3 SELECTION OF THE ENDMEMBERS 4 RESULTS
The analysis of the field spectra provides four endmem- Water surfaces and vegetation areas were masked in the
bers which represent the major substrate types of the study image. The results of the spectral unmixing are shown in
area: sand, stone pavement with flintstones, nummulite- Fig. 4 to 6. Four selected areas were checked by ground
covered surfaces and gypsum containing nummulitic truth. In these areas field spectroscopy data were acquired,
limestone (Fig. 3, left). These endmembers have small The spectral unmixing yielded plausible results for the
differences in their spectral signature. Sand mainly consists surface fractions of these areas.
of SiO, that shows no absorption bands and an increasing
reflectance up to 2um wavelength. The spectral signature
of the sand shows weak bands from ferric ions (0.45, 0.6,
0.9um) and the deep band of calcite at 2.3um. The spec-
trum of the stone pavement consisting of sand and flint-
stones has the weak ferric ion bands from the sand and an
overlapping band from opal and calcite at 2.3um. The
nummulite-covered surfaces are very bright. They have
weak ferric ion bands and a strong band due to the large
calcite content. The nummulitic limestone also shows
these bands and in addition strong bands from gypsum at
1.5, 1.8um and in the range from 2.0 to 2.4um.
In the following we show half-tone images corresponding
to the fraction of the various endmembers. The fraction of
shadow (Fig. 4) arises due to the relief and the surface
roughness. On average the four endmembers have a slightly
too small reflectance. Therefore, the fraction of shadow
lies between -2 and -1196. Strongly negative fractions of
shadow indicate areas which are overexposed due to their
particular arrangement with respect to the incident sun
light. These are dunes and slopes or surfaces with very
small surface roughness, e.g. nummulite-covered areas. If
shadow appears over extended areas it is due to the sur-
face roughness. If shadow appears as a line contrast it
Convolution of the spectra with the sensor response func- comes from large variations in the relief.
tion of the LANDSAT-TM (Fig. 3, right) shows that the
spectral differences of the endmembers are preserved in
the broad TM bands (TM1-TM5 and TM7). In the spec-
trum of sand reflectance increases up to TMS and de-
creases for TM7. The reflectance of the stone pavement
increases more strongly up to TMS and decreases strongly
to TM7. The nummulite-covered surfaces show the largest
reflectance and the largest decrease from TM5 to TM7.
The nummulitic limestone is the only endmember for which Areas with large fractions of stone pavement (Fig. 6, top)
reflectance decreases form TM4 to TMS and further on to are found predominantly on top of the escarpments be-
TM7. cause the flint horizons occur in the younger layers. Areas
with a small fraction of stone pavement appear at the
pediments in combination with nummulite-covered surfaces.
Sand has the largest fractions in the whole image (Fig. 5).
Dune belts and individual longitudinal dunes between the
escarpments yield nearly 10096 of sand. Along the slopes
thin ribbons are observed with a 60 - 10096 sand fraction.
These ribbons arise due to an increased deposition of sand
at the foot of the slope. Sand is also contained in most
other pixels, however, with significantly smaller fractions.
For the spectral unmixing TM spectra were determined for
every endmember by selecting pixels in the TM image in
the neighborhood of the field spectrometer measurements
(EIBL, 1995).
nummulite-covered "kj
surfaces
absolute reflectance (%)
absolute reflectance (%)
Cc
o
|
$0 sand
20 (^
i —(Q— stone pavement
7 10 T —E- nummulitic limestone Be
0 : | | T | | T | 0 T | T I T | T I BA in
0.4 0.8 1.2 1.6 2.0 2.4 0.4 0.8 1.2 1.6 20 24 pl
wavelength (um) wavelength (um) er
t
Fig. 3: Spectra (left) of the major substrate types and their TM simulations in the ne
LANDSAT-TM band 1-5 and 7 (right) sh
Stc
me
228
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
