- determination of the relationships between the geometric
resolution of MOMS-02 data and the dimensions of dune
forms,
- evaluation of the stereoscopic capability of MOMS-02
stereo-images in the studies of dune morphometry,
- determination of the relationship between the dune forms
and factors controlling their formation and development.
4. APPLICATION OF MOMS-02 DATA FOR ANALYSIS
OF SAND DESERT MORPHOLOGY
The selected scenes have been digitally processed on the
Intergraph MicroStation and the processed images were
analysed on the monitor screen. In the case of scene 14,
recorded in mode 1, over the Great Sand Sea, the main
objective was to evaluate and to compare the data recorded in
the high resolution channel 5 (4.5 m) with the images taken in
channels 6 and 7 which have a resolution of 13.5 m. The
images from high resolution channel 5 and one of the off-
nadir channels 6 or 7, were simultaneously projected on the
monitor screen at the same scale. The results of the processing
and enhancement indicated that the maximum scale on the
monitor, with good visibility and interpretability of sand
desert forms is ca 1:8,000 for the images recorded in channel
5 and 1:16,000 for the images acquired in channels 6 and 7.
The images presented on the screen at the smaller scale permit
to analyse the general dune forms and trends, as in the case of
the Landsat data. On the images monitored at larger scales,
small dune forms as well as zibars can be detected in
interdune areas. Owing to the high resolution, the possibility
of detection of the small simple or composite dune forms
exists. In this respect the MOMS-02 data are more
advantageous than other sources of satellite remote sensing
data.
The analysed scene 14 overlays a terrain swath ca 37 km wide
(NS) and ca 110 km long (WE). The images recorded in
channels 6 and 7 cover the same terrain. The images
registered in channel 5 consist of 3 sub-scenes, described as a,
b and c. After geometric matching, these tree sub-scenes
correspond with the same part of terrain surface as the images
from channels 6 and 7. The longer axis of image strip extends
almost W-E direction, from the boundary between As Sarir
and the Great Sand Sea up to the eastern boundary of sand sea.
The different sand dune forms appearing in this part of sand
sea are easly discerend in this scene. On the western border,
linear dunes and their components represented by very open
crescentic dunes are clearly visible. That part of image which
covers the central part of the sand sea, where two trends of
dunes can be distinguished, is very instructive. On the image,
presented on the monitor screen at smaller scale, only the
larger dune forms representing subdued sandridges, are
visible, but after enlargement, the contemporay dune trends,
represented by linear dunes, can also be detected. These linear
dunes are developed in the bi-directional wind regime which
governs this part of sand sea. This is very spectacular part of
the Great Sand Sea, in which the transition process, from one
dune form to another, can be observed. Zibars covering the
interdune areas can also be detected, thus indicating the
strongest winds.
In the northern part of Ramlat Zaltan and particularly in
Ramlat Zaqqut, the MOMS-02 images recorded on scenes 22
and 23 were investigated. These focused on the evaluation of
the interpretability of the different dune forms which appear in
this area (mainly star dunes, crescentic chains and linear
dunes), in comparison with the Metric Camera photographs
and on the evaluation of the stereoscopic capability of th;
stero mode of MOMS-02 system.
The MOMS-02 data were transformed onto the Metric Came,
space photograph No.158 from 1983, which was converted ty
the digital form on the photogrammetric scanner PhotoScg,
PS-1. The data registered in the high resolution channe] § |g
to determine the small linear dunes and crescentic chains as
well as very well displayed zibars. The analysis of these data
led to the establishment of the relationship between dune
forms and zibars and their relationship to the wind regime in
this region. The application of stereo-images led to the
establishment of the relationship of the linear dunes to terrain
topography and their preferencial development along small
terrain depressions. The trend of linear dunes thus recognized
is evidently determined by terrain topography; dune trends are
obviously tailored and adjusted to terrain topography. Such
trends can also be related to the secondary surface winds
affected by terrain topography. Composite linear dunes,
formed by connected, very open, crescentic dunes were also
detected from these high resolution data.
The most spectacular dune forms which are present in the
region of Ramlat Zaqqut are so called star dunes. Owing to the
high resolution and stereoscopic capability of the MOMS-0)
images, it was possible to study different types of star dunes;
single star dunes, twin-star dunes and chains of star dunes,
their connection with other forms and their relationship to the
terrain topography.
5. COMPARATIVE ANALYSIS OF DUNE FORMS
REGISTERED BY METRIC CAMERA IN 1983 AND
MOMS-02 DATA FROM 1993
The space photograph No 158, taken by Metric Camera on 2
December 1983 and the scene 23 acquired by MOM-()
system from the orbit 91 in mode 1/30 in May 1993, were
used for detection of time changes in the dune morphology in
Ramlat Zaqqut. The Metric Camera photograph was converted
into digital form, and then both sets of data were processed on
an Intergraph MicroStation.
The detailed comparative analysis indicated several changes in
dune forms. The new trend of linear dunes, which were not
depicted on Metric Camera photograph can be easy detected
on MOMS-02 scene. This new trend is presumably related to
the changes in the wind regime in this region. Significant
changes were also detected in the forms of star dunes
surrounding the horseshoe-shaped depression in Ramlat
Zaqqut. The changes in star dune arms are quite obvious. The
performed analysis indicated significant dynamics of dune
forms and confirmed the contemporary precesses of dum
development in this part of Libyan Sahara.
6. CONCLUSIONS
The main conclusions, derived from the performed
investigations are as follows:
- The MOMS-02 images fill the gap between the small scale
aerial photographs and the medium resolution satellite data;
- MOMS-02 data due to the high geometric resolution allow o
detect and analyse all types of dune and their relationships;
- The repetative MOMS-02/P data can be used for monitor
of sand desert dynamics and for prediction of future
develoment of desert areas and their influence 0
desertification precesses.
418
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
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