International Archives of the Photogrammetry, Remote Sensing and Spatial Information Science, Volume XXXVIII, Part 8, Kyoto Japan 2010

EVALUATION OF THE FEASIBILITY OF WEB CAMERASTO MONITOR COASTAL AREAS
M. Ishihara a, *, H. Oguma a, N. Namizaki a, and H. Yamano aa

National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan - (ishihara.mitsunori,oguma, namizaki.naoko, hyamano)@nies.go.jp
Commission VIII, WG VIII/9

KEY WORDS: Web camera, coastal area, coral reef, shoreline, monitoring, infra-red image
ABSTRACT:Global and regional environmental changes can cause serious effects on coastal areas. Web camera has an effective possibility forthe monitoring of these phenomena, because they can acquire continuous data in relatively large areas. This research is evaluating
the feasibility of web cameras to monitor coastal areas (benthic cover of a coral reef and shorelines). Composite visible images wereuseful for decrease of effects of halation and wave, it was found that they were effective way of monitoring under water. We wereable to extract waterline accurately using near-infrared images, it was found that they were effective way of monitoring the change ofshorelines by rising sea levels and intensified typhoon activities.
1. INTRODUCTION

2.2 Data

Global and regional environmental changes (global warming,extreme weather, terrestrial runoff, etc.) can cause serious
effects on coastal areas. For example, coral bleaching is causedby combined stresses including rising sea temperatures and redsoil runoff from the adjacent land. Moreover, shoreline changes
are enhanced by rising sea levels and intensified typhoon
activities. Several methods have been used to monitor thesechanges so far. In-situ monitoring includes observation ofcoverage of corals and seaweeds by snorkeling or diving andtracking shorelines by GPS (Global Positioning System)(Kayane et al., 2002; Harii and Kayann, 2003; Kench andBrander, 2006). These monitoring methods, however, aredifficult to observe the changes in large areas with highfrequency, though detailed examination is possible. On the other
hand, large-scale monitoring includes mapping of coral reefs
and shorelines by using aerial photographs and satellite data
(Palandro et al., 2003, 2008; Yamano and Tamura, 2004;
Yamano et al., 2006). These monitoring methods, however, aresynoptic, and the accuracy is generally low. Web cameras could
serve as a complementary method for these monitoring methods,
because they can acquire continuous data in relatively large
areas. In this study, we evaluate the feasibility of web cameras
to monitor coastal areas (benthic cover of a coral reef andshorelines).

To observe the coastal area, we installed a web camera (M12D,MOBOTIX AG, Germany) at the rooftop of a building (19 ma.s.l.). The web camera has two lenses and can capture visible
and near-infrared images at the same time (Figure 1). The
captured images were contained coral reefs, shorelines andseaweeds. We conducted the observation four times in several
season (18-19 May, 13 July, 29-31 August 2009 and 20-22
February 2010).
Ground-truthing for benthic cover and shoreline positions wasperformed by observing coverage of corals and seaweeds with aquadrat and by tracing shorelines, respectively, during thecamera monitoring.

Figure 1. Example of captured near-infrared (left) and visible(right) images.
2.3 Data analysis

2. MATERIALS AND METHODS

We produced 1-hour composite images to remove effects ofhalation and wave. The composite was based on minutely
images with the highest digital number (DN) value of anindividual pixel, because the DN value of the pixel influenced
halation and wave decreased.
We picked up DN values around water areas from near-infrared
image and made a histogram of these DN values. We used themaximum value around the tracing shorelines to define thethreshold value for delineating the land-water boundary(waterline). The waterlines were extracted as contours from thethreshold value.

2.1 Study site
Our study site is part of Sesoko Station, University of theRyukyus, Japan (26º38'12''N, 127º51'54''E), which is located in
the northern part of main island of Okinawa.

