Coral reefs worldwide are suffering from severe and rapid degradation (Bryant et A, 1998; Hoegh-Guldberg, 1999). Quick, consistent, large-scale assessment is required to assess and monitor their status (e.g., USDOC/NOAA NESDIS et al., 1999). On-going systematic collection of high resolution digital satellite data will exhaustively complement the relatively small number of SPOT, Landsat 4-5, and IRS scenes acquired for coral reefs the last 20 years. The workhorse for current image acquisition is the Landsat 7 ETM+ Long Term Acquisition Plan (Gasch et al. 2000). Coral reefs are encountered in tropical areas and cloud contamination in satellite images is frequently a problem (Benner and Curry 1998), despite new automated techniques of cloud cover avoidance (Gasch and Campana 2000). Fusion of multidate acquisitions is a classical solution to solve the cloud problems. Though elegant, this solution is costly since multiple images must be purchased for one location; the cost may be prohibitive for institutions in developing countries. There are other difficulties associated with fusing multidate images as well. For example, water quality or surface state can significantly change through time in coral reef areas making the bathymetric processing of a mosaiced image strenuous. Therefore, another strategy must be selected to detect clouds and improve coral reefs mapping. Other supplemental data could be helpful and cost-effective for distinguishing clouds and generating the best possible reef maps in the shortest amount of time. Photographs taken from the 1960s to the present from the Space Shuttle and other human-occupied spacecraft are one under-used source of alternative multitemporal data (Lulla et al. 1996). Nearly 400,000 photographs have been acquired during this period, an estimated 28,000 of these taken to date are of potential value for reef remote sensing (Robinson et al. 2000a). The photographic images can be digitized into three bands (red, green and blue) and processed for various applications (e.g., Benner and Curry 1998, Nedeltchev 1999, Glasser and Lulla 2000, Robinson et al. 2000c, Webb et al, in press)

Andrefeouet, Serge

Robinson, Julie

NASA Technical Reports Server

Source of Acquisition
NASA Johnson Space Center
Improving cloud detection in satellite images
of coral reef environments
using Space Shuttle photographs and High-Definition Television
Serge Andrefouk (1), Julie A. Robinson (Z)
t University of South Florida
Remote Sensing Biological Oceanography Lab.
College of Marine Science
140, 7 t' Av. South
St Petersburg FL 33701, USA
2 Office of Earth Sciences
Johnson Space Center
2400 NASA Road 1, C23
Houston, Texas 77058
To be submitted to:
International Journal of Remote Sensing
Letters
September 2000
https://ntrs.nasa.gov/search.jsp?R=20100035183 2019-08-30T12:14:23+00:00Z
Short title:
Detection of clouds in coral reefs with Shuttle imagery
Key-words: Landsat, SPOT, Space Shuttle, HDTV, astronaut photography, coral
reefs, atolls
Introduction
Coral reefs worldwide are suffering from severe and rapid degradation (Bryant et
A, 1998; Hoegh-Guldberg, 1999). Quick, consistent, large-scale assessment is required
to assess and monitor their status (e.g., USDOC/NOAA NESDIS et al., 1999). On-going
systematic collection of high resolution digital satellite data will exhaustively
complement the relatively small number of SPOT, Landsat 4-5, and IRS scenes acquired
for coral reefs the last 20 years (e.g., Andr fou t et al. 1999 "different citation??). The
workhorse for current image acquisition is the Landsat 7 ETM+ Long Term Acquisition
Plan (Gasch et al. 2000).
Coral reefs are encountered in tropical areas and cloud contamination in satellite
images is frequently a problem (Benner and Curry 1998), despite new automated
techniques of cloud cover avoidance (Gasch and Campana 2000). Fusion of multidate
acquisitions is a classical solution to solve the cloud problems. Though elegant, this
solution is costly since multiple images must be purchased for one location; the cost may
be prohibitive for institutions in developing countries. The are other difficulties
associated with fusing multidate images as well. For example, water quality or surface
state can significantly change through time in coral reef areas making the bathymetric
processing of a mosaicked image strenuous. Therefore, another strategy must be selected
to detect clouds and improve coral reefs mapping. Other supplemental data could be
helpful and cost-effective for distinguishing clouds and generating the best possible reef
maps in the shortest amount of time.
