lergy from the
an antenna,
t cloud cover
and weather
signal film
r returns in
:ed to produce
ith covers an
'al resolution
as in range
geometry.
lal from the
of the radar
netry and the
ric constant,
station cover,
is generally
of magnitude,
f, i.e. image
).
neasure of the
naterial upon
Variations in
determined by
rock, soil or
itions signal
considerable
discontinuity
occurs. The
, however, is
hy and surface
defined by the
ation geometry
look direction
ge signatures.
at an angle
ult in radar
t displacement
radar antenna
radar signal
feature first
irregularities
res, and is
ncidence angle
IR-A) a flat
face roughness
e scattering,
ngle equal and
The resulting
:es with an
1.3 cm disperse
:ering response
the image. A
ical relief is
elatively high
amount of radar backscatter is received by the radar
antenna resulting in light grey signatures on the
image.
Vegetated areas appear rough and hence in bright
tone on the SIR-A image. They are considered as
extremely diffuse scattering surfaces due to their
complex geometric characteristics, i.e. plant
physionomy, but also because of their electrical
properties. Geometrical characteristics which
influence the radar backscatter include roughness and
discontinuities at the plant canopy itself and within
the volume, e.g. foliage and branches. The
electrical properties of vegetation are a function of
the target permittivity and conductivity, i.e. the
dielectric constant of the biomass.
5 IMAGE INTERPRETATION
The SIR-A image analysis of western Xinjiang (Plate
1) focused on the following features: physiographic
regions, water supply, and land use patterns. A
visual interpretation method was chosen to assess
such image elements as tone, texture and geographical
context. Thematic maps were produced at an original
scale of 1:500,000 using various transparency
overlays. Locations mentioned in the text, e.g.
(A-7), refer to Plate 1, Figure 4 and Figure 5.
The analysis was supported by ground observations
made during a field trip in October 1984, by data
extracted from topographical and geological maps
(0NC, G-7, 1:1,000,000 and Geological Map of China,
1:2,500,000), and by material presented in the
literature.
Table 1. SIR-A image interpretation chart.
PHYSIOGRAPHIC REGIONS
& Terrain Categories
IMAGE
TONE
IMAGE
TEXTURE
BOUNDARY
" RELIEF 2 '
Impression
DRAINAGE 3 ' TECTONIC
pattern/density CONTROL
1 SANDY DESERT
dark
smooth
+/-
internal
nona
2 ALLUVIAL PLAIN
medium
fine-grain
+/-
parallel (1)
moderate
Floodplains
dark-grey
smooth
+
braided (1)
moderate
Sand Dunes
dark
smooth
+
internal
n.a.
3 ALLUVIAL FANS
medium
fine
+
distrib. (m)
none
Floodplains
light-grey
fine-grain
+
--
distrib. (h)
none
Old Terraces
dark
smooth
+ +
+
parallel (m)
none
4 DENUD. MOUNTAIN
& HILL REGION
medium/
light-grey
fine
+
+
dendritic/
parallel (h)
high
5 EROSIONAL HIGH
MOUNTAIN REGION
dark-grey/
light-grey
coarse
+
+ +
sub-parallel/
dendritic (m)
moderate
6 NIVAL-ALPINE
REGION
dark/light
coarse
+ +
+ +
sub-parallel/
dendritic (1)
moderate
Snovfields
light
fine-grain
-
-
n.a.
n.a.
Glaciers
med./light fine-grain
+
+/-
n.a.
n.a.
''Boundary:
+ + ■ very sharp
^Relief Impression:
+ + ■ very good
¿ \
^Drainage density:
h - high
m “ medium
— - ■ transitional - - - none
n.a. - not applicable
5.1 Physiographic Regions
On the SIR-A image a number of terrain categories and
various sub-categories may be recognized (Table 1)
and grouped into six major natural physiographic
regions (Figure 4). These regions are oriented in a
SW-NE direction. A distinct boundary divides the
moderately to highly reliefed uplands of the Pamirs
and the lowlands of the Tarim Basin.
