Automatic near real-time flood detection in high resolution X-band synthetic aperture radar satellite data using context-based classification on irregular graphs [Elektronische Ressource] / Sandro Martinis

Sandro Martinis

Informations

• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2010
• Nombre de lectures: 22
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

Automatic near real-time flood detection in high resolution
X-band synthetic aperture radar satellite data using
context-based classification on irregular graphs


Dissertation
der Fakultät für Geowissenschaften
der Ludwig-Maximilians-Universität München











Sandro Martinis

Eingereicht am: 15.09.2010























1. Gutachter: Prof. Dr. Ralf Ludwig
2. Gutachter: . Richard Bamler

Tag der Disputation: 06.12.2010


Abstract

This thesis is an outcome of the project “Flood and damage assessment using very high
resolution SAR data” (SAR-HQ), which is embedded in the interdisciplinary oriented
RIMAX (Risk Management of Extreme Flood Events) programme, funded by the Federal
Ministry of Education and Research (BMBF). It comprises the results of three scientific
papers on automatic near real-time flood detection in high resolution X-band synthetic
aperture radar (SAR) satellite data for operational rapid mapping activities in terms of disaster
and crisis-management support.
Flood situations seem to become more frequent and destructive in many regions of the
world. A rising awareness of the availability of satellite based cartographic information has
led to an increase in requests to corresponding mapping services to support civil-protection
and relief organizations with disaster-related mapping and analysis activities. Due to the
rising number of satellite systems with high revisit frequencies, a strengthened pool of SAR
data is available during operational flood mapping activities. This offers the possibility to
observe the whole extent of even large-scale flood events and their spatio-temporal evolution,
but also calls for computationally efficient and automatic flood detection methods, which
should drastically reduce the user input required by an active image interpreter.
This thesis provides solutions for the near real-time derivation of detailed flood
parameters such as flood extent, flood-related backscatter changes as well as flood
classification probabilities from the new generation of high resolution X-band SAR satellite
imagery in a completely unsupervised way. These data are, in comparison to images from
conventional medium-resolution SAR sensors, characterized by an increased intra-class and
decreased inter-class variability due to the reduced mixed pixel phenomenon. This problem is
addressed by utilizing multi-contextual models on irregular hierarchical graphs, which
consider that semantic image information is less represented in single pixels but in
homogeneous image objects and their mutual relation. A hybrid Markov random field (MRF)
model is developed, which integrates scale-dependent as well as spatio-temporal contextual
information into the classification process by combining hierarchical causal Markov image
modeling on automatically generated irregular hierarchical graphs with noncausal Markov
modeling related to planar MRFs. This model is initialized in an unsupervised manner by an
automatic tile-based thresholding approach, which solves the flood detection problem in
large-size SAR data with small a priori class probabilities by statistical parameterization of
local bi-modal class-conditional density functions in a time efficient manner.


Experiments performed on TerraSAR-X StripMap data of Southwest England and
ScanSAR data of north-eastern Namibia during large-scale flooding show the effectiveness of
the proposed methods in terms of classification accuracy, computational performance, and
transferability. It is further demonstrated that hierarchical causal Markov models such as
hierarchical maximum a posteriori (HMAP) and hierarchical marginal posterior mode
(HMPM) estimation can be effectively used for modeling the inter-spatial context of X-band
SAR data in terms of flood and change detection purposes. Although the HMPM estimator is
computationally more demanding than the HMAP estimator, it is found to be more suitable in
terms of classification accuracy. Further, it offers the possibility to compute marginal
posterior entropy-based confidence maps, which are used for the generation of flood
possibility maps that express that the uncertainty in labeling of each image element. The
supplementary integration of intra-spatial and, optionally, temporal contextual information
into the Markov model results in a reduction of classification errors. It is observed that the
application of the hybrid multi-contextual Markov model on irregular graphs is able to
enhance classification results in comparison to modeling on regular structures of quadtrees,
which is the hierarchical representation of images usually used in MRF-based image analysis.
X-band SAR systems are generally not suited for detecting flooding under dense
vegetation canopies such as forests due to the low capability of the X-band signal to penetrate
into media. Within this thesis a method is proposed for the automatic derivation of flood areas
beneath shrubs and grasses from TerraSAR-X data. Furthermore, an approach is developed,
which combines high resolution topographic information with multi-scale image
segmentation to enhance the mapping accuracy in areas consisting of flooded vegetation and
anthropogenic objects as well as to remove non-water look-alike areas.
Contents

Contents

List of figures ............................................................................................................................ I
List of tables.............................................................................................................................II
List of acronyms .................................................................................................................... III
1 Introduction .........................................................................................................................1
1.1 Motivation.....1
1.2 Objectives.......................................................................................................................6
1.3 Structure.........................................................................................................................7
2 Synthetic Aperture Radar...................................................................................................8
2.1 Basic principles and properties of imaging radar systems .............................................8
2.1.1 Basic principles of imaging radar systems ..........................................................8
2.1.2 Resolution in range..............................................................................................9
2.1.3 Resolution in azimuth ..........................................................................................9
2.2 SAR Signal...................................................................................................................11
2.2.1 Radar equation...................................................................................................11
2.2.2 System specific properties .................................................................................12
2.2.3 Object specific properties ..................................................................................13
2.2.4 Speckle effect.....................................................................................................15
2.3 Geometric effects.........................................................................................................15
2.4 TerraSAR-X.................................................................................................................16
3 Interaction between SAR signal and water bodies .........................................................18
3.1 Smooth open water.......................................................................................................18
3.2 Rough open water........................................................................................................20
3.3 Flooded vegetation22
3.4 Floods in urban areas ...................................................................................................25
4 State of the art in SAR-based water detection ................................................................27
Contents

5 Publications ........................................................................................................................34
5.1 Paper 1..........................................................................................................................34
5.2 Paper 2......47
5.3 Paper 3......61
6 Summary and outlook .......................................................................................................81
References ..........................................................