Photographs taken from the 1960s to the present from the Space Shuttle and other
human-occupied spacecraft are one under-used source of alternative multitemporal data
(Lulla et al. 1996). Nearly 400,000 photographs have been acquired during this period,
an estimated 28,000 of these taken to date are of potential value for reef remote sensing
(Robinson et al. 2000a). The photographic images can be digitized into three bands (red,
green and blue) and processed for various applications (e.g., Benner and Curry 1998,
Nedeltchev 1999, Glasser and Lulla 2000, Robinson et al. 2000c, Webb et al, in press).
From this unique database, several applications to studies of coral reefs have been made.
Georeferenced astronaut photographs have been used as surrogates to simple shoreline
base maps for pilot coral reef studies (Robinson et al. 2000a). The photographs have been
used as a source of opportunistic observations of phytoplankton blooms in coral reef
regions (Kuchler and Jupp, 1988). Recent tests of High Definition Television (HDTV)
technology on the Space Shuttle extended the kinds of Earth observation images that can
be acquired (Robinson et al. 2000b).
Astronaut photography is distributed at cost of reproduction; images for scientific
research are digitized on request; and digital images are made available for free search
and download via the Internet (http://eolisc.nasa.gov/sseop) . However, there are a
number of challenges to using the data for remote sensing including variability in look
angle, sun elevation angle, altitude, variety of lenses used, and film processing (e.g.,
Webb et al. in press, Robinson et al. 2000b). Both digitized Space Shuttle photographs
and HDTV suffer from the lack of accurate systematic geometric corrections from which
georeferenced products could be derived automatically (Zheng et al. 1997). For this,
existing maps must be used as a reference. This is a severe limitation for mapping and
inventory applications in areas such as coral reefs where reliable, updated maps at the
right scale seldom exist, and where independent geographic features to use as tie points
can be limited. Therefore, for this reason, Landsat or SPOT products must be favored as a
primary source of data for reef mapping.
Here, we illustrate the complementarity of satellite images and astronaut
photography. We used astronaut photography and HDTV for cost-effective detection and
removal of clouds from remote sensing classifications based on satellite data.
Study Area and Methods
In the atolls of French Polynesia specific geomorpho logical features (coral
pinnacles) make the presence of small clouds a critical hindrance to accurate reef
mapping. Even with clear sky, occurrence of small clouds is common and they look
extremely similar to the top of coral pinnacles rising vertically from the bottom of atoll
lagoons (figure l). The top of a pinnacle may reach the surface and provide a strong
optical signal or the top may be submerged deeper, providing only an attenuated signal.
Although pinnacles represent only a fraction of the surface of an atoll, they cannot be
neglected for several reasons. First, from a human-use point of view, they are navigation
hazards and must appear on maps. Second, from a biological point of view, pinnacles are
a center of biodiversity and species abundance, and their presence or absence may
explain the differences between atolls in the population structure of benthic communities
(Adjeroud et al. 2000). Third, from a geochemical point of view, the fluxes of carbon,
carbonate and nutrients around pinnacles are not negligible in the budget of an atoll
lagoon (Charily et al. 1998).
Depending on the depth of the pinnacles, and the density and size of the cloud, it
may be possible to readily discriminate pinnacles from clouds. However, there is no
single rule and processing for automatic cloud filtering (e.g., classification, segmentation,
ratios, ISH transformation, shadow matching, etc.) that may perform well on one image
may fail on another (Loubersac et al. 1990). Multidate data becomes a necessity for
distinguishing pinnacles from small clouds.
"Paragraph on SPOT data acquisiton and coverage to date.
The database of astronaut photography reveals extensive coverage of French
Polynesian atolls with a number of high-quality images available; almost all the 84 atolls
of this oceanic region have been photographed multiple times. We searched the database
to identify images acquired in the past, and digitized them at 2400 ppi (10.6 µm / pixel).
