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I CÓ EDS HE I EHE AA
LAND-WATER BOUNDARY DELINEATION FROM ERS-1 SAR
FOR CONSTRUCTION OF DEM IN THE INTER-TIDAL ZONE
Y. Wang, Ph. D. candidate
B. N. Koopmans, Associate Professor
International Institute for Aerospace Survey and Earth Sciences (ITC)
350 Boulevard 1945, P. O. Box 6, 7500 AA Enschede, The Netherlands
Tel:+31-4874248, Fax:+31-4874336, E-mail:yiman@itc.nl
Commission VII, Working Group 8
KEY WORDS: SAR, DEM/DTM, classification, coast, mapping, sea surface modeling, Global Classification filter, inter-tidal
Wadden Sea.
ABSTRACT
Topographic mapping of the inter-tidal area is very important in the viewpoint of human economic activities and the environmental
management in the coastal areas. The ERS-1 radar satellite provides an opportunity for testing the “water line procedure” for
rapidly updating of the topographic maps in the Wadden Sea, The Netherlands. Water lines from low tide to high tide can be
delineated from sequential SAR images acquired at different tidal stages. The height values along those water lines can be
extracted from respective water surface model at the time of image acquisition. The DEM of the inter-tidal flat area then can be
constructed by interpolation from those height values.
For land/water classification, the speckle effect on the SAR imagery has to be treated first. Here we present a concept of global
classification filtering. By the Global Classification Filter, the speckle is reduced by grouping all the pixels of the image which
have the similar characteristics space (variables such as the local mean and variance) with the T-test method. It was proved to be
efficient in speckle reduction and edge preservation for accurate water line delineation.
Water surface model was derived from an existing tidal simulation model with corrections according to the real tidal measurements
from gauges and the water surface curvature over the flats learnt from our field measurement. The combination of water lines and
water surface models yields the height values along the water lines.
Water lines delineated from 9 ERS-1 SAR images acquired during 1993 were used for the construction of a water line DEM
(WALDEM). For validation the WALDEM was compared with the DEM obtained by bathymetric echo sounding survey. The
height differences between the WALDEM and the bathymetric DEM on basis of pixel-by-pixel comparison give an average of -5 cm and a
standard deviation of 28 cm. 73% of the total pixels of WALDEM is within the accuracy of 30cm, and 9396 is within 50 cm.
1. INTRODUCTION The Wadden Sea is one of the most important and highly
sensitive coastal tidal areas in the world. This large tidal area
Repeated topographic mapping of the inter-tidal flat area is provides an important habitat for migration birds and for part
important in respect to ship routing, sound management of ^ or full life cycle of many fish, shrimp and shell fish species.
ecosystem balance and recreation activities, change monitoring s Er
and erosion-sedimentation studies, etc. due to the active :
morpho-dynamic changes in the tidal area. At present the ES | (AMELAND— V
mapping has been carried out with ship-borne echo sounding NORTH SEA
and terrestrial leveling. They are constrained by weather and
tidal conditions and very time and money consuming.
Z
5345 + VLIELAND
The ERS-1 radar satellite provides an opportunity for testing a
DEN Al
rapid mapping method —water line procedure— using SE SCH SRONDEN @ SS HARLINGEN RRUNARDEN
sequential SAR data over the western Dutch Wadden Sea. It
requires several SAR images representing different stages of
the tidal cycle for the extraction of the water lines, and obtains FRIESLAND
the height values of these water lines through water surface "E N
modeling at the time of the image acquisition. Interpolation Ir i
between the height points will derive a DEM of the flat area foe : dta
within the tidal range. jen: B GOLL ND P pron
The Wadden Sea, including a string of barrier islands, is a tidal ris Tidal Channels Vidal Divide
flat area of 8000 square kilometres along the North Sea coast QF 1 pormanent aus stations
of the north Netherlands, northwest Germany and southwest Hp Era AT. i in Kornwerderzand B.
Denmark. The study area was selected in the western part of
the Dutch Wadden Sea (figure 1). Figure 1. Study area (A) in the western Dutch Wadden Sea.
763
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
