NH
Natural Hazards

Archives / 2018 / February

The fantastic world of OBIA!

For today blog, we have interviewed Clemens Eisank about OBIA and its application in Natural Hazard.

Dr. Clemens Eisank is Remote Sensing Specialist & Project Manager at GRID-IT Company in Innsbruck (Austria). He obtained his Ph.D. from the Department of Geoinformatics – Z_GIS at Salzburg University in 2013. In his Ph.D. research, he proposed a workflow for automated geomorphological mapping with object-based image analysis (OBIA) methods. His research interests include remote sensing based mapping/monitoring of natural hazards and the development/automation of the related information extraction workflows and tools. During his career, he has worked on several research projects on natural hazards related topics.

 

1) Hi Clemens, can you tell us what OBIA is in simple words?

OBIA is short for Object-Based Image Analysis. OBIA is a powerful framework for the analysis and classification of gridded data, especially images. A typical OBIA workflow includes two steps:

  1. Segmentation to generate so-called “objects”. Objects are created by merging adjacent grid cells (pixels) of the input grid layer(s) based on specific criteria. For merging pixels into objects, many segmentation algorithms evaluate the similarity of pixel values against a user-defined threshold (fig. b).
  2. Classification of objects. Objects are defined by a plethora of attributes, including spectral (e.g. pixels mean brightness), geometrical (e.g. maximum slope) and spatial properties (e.g. relative border to class X). Based on statistical learning or knowledge models the best set of attributes is identified for each target class and used for object-based classification of the input grid layers (fig. c).

 

Image in fig. a is segmented in different objects (fig. b), which are classified as one class, i.e. “moraine deposits” (fig. c). Image credit: Gabriele Amato & GRID-IT Company.

In general, the segmentation and classification steps are applied in a cyclic manner to obtain an accurate classification result. Compared to pixel-based classification, OBIA results are more realistic, more accurate and visually more appealing, since the “salt-and-pepper” effect, which is typical in pixel-based classifications, is avoided. Moreover, classification results are typically in vector format allowing for straightforward integration with other GIS layers.

One reason for the success of OBIA may be the fact that it mimics human perception: humans perceive the world as an assemblage of discrete entities such as trees, mountains, buildings; they name these entities and distinguish them by properties such as colour, shape and spatial setting. In the OBIA world, objects are the digital representation of the perceived real-world entities, and the digital properties can be directly associated with the properties that humans use to distinguish different categories of real-world entities.

2) Why do you think OBIA can give a contribution in the field of Risk Assessment?

For a proper risk assessment, a comprehensive geodatabase with all kinds of layers ranging from terrain data and geology to images has to be established for the region of interest. OBIA is a great framework for integrating all these data coming at different spatial resolutions and for extracting the relevant risk information (e.g. risk zones), especially via the use of “objects”. By analysing multi-temporal data, natural-hazard objects such as landslides can be identified as polygons or the evolution of natural hazard objects can be monitored, including the change in attributes such as shape, which may help to improve the understanding of the relevant surface processes.  Other application scenarios are that natural hazard polygons, which have been extracted by OBIA, are used (1) as constraining areas for optimization of susceptibility models or (2) as basis for the mapping of risk “hot spots”.

3) In which kind of Natural Hazard do you think OBIA can provide the best performance?

OBIA performs best in detection scenarios’ changes:  in other words  when two or more images of the same area are compared. One prominent application example is the automated mapping of a new landslide to create event-based landslide inventories: a post-event image is segmented (ideally, in combination with terrain layers) and bare soil objects are automatically extracted using spectral, terrain and other thresholds. The extracted bare soil objects are put on top of a pre-event image and pre-event object properties are recorded. If a significant difference between pre- and post-event object properties (e.g. NDVI) is observed, these objects are regarded as new landslide areas and added as polygons to the existing landslide inventory. By repeating this extraction, a complete detailed landslide inventory can be established at national and regional scale. Such an inventory will be more objective than inventories usually created by multiple people with different background and experience.

4) In this regard, could you tell us something about the research projects you are involved in?

We have just finished a project on improving object-based landslide mapping (Land@Slide). The focus was on increasing the robustness and flexibility of object-based landslide mapping algorithms. These algorithms  should deliver high-quality landslide mapping results for different kinds of optical satellite data with varying spatial and spectral resolution. In close cooperation with potential end-users, we gathered the desired requirements and identified application scenarios ranging from landslide rapid mapping to inventory mapping. The improved algorithms were implemented in a prototypical web processing service, which allows the users to map landslides online by themselves.

Currently, I am managing the MorphoSAT project which can be seen as follow-up project of my PhD research. The idea is to bring digital geomorphological mapping to the next level. The motivation of this research is that digital geomorphological information is required for many applications (also in natural hazards), but is only rarely available for most regions of the world. We are positive that we can provide improved algorithms based on OBIA for automated extraction of geomorphological features (e.g. landforms, process domains) in digital terrain layers.

5) Which are the OBIA future perspectives?

I think the impact of OBIA will constantly increase in the future. OBIA has proven to be a powerful framework for integrative 2D data analysis. However, additional functions and methods are needed to analyse data in 3D and 4D, especially for multi-temporal LiDAR point clouds. OBIA will be of great value also for tracing objects in video streams such as the already recorded ones by mini satellites. We will hopefully also see new OBIA software tools that can reach the quality of commercial products. Especially in the open-source world, OBIA tools are still rare. Free tools are needed to widen the user community and to strengthen the position of OBIA as an innovative geodata analysis framework, also in the field of natural hazards.