We also obtained digital still images extracted from HDTV video acquired during tests
on Space Shuttle mission STS-93 (see details on the projects and on still image
deinterlacing in Robinson et al. 2000b).
Processing of the Space Shuttle images for cloud-filtering began with geometric
rectification of the Shuttle image using the satellite image as a reference (a tutorial for
this process is provided by McRay et al. 2000). The spatial resolutions are similar (table
1) and control points along the rim of the atoll were easy to select as illustrated on several
types of atoll rims in figure 2 (rim typology in Andr fou t et al. in press). This stage was
made more effective by using images with high spatial resolution (low altitude and long
lens) and low distortion due to oblique look angle, as the image in figure 3.
Next, the near infra-red bands in the satellite image (XS3 in SPOT XS data or
TM4 in Landsat TM/ETM+ data) and the red channel of the digitized photograph or
HDTV were stretched and thresholded, to produce two binary images featuring the
background/lagoon in white (255) and the clouds/pinnacles in black (0. The final stage
was to inspect simultaneously the two binary images. A cloud-flag was set for any pixel
that was black in both images. Since the geometric correction may not be perfect, a
tolerance factor, N, can also be set to allow inspection in a N x N window instead of per
pixel (N can be estimated using the RMS error of the geometric correction). Note that
once the presence of pinnacles is confirmed, they may be used as new control points to
refine the geometric rectification, by providing references not only along the rim of the
atoll, but also in the lagoon.
Results and conclusions
Illustrations of the technique presented here will focus on two SPOT images
acquired 5 November and 17 September 1989. These data were combined with a
deinterlaced HDTV still image of Amanu atoll acquired 25 July 1999 (STS093-HDTV-
5166), from 285 km, with a spatial resolution of 27.7 m / pixel. Figure 3 shows where
coral pinnacles and clouds were detected on Amanu atoll.
Figure 4 illustrates cloud/pinnacle discrimination on Takaroa atoll using a Landsat
7 ETM+ image acquired **ZZ/ZZ, IZZ and a digitized photograph (STS093-717-??
**Serge add number of the image used 73, 74 or 75) acquired from the Space Shuttle
on 25 July 1999. The complementarity of the two sources of information, satellite and
Shuttle, for cloud detection is well demonstrated by these two examples.
This method has served well for distinction of clouds and pinnacles in a number
of cases. One condition for its success is that both images be relatively cloud-free (as in
figures 3 and 4). If clouds were very abundant, they could occur simultaneously on same
locations on the two images and be interpreted as pinnacles. However, this event is
unlikely using images with very low cloud cover. It is also possibile to use a third
astronaut photograph for further refinement.
We found that identifying deep pinnacles could be a problem since their
signatures in the astronaut photographs and satellites images could be weak. In this case,
the processing can be applied also to the XS2 (SPOT-HRV), TM3 (Landsat TM/ETM+)
and Green (astronaut photographs/HDTV) channels of the images to have a better signal.
Finally, for small clouds occurring on satellite images such as this in figure 4 and
5, the information may be interpolated from the neighborhood to construct a cloud-free
image, although this is not recommended when clouds get bigger. Even if pinnacles are
obviously absent, there may exist other deeper structures with smooth bathymetric
variation (e.g., sand dunes) and it would be preferable to simply indicate the area as
pinnacle-free .
In the cases presented here, adequate maps are not available, ground-truthing is
problematic in such remote areas, and a quantitative accuracy assessment of this process
is difficult. However, we feel extremely confident with such simple techniques. The key
is to have access to multidate relatively cloud-free images. This study demonstrates that
with simple methodology, astronaut photographs serve as complementary supplemental
data that can improve mapping based on satellite data. The cost-effectiveness of this
approach is an important factor for tropical developing countries eager to map their vital
coral reef environments.
References
Adjeroud M., Andr fou t S., Payri C., and Orempuller J., 2000, Physical factors of
differentiation in macrobenthic communities between atoll lagoons in the Central
Tuamotu Archipelago (French Polynesia). Marine Ecology Progress Series, 196,
39-48.