* Corresponding author. E-mail: ishihara.mitsunori@nies.go.jp (M. Ishihara)

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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Science, Volume XXXVIII, Part 8, Kyoto Japan 2010

3. RESULTS AND DISCUSSION

4600

Count of pixels

3.1 Composite images
Figure 2 shows composite visible images from 13:00 to 13:59
local time (a total of 60 images) at 19 May, 13 July, 29 August
2009 and 21 February 2010. Visibilities under water ofcomposite images were different from the weather conditions.
We were able to confirm patches of coral and seaweed moreclearly in the clear days (13 July 2009 and 21 February 2010)
than the cloudy days (19 May and 29 August 2009). However,
we confirmed decrease of effects of halation and wave by using
composite images compared to original images. Moreover, thedistribution of coral and seaweeds obtained from groundverifications and the image analysis is corresponding, then it
was found that composite images was effective way ofmonitoring under water.

45002001000050100150DN

200250Figure 2. Histogram of digital numbers (DN) values of thenear-infrared image captured on 21 February 2010.
References
Harii, S., and Kayanne, H., 2003. Larval dispersal, recruitment,and adult distribution of the brooding stony octocoral Heliopora
coerulea on Ishigaki Island, southwest Japan. Coral Reefs, 22(2), pp. 188-196.
Kayanne, H., Harii, S., Ide, Y., and Akimoto, F., 2002.
Recovery of coral populations after the 1998 bleaching onShiraho Reef, in the southern Ryukyus, NW Pacific. Marine
Ecology Progress Series, 239, pp. 93-103.
Kench, S.P., and Brander, W.R., 2006. Response of reef island
shorelines toseasonal
climateoscillations: South
Maalhosmadulu atoll, Maldives. Journal Geophysical Research,
111 (F01001), doi:10.1029/2005JF000323.
Palandro, D., Andréfouët, S., Dustan, P., and Müller-Karger,E.F., 2003. Change detection in coral reef communities using
Ikonos satellite sensor imagery and historic aerial photographs.
International Journal of Remote Sensing, 24 (4), pp. 873-878.

Figure 1. Composite visible images from 13:00 to 13:59 local
time at (a) 19 May, (b) 13 July, (c) 29 August 2009 and (d) 21February 2010.

Palandro, D., Andréfouët, S., Hu, C., Hallock, P., MüllerKarger, F.E., Dustan, P., Callahan, M.K., Kranenburg, C., andBeaver C.R., 2008. Quantification of two decades of shallowwater coral reef habitat decline in the Florida Keys National
Marine Sanctuary using Landsat data (1984–2002). RemoteSensing of Environment, 112 (8), pp.3388-3399.

3.2 Extracting the waterlineFigure 3 shows histogram of digital numbers (DN) values
around water areas of the near-infrared image captured on 21
February 2010. We decided that the threshold value was 246(DN) as the maximum value around the tracing shorelines. It isclear that the threshold value can delineate the land-waterboundary (waterline). Moreover, we extracted the waterlines ascounters from the threshold value, and the mean error between
extracted waterline and ground-truthing tracing shoreline was
1.3 pixels (about 30 cm). Therefore, the monitoring using nearinfrared images was effective for detection of waterline.

Yamano, H., and Tamura M., 2004. Detection limits of coral
reef bleaching by satellite remote sensing: Simulation and dataanalysis. Remote Sensing of Environment, 90 (1), pp. 86-103.
Yamano, H., Shimazaki, H., Matsunaga, T., Ishoda, A.,
McClennen, C., Yokoki, H., Fujita, K., Osawa, Y., and Kayanne,
H., 2006. Evaluation of various satellite sensors for waterline
extraction in a coral reef environment: Majuro Atoll, MarshallIslands. Geomorphology, 82 (3-4), pp.398-414.

4. CONCLUSION
This research is evaluating the feasibility of web cameras tomonitor coastal areas (benthic cover of a coral reef andshorelines). Composite visible images were useful for decreaseof effects of halation and wave, it was found that composite
visible images were effective way of monitoring under water.
We were able to extract waterline accurately using near-infrared
images, it was found that near-infrared images were effective
way of monitoring the change of shorelines by rising sea levels
and intensified typhoon activities.

Acknowledgements
This work was supported by Grant-in-Aid for Scientific
Research on Innovative Areas "Coral reef science for symbiosis
and coexistence of human and ecosystem under combinedstresses" (No. 20121004) of the Ministry of Education, Culture,Sports, Science and Technology (MEXT), Japan and Japan
Science and Technology Agency (JST)/Japan International
Cooperation Agency (JICA), SATREPS (Science andTechnology Research Partnership for Sustainable Development).

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