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 
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