Andr fou t S., Claereboudt M., Matsakis P., Pag s J., and Dufour P., in press, Typology
of atolls rims in Tuamotu archipelago (French Polynesia) at landscape scale using
SPOT-HRV images. Int. J. Remote Sensing.
Andr fou t, S., Muller-Karger, F., and Hu, C., 1999, Landsat 7: long-term large-scale
reef studies, Reef Encounter, 26, 27-29.
Benner T. C., and Curry J. A., 1998, Characteristics of small tropical cumulus clouds and
their impact on the environment. J. Geophysical Research-Atmospheres, 103,
28753-28767,
Bryant, D., Burke, L., McManus, J., and Spalding, M., 1998, Reefs at Risk: A Map-Based
Indicator of Threats to the World's Coral Reefs (Washington D. C.: World
Resources Institute, International Center for Living Aquatic Resources
Management, World Conservation Monitoring Centre, and United Nations
Environment Programme).
Charpy L., Charpy-Roubaud C., and Buat P., 1998, Excess primary production,
calcification and nutrient fluxes of a patch reef ( Tikehau atoll, French Polynesia).
Marine Ecology Progress Series, 173, 139-147.
Gasch J., Arvidson T., Goward S. N., Andr fou t S., Hu C., and M ller-Karger F. E.,
2000, An Assessment of Landsat 7/ETM+ Coverage of Coral Reefs Worldwide.
Proceedings of IGARSS 2000, Honolulu, Hawaii, paper TU 14-11.
Gasch J., and Campana K. A., 2000, Cloud avoidance in space based remote sensing
acquisition. Proceedings of SPIE/Aerosense 2000, Orlando, Florida, paper 4049-
36.
Glasser M. E., and Lulla K. P., 2000, NASA astronaut photography depiction of the
spatial and temporal characteristics of biomass burning. Int. J. Remote Sensing,
21, 1971-1986.
Hoegh-Guldberg, O., 1999, Climate change, coral bleaching and the future of the world's
coral reefs, Marine and Freshwater Research, 50, 839-866.
Kuchler D. A., and Jupp D. L. B., 1988, Shuttle photographs captures massive
phytoplankton bloom in the Great Barrier Reef. Int. J. Remote Sensing, 9, 1299-
1301.
Loubersac L., Wibaux B., Chenon F., and Varet H., 1990, A method for SPOT images
segmentation: application to identification of morphologic contours in atoll
environments. Proceedings of Pix'Iles 90, Noum a-Tahiti, pp. 321-328/647-648.
Lulla K. L., et al., 1996, The Space Shuttle Earth Observations Photography Database: an
underutilized resource for global environmental sciences. Environmental
Geosciences, 3, 40-44.
McRay, B., J. Scott, and J. A. Robinson. 2000. Technical applications of astronaut
orbital photography: how to rectify multiple image datasets using GIS. Available
at http://eol.isc.nasa.gov/newsletter/gis/
Nedeltchev N. M., 1999, Space Shuttle photography: a powerful tool of remote sensing
investigation of human effects on the Earth. Int. J. Remote Sensing, 20, 183-188.
Robinson, J. A., Feldman, G. C., Kuring, N., Franz, B., Green, E., Noordeloos, M., and
Stumpf, R. P., 2000a, Data fusion in coral reef mapping: working at multiple
scales with SeaWiFS and astronaut photography. Proceedings of the 6th
International Conference on Remote Sensing for Marine and Coastal
Environments, Vol. 2, pp. 473-483.
Robinson J. A., Holland S. D., Runco S. K., Pitts D. E., Whitehead V. S., and Andr fou t
S., 2000b, High-definition television (HDTV) images for Earth observations and
Earth science applications. Report NASA/TP-2000-210189, Johnson Space
Center, Houston, Texas.
Robinson, J. A., McRay, B., and Lulla, K. P. 2000c. Twenty-eight years of urban
growth in North America quantified by analysis of photographs from Apollo,
Skylab and Shuttle-Mir. In Dynamic Earth Environments: Remote Sensing
Observations from Shuttle-Mir Missions. John Wiley & Sons, New York, pp. 25-
42.
