Although LANDSAT images of our planet represent a quantum improvement in the availability of a global image-data set for independent or comparative regional geomorphic studies of landforms, such images have several limitations which restrict their suitability for quantitative geomorphic investigations. The three most serious deficiencies are: (1) photogrammetric inaccuracies, (2) two-dimensional nature of the data, and (3) spatial resolution. These deficiencies are discussed, as well as the use of stereoscopic images and laser altimeter data

Williams, R. S., Jr.

English

NASA Technical Reports Server

QUANTITATIVE GEOMORPHOLOGIC
STUDIES FROM SPACEBORNE PLATFORMS*
Richard S. Williams, 3r.
U.S. Geological Survey
Reston,Virginia22092
Summary
The five spacecraft in the Landsat series, the first of which was launched on 23
3uly 1972, provided geomorphologists with their first access to a complete set of
medium-resolution, multispectral images of most of the land and shallow-sea areas
of the Earth. These revolutionary new data about our planet provide a valuable
source of information on landforms and geomorphic processes to those scientists
involved in comparative geomorphic studies of landforms on a global basis. One of
the first geologists to take advantage of this new data source was Ed McKee and
his colleagues in U.S. Geological Survey (USGS) Professional Paper I052 (1979),
entitled "A study of global sand seas." Another global study, entitled "Satellite
image atlas of glaciers," in which Landsat images are the principal source of data,
is in preparation by the author and more than 50 other scientists.
An important aspect of the Landsat image to geomorphologists is the fact that
many areas of Earth now have multiple sets of coverage spanning a 13-year
interval. For those geomorphic processes that produce areal changes resolvable on
Landsat images, quantitative, two-dimensional measurements can be made.
Sequential Landsat images have been used to determine the velocity of outlet
glaciers, calculate changes in area of ice caps, delineate changes in areal extent of
barrier beaches and islands following severe storms, determine the areal extent of
new lava flows and deposits of tephra, and so on.
Although Landsat images of our planet represent a quantum improvement in the
availability of a global image-data set for independent or comparative regional
geomorphic studies of landforms, such images have several limitations which
restrict their suitability for quantitative geomorphic investigations. The three
most serious deficiencies are: (I) photogrammetric inaccuracies, (2) two-dimen-
sional nature of the data, and (3) spatial resolution.
Landsat is not a mapping system in the strict sense, although Landsat images have
been used to produce maps of many areas, either as a single image or as image-
mosaic maps. Landsat image-maps significantly improve in our knowledge of
poorly mapped regions. To serve as a true map, however, Landsat images must be
related to the figure of the Earth, either by geometrically correcting the data or
by fitting a recognized geomorphic grid to the data. Such a grid uses identifiable
features on the image, whose geographic locations are known from independent
geodetic surveys, for geometric and geographic control. Even with independent
geodetic control, most Landsat image maps have inaccuracies in position of I00 m
or more in well mapped areas and I krn or more in poorly mapped areas. Although
some geomorphological studies only require measurements of relative change, such
as measurement of the fluctuation in termini of a glacier, many studies require
precise geographic location (x,y or 16,}0.
*Discussion commentary for Theme V, Other Themes in Mega-Geomorphology,
Workshop-Conference on Global Mega-Geomorphology, Oracle Conference Center,
Arizona, 13-17 3anuary 1985, which was sponsored by the National Aeronautics and
Space Administration and the International Union of Geological Sciences.
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https://ntrs.nasa.gov/search.jsp?R=19850024068 2020-03-20T18:09:40+00:00Z
Most landforms have a unique three=dimensional character. However) Landsat
images (and most other space images and photographs available to the scientific
community) can only be used in geomorphic studies of a two-dimensional (x)y)
nature) and then only in a relative sense because of a lack of precise geographic
position. Future geomorphic studies of the Earthts subaerial landforrns will require
satellite data that can be analyzed stereoscopically. The third dimension) z) is the
missing element in quantitative regional and global geomorphic studies.
The other limiting factor of Landsat images) and most other satellite images and
photographs) is the inadequate spatial resolution for many types of geomorphic
studies. The Landsat I-5 multispectral scanner (MSS) and Landsat I and 2 return
beam vidicon (RBV) images have a picture element (pixel) size of 79 m; the Landsat
3 RBV and Landsat t_and 5 thematic mapper (TM) images have a pixel size of about
30 m. The standard photogrammetric definition of resolution is "the minimum
distance between adjacent features) or the minimum size of a feature9 which can
be detected by remote sensing. For photography, this distance is usually expressed
in lines (line pairs) per millimeter recorded on a particular film under specified
conditions" (Manual of Photogrammetry_ American Society of Photogrammetry)
fourth edition) 1980). An approximate way to convert from pixel size to
photographic resolution is to multiply the pixel dimension by 2.8; hence Landsat
MSS and RBV (Landsat I and 2) images have a theoretical photographic resolution
of 221 m; Landsat 3 RBV and Landsat 4 and 5 TM images have a resolution of 8t_m.
This means that only large landforms and geomorphic processes that produce
significant change will be recorded on Landsat images. Unless a geomorphologist
knows an area well through field work or analysis of large-scale aerial photographs
s/he may not be aware_ or even suspect_ that important geomorphic features are
present or that geomorphic processes have left their mark. Astrogeologists have a
similar problem in their analysis of landforms on other planets and moons. My
geomorphic studies of Icelandic volcanoes) for example_ provide an insight into the
