines
are generally better visible on FCC).
5. REQUIREMENTS OF REMOTE SENSING DATA FOR SMALL-
SCALE SOIL AND LAND RESOURCE SURVEYS
Three main aspects have to be taken into account
to define the requirements and to assess the use
fulness of different types of remote sensing data
for a specific survey:
- costs (for production and interpretation),
- 'handling' qualities (time and instruments re
quired for interpretation and transfer of the
interpretation results to the base map),
- information content (related to the specific
survey task).
Cost and handling requirements are almost the
same for most types of surveys, i.e. the material
should be cheap and easy to handle. Comparing aeri
al photographs and satellite imagery concerning
these two points, the prevailing advantages of the
latter are generally accepted and need not be dis
cussed here.
The more interesting question is which typo of
remote sensing data supplies the surveyor with the
highest amount of information for his specific sur
vey tasks.
In a land resource survey at a scale of 1:250,000
the main mapping task is the delineation and iden
tification of broad homogeneous units, mainly based
on climate and physiography.
In a small-scale soil survey at a scale 1:100,000
the surveyor has different mapping tasks:
- delineation of soil boundaries,
- identification of the soils in each mapping
unit,
- assessment of the relative portion (percentage)
of each soil within the mapping unit.
The following chapters demonstrate that the uti
lity of different remote sensing data for these
survey tasks depends on many factors and varies
from area to area.
6. GENERAL RESULTS IN DIFFERENT AGRO-ECOLOGICAL
ZONES
The. survey area has been subdivided into thirty-
six agro-ecological land units, grouped into five
major agro-ecological zones. The coding of the five
agro-ecological zones was adopted from an existing
small-scale map for the whole country (Samki 1982),
but the zone boundaries were modified considerably.
Table 2 gives a summary of the most important
characteristics of these five zones.
6.1 Zone 16
Zone 16 covers the warmest and driest part of the
Iringa Region. Part of the zone consists of rocky
hills with Miombo woodland and stony shallow soils.
A great portion of the zone is almost flat and co
vered by an open, degraded, rather uniform Acacia-
thornbush vegetation. Due to the dry climate and
overgrazing there is very little grass cover in the
dry season resulting in much bare soil being expos
ed. In these flat areas heavy dark to yellow brown
sandy clay loams and dark cracking clays are the
prevailing soils.
In this zone the satellite imagery was of much
greater use than the aerial photographs. While the
major boundaries (i.e. the boundaries between the
agro-ecological land units) could easily be mapped
from either typo of remote sensing data, the aerial
photographs were almost useless for any further
subdivision of the units. This was mainly due to
the almost flat relief and the rather homogeneous
vegetation.
Main mapping supports in this zone were the spec
tral characteristics of the exposed bare soil sur
face. These spectral differences are excellently
displayed by the IHS image (see Figure 1 IHS). Due
to high albedo values which reduce the colour con
trasts the spectral differences were much less pro
nounced on the FCC (Figure 1 FCC). Textural differ
ences, however, were more clearly visible on the
FCC. Hence, drainage lines could be better identi
fied on the FCC (Figure 1 FCC), but little use
could be made of this advantage due to the very
uniform drainage system.
Major interpretation problems in this zone were
caused by an area covered with dense bush and some
areas where a veneer of light sand covers a dark
loam subsoil (see Chapters 7 and 8).
6.2 Zone 8
Zone 8 lies about 500 metres higher than zone 16.
Therefore the area is less arid and considerably
cooler, but it still belongs to the drier part of
the Iringa Region.
Characteristic for zone 8 are piediplains with
Table 2.
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Physiogra
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