“Twenty or more Leagues Under the Sea”: A journey to understand submarine canyons

“Twenty or more Leagues Under the Sea”: A journey to understand submarine canyons

As NhET, we have the pleasure to have Mauro Agate as our guest and interviewee today. We discuss about submarine canyons and related geo-hazards. Further details will be available at: http://www.sciencedirect.com/science/article/pii/S0967064513002488  for a scientific-oriented audience;

or https://www.youtube.com/channel/UCwWErQNoZYpJhhkxPa82x5g for a broader audience.

 

Dr. Mauro Agate is a marine geologist and a stratigrapher working at Earth and Marine Sciences Department of Palermo University (Italy) where he teaches Marine Geology and Sedimentology. He has taken part in more than 25 oceanographic cruises in the central Mediterranean Sea. He focuses on: i) effects of sea level change on sedimentary dynamics of shelf-to-slope systems; ii) geomorphological and geological mapping of seafloor; iii) origin and evolution of submarine canyon systems; iv) tectono-sedimentary evolution of continental margins.

He has contributed to several research projects: VECTOR (Vulnerability of Italian marine coasts and ecosystems to climatic change); CARG (Geological mapping); MAGIC (Marine geohazards along the Italian coasts); EMODNet (European marine observation and data network).

 

 

Interview:

1) Which is the main topic of the research we are going to discuss today?

I will focus on submarine canyons as they are one of the most widespread morphologies shaping the ocean floor. These erosional features may extend seawards across continental margins for hundreds of kilometres (> 400 km) and occasionally have canyon wall heights of up to 5 km, from canyon floor to canyon rim. Submarine canyons play a key role in the framework of oceanographic and sedimentary processes, acting as conduits for the transfer of sediment from mainland to the deep sea, and controlling meso-scale oceanographic circulation as well as controlling the functioning of specific benthic habitats.

2) How do submarine canyons originate?

For years, the origins of submarine canyons have been the subject of debate among investigators, and various ideas have been proposed. As already suggested by Shepard in a famous paper dated 1981, multiple causes may contribute to the origin of a canyon. Canyons are fundamentally erosive features, yet some of them show a very complex evolution characterized by alternating erosive and depositional stages.

It is important to separate two main types of submarine canyons, because the origin, evolution and quality of related ecosystems are very different: a) shelf-indenting, sediment fed canyons and b) slope-confined, retrograding canyons. Marine geological and geophysical research documented as slope-confined canyons can retrogressively develop up to the shelf edge, and recent studies, also based on numerical modelling, suggested that these canyons formerly originated from downslope-eroding sediment flows. Some shelf-indenting canyons may cross the continental shelf in its entirety and link to current fluvial networks. Submarine canyons display many similarities with exposed river systems, but also relevant differences.

3) Why it is important to understand submarine canyon origins and evolution?

The unravelling of submarine canyon dynamics has been driven by the need to plan safe routes along which to place cables and pipelines across the seafloor. Often, at the down-slope end of the canyon, sedimentary submarine fans may occur. These represent modern analogues for ancient deposits of economic significance (hydrocarbon source-rock and reservoir). Moreover, oceanographic implication of canyon activity, by mixing of shallow water with upwelling of deep water can also enhance local primary productivity: consequently, commercially relevant fisheries are commonly located at the heads of submarine canyons.

Further on, among deep ocean geomorphic features, the heads of some shelf-incising submarine canyons have been identified as supporting ecosystems (e.g. cold-water coral communities). These are especially vulnerable to human activities, mostly as consequence of water acidification caused by anthropogenic climate change and bottom trawl fisheries. In particular, the trawling practise could have an enormous impact on canyon dynamics by altering deep-sea sediment transport pathways and ecosystems.

Ultimately, a more complete understanding of the canyon activity may help us in preventing some natural and anthropogenic hazards.

4) What types of hazards are related to the underwater canyons?

Two main types of hazards are associated with the presence of submarine canyons and their related processes: landslides and dispersion of pollutants.

Submarine slides can be generated by the failure of canyon wall and head-scarp. Usually mass wasting phenomena occurring inside the canyon are not very large in size. However, they can be dangerous for cables and pipelines. Moreover, in some canyons, the headward erosion driven by downslope-cutting sediment flows and the following landward shift of the canyon head may come to threaten harbor facilities or other anthropic settlements located along the coast. In 1979 a landslide in the head of the Var Canyon in the Gulf of Lions, involved a volume of about 9 million m3 of material and caused a tsunami wave of about three meters that damaged part of the work to extend the airport of Nice and killed 10 people. Similarly, in southern Italy, along the Tyrrhenian coast of Calabria, in 1977 a submarine landslide in the head of Gioia Tauro canyon mobilized about 5 million m3 of material generating a tsunami and a turbidity current which caused serious damage to the port and the break of submarine cables. Even away from settled regions, the retreat of the canyon heads can cause extensive damage such as the sudden disappearance of entire stretches of sandy coastline.

If the canyon walls and head were stable, however, the same morphology of a submarine canyon could represent a threat in case of earthquakes or tsunamigenic landslides because the bathymetric pattern of the canyon can amplify (or simply not mitigate) a possible tsunami wave. Such different effects of sea floor bathymetry on tsunami characteristics have been documented. Examples are the tsunami in 1998 affecting the island of Papa New Guinea and the Indian Ocean tsunami on 26 December 2004 affecting on the Bangladesh coast.

As concerns dispersion of pollutants, at present not many studies have been carried out. However, there is growing evidence that sediment transport along the canyon can contribute to contamination of pelagic sediments by industrial waste and chemical pollutants coming from coastal areas and subsequent accumulation in deep-sea fauna.

5) What are the most advanced methodologies for the investigation of submarine canyons and what further discoveries do you expect from the upcoming research?

During the past two decades, the wide-spread use of the Multibeam echo-sounder devices in underwater geological surveys have provided wonderful