problem. Of the 25 discrete types of subaerial volcanoes in Iceland, only 7 types of
volcanoes can be unambiguously identified on Landsat MSS images. The other 18
are "missing") either because of the lack of stereoscopy (third dimension) or
because of inadequate spatial resolution or a combination of both. Except in a
gross sensep Landsat MSS images cannot be used to study Iceland's volcanoes.
Although the increase in resolution of the Landsat 3 RBV image adds three more
types of volcanoes) the combination of still inadequate spatial resolution and non-
stereoscopic nature of the data still limits the usefulness of the Landsat image for
such geomorphic studies. Ed McKee's research on sand seas was successful)
because dunes or dune groups had areal and(or) linear dimensions large enough to be
resolved spatially on the Landsat image. The third dimension was not that critical;
the x-y geometric configuration of dunes and dune groups was more important.
This is one of the reasons that we have been analyzing Landsat images of glaciers.
Although the third dimension (z) is very important to glaciological studies_
information on the x-y geometric configuration of ice sheets) ice caps) and tidal,
valley_ and outlet glaciers is also valuable. Measurements of area) changes in area)
and fluctuations of glacier termini can also be determined from two=dimensional
satellite images) such as Landsat.
To effectively carry out regional and global quantitative geomorphologic studies of
the landforms of the Earth, it is necessary to develop specialized satellites which
will carry optimum instrumentation. The two most important systems needed are:
(I) an imaging instrument capable of acquiring photogrammetrically accurate
stereoscopic images of the Earth's surface) with a ground resolution of _-I0 m per
line pair) adequate for l:50_000-scale topographic mapping (at a scale of 1:509000))
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and (2) a laser altimeter capable of measuring elevation differences of __10 cm.
Both satellite systems will need the type of positional accuracy achievable with the
Global Positioning System (GPS) (projected to be a few meters in x_ Y9 or z) and
with accurate attitude control of the spacecraft achievable with stellar sensors.
Both the stereoscopic imager and the laser altimeter need to be operated for at
least 10 years to build up a basic global set of cloudfree data. After 10 years the
stereoscopic imager should be operated repetitively to acquire new data sets for
comparative studies of changes in landforms. The laser altimeter should also be
operated after 10 years to support similar types of studies_ such as studies of
ground subsidence caused by groundwater withdrawal_ ground subsidence or eleva-
tion caused by tectonic forces_ inflation and deflation of volcanic calderas caused
by intrusive and extrusive volcanic activity_ changes in elevation of ice sheets and
ice caps9 and so on.
The new black-and-white_ color_ and color-infrared photographs from the Large
Format Camera (LFC)_ which was flown in October 19g_ on the Space Shuttle (STS
Mission 41-G)_ provide superb examples to geomorphologists of the type of satellite
data that can satisfy their needs. Frederick 3. Doyle of the U.S. Geological
Survey (written communication_ 1995)_ stated that a stereo model of LFC photo-
graphs covers about 170 by 170 krn with a ground resolution of -" I0 m (high
resolution black and white film) and has an internal photogrammetric precision of
_-5 m in x_ y_ and z coordinates. By orienting the stereo model to 3 or _ ground
control points (GCP's) an accuracy of about 8 m can be obtained in geographic
position and terrain elevation. For comparison_ the internal geometric accuracy of
the thematic mapper (TM) image is 30 mr using about 20 GCP's. In future
operations of the LFC with the Global Positioning System (GPS) and an electro-
optical_ stellar-attitude sensor_ the stereo model would have the same internal
accuracy_ and the absolute position and elevation anywhere in the world could be
determined with an accuracy of about 20 m without any GCP's. Although the LFC
is capable of producing adequate image data_ it is not likely to be flown on the
Space Shuttle very often and also_ because of orbital limitations of the Space
Shuttle, coverage is not likely to be obtained on a systematic basis, What is needed
is a dedicated free-flyer spacecraft_ in near-polar orbit_ to acquire global data on a
systematic basis. Our experience with using Landsat images in glacier studies is
that about 150 data sets (acquisition for about 10 years at 22 orbits per year over
the same location) are necessary in some glacierized regions to obtain optimum_
cloudfree images under the right seasonal conditions.
The U.S. Geological Survey has proposed a mapping satellite_ Mapsat (also called
the Orbital Mapping System (OMS) by the International Society for Photo-
grammetry and Remote Sensing_ which has carefully studied and accepted the
Mapsat concept)_ to acquire multispectral stereoscopic images of the F_arth on a
long term_ systematic basis. With a stellar sensor for attitude control and the GPS
for positional control_ Mapsat or OMS would provide the needed image data. A
NASA scientific panel has proposed a Laser Atmospheric Sounder and Altimeter
(LASA) which would acquire the needed data on landform elevation. Unless LASA
were carried on Mapsat or OMS_ it_ too_ would need to be carried on a satellite
with a good stellar sensor and use of GPS.
Both the stereoscopic images and the laser altimeter would provide important new
sources of data to geomorphologists involved in landform studies. Stereoscopic
images would provide the areal coverage and accurate x_ y_ and z geometry of
landforms. The laser altimeter would provide detailed elevation information
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which, at _- IO cm, would revolutionize comparative regional and global landform
studies. Even our best topographic maps rarely show contours of I m and usually
provide data ranging from 5- to IO0-m contour intervals. Space data of this kind
would permit not only the study of megageomorphology but would also be useful in
microgeomor phological studies.
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Quantitative geomorphologic studies from spaceborne platforms

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