USDOC/NOAA/NESDIS, NOAA/CSC, and ICLARM, 1999, Advancing remote sensing
technologies for the sustainable management of coral reefs, resolution and
recommendations for action, International Workshop On The Use Of Remote
Sensing Tools For Mapping And Monitoring Coral Reefs, June 7-10, 1999, East-
West Center, Honolulu, Hawaii, USA. Available at
http://www.coral.noaa.gov/corviI/coral_reefs/resolution.html
Webb E. L., Evangelista M. A., and Robinson J. A., in press, Digital land-use
classification using Space Shuttle acquired orbital photographs: a quantitative
comparison with Landsat TM imagery of a coastal environment, Chantaburi,
Thailand. Photogrammetric Engineering Remote Sensing.
Zheng Q., Klemas V., Yan X. H., Wang Z., and Kagleder K., 1997, Digital
ortho rectification of Space Shuttle coastal ocean photographs. Int. J. Remote
Sensing, 18, 197-211.
Table 1. Best possible digital spatial resolution (pixel width in m) for the most common
cameras used to acquire images of French Polynesia from the Space Shuttle. The spatial
resolution given is for an image taken looking directly toward spacecraft nadir with
values representing different lenses and spacecraft altitudes. (after Robinson et al. 2000b,
unpublished data).
{**Note to Serge this table has been removed from the Webb et al. paper
because a larger more complete version is going to be in the big resolution paper which is
due to be submitted ASAP. 1 ve edited it so that it doesn t conflict with that publishing,
but does provide supplemental information needed here. }
Minimum ground distance represented by 1
pixel (m) as a function of altitude
Lens focal length (mm) Minimum	 Median	 Maximum
Camera (222 km)	 (326 km)	 (611 km)
Hassleblad a	100 23.5	 34.5	 64.7
250 9.4	 13.8	 25.9
Sony HDW-700	 15 x, zoom to 8 mm 218	 320	 601
15 x, zoom to 120 mm 14.5	 21.3	 40.1
a Film camera with 55 x 55 mm original image size, digitized at 2400 ppi (10.6µm/pixel)
to 5198 x 5198 pixels.
b Digital HDTV camera with 14.8 x 8.31 mm (1920 x 1035 pixel) original image size.
Figures
Figure 1: Top: the picture, taken from a commercial airplane, illustrates small low
altitude clouds (C) and coral pinnacles (P1 in subsurface and P2 deeper) in the lagoon of
Apataki atoll. Bottom: a typical coral pinnacle, photographed in Raroia atoll lagoon.
Figure 2: Comparisons of atoll rims view. Top: a section of the rim of Amanu
atoll viewed by SPOT-HRV (XS3-1) and Space Shuttle HDTV (deinterlaced still image,
STS093-HDTV-5166). Bottom: a section of the rim of Rangiroa atoll viewed by Landsat
ETM+ Bands 4,3,1 and Space Shuttle photograph RGB (Hasselblad camera, STS080-
750-76).
Figure 3: Pinnacles (1) and clouds (2) in HDTV (deinterlaced still STS093-
HDTV-5166), and SPOT-HRV images of Amanu atoll. Images have been stretched to
highlight the pinnacles and clouds. The letter A marks the very large pinnacle with an
arrowhead shape. Note that on the mosaicked SPOT images, the shadows of clouds are
visible on the western part of the image, providing an indirect control of the accuracy of
the processing.
Figure 4: Pinnacles (1) and clouds (2) in digitized Shuttle photographs
(Hasselblad STS093-717-?? **Serge add number of the image used 73, 74 or 75 ) and
Landsat ETM+ Bands 4,3,2 images of Takaroa atoll.
Figure 1 
Figure 2
J
HDTV
SPOT
Figure 3
Figure 4


Improving Cloud Detection in Satellite Images of Coral Reef Environments Using Space Shuttle Photographs and High-Definition Television

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