A uniform dark tone and a smooth texture in the SE
part of the study area characterize the sandy desert
as a marginal part of the Taklimakan Desert. Dune
formation or drainage channels can not be identified,
indicating that the incident radar energy is largely
absorbed by a continuous cover of loose, dry sand.
As a result the backscatter signal is very low. The
ragged boundary towards the alluvial plain is
partially transitional.
Medium grey tones and a fine granular texture are
the dominant image characteristics of the alluvial
plain. Clay, silt and loess deposits which are in
part deformed by wind erosion contribute to moderate
radar backscattering. Low density and sub-parallel
drainage channel sections appear as irregular dark
and dark-grey bands (H-10). During the dry season
most of the active river channels do not contain
water. Their fine-grain sediments and smooth surface
morphology produce low backscatter signals. Sporadic
alignments of light grey tone and crescent shapes
(F-6/7) delineate cliffs of fluvial terraces which
act as corner reflectors when oriented towards the
radar. The drainage network in the alluvial plain is
generally oriented in a E to ESE direction. Small
elongated streaks of dark-grey tone mark the present
position of various groups of sand dunes (D-4). The
dunes are almost exclusively oriented in a SE
direction which corresponds well with the prevailing
regional wind direction. The most outstanding
features within this alluvial plain are linear and
rectangualar outlines of the oases. These are dealt
with in a seperate section.
Along the base of the Pamirs various shades of grey
tone as well as smooth and fine-grain image texture
suggest different radar backscatter intensities
within a zone of interlocked alluvial fans. Their
individual delta-shaped outlines and the distributary
pattern of the drainage channels are the principal
recognition elements on the SIR-A image.
Medium-grey, dark-grey and dark tones allow for
further subdivision into active channels (G-14),
Pleistocene fan deposits (E-13), and older eroded
fluvial and/or fluvio-glacial sediments (1-21),
respectively. The different grey tone levels
probably account for progressive accumulation of
aeolian sand or loess deposits on the geneally coarse
gravel fans. At the funnel-shaped head of the fans,
Quaternary fluvial and fluvio-glacial terraces are
frequently undercut by stream erosion during the
meltwater season (F-18). The two largest fans within
the study area extend over an area of 360 km 2 (F-14)
and 380 km 2 (H-16). Gravel bodies such as these have
an excellent water storage capacity. The allocation
of oases along the foot of the alluvial fans is a
clear indicator of the present ground water horizon.
The denudational mountain and hill region
represents terrain which is highly dissected by a
sub-parallel and sub-dendritic drainage pattern.
Numerous small valley sections are oriented toward
the radar and produce an overall light-grey tone and
a fine image texture. Partial coverage of aeolian
sediments and the low resistance of the underlying
material result in intensive erosion and high density
drainage (D-17/18). Tracts of elongated ridges
represent more resistant lithologic units (F-17) or
hint at folded sedimentary rock structures (B-15).
Various linear and partly angular features between
this upland area and the lowlands indicate a high
degree of tectonic control by a major NW oriented
boundary fault (A-14/ H-21) which suffered lateral
displacement (G-19).
In the erosional high mountain region, topographic
relief is expressed in light grey tone where slopes
are facing the radar look direction. Slopes oriented
away from the sensor appear dark. The moderate to
coarse image texture is a function of drainage
density which in turn reflects a higher resistance of
lithology to erosion compared to the previous region.
Crestlines of ridges are irregular and sharp in the
eastern portion of the mountain area (G-22/23),
whereas higher uniformity charcterizes the Sarikol
Mountains (D-28). Accumulation of Pleistocene
fluvial and fluvio-glacial sediments occurs in a
major NNW oriented intermontane basin (D-26). Its
partly dissected terrain has low relief, giving
smooth image texture and dark grey tone against which
the dark outlines of a lake (D-26/27) barely
contrast. Recent floodplains (C-26) appear in
lighter tones and a fine granular texture. The NW
extension of this tectonically controlled basin, or
graben system, suffered lateral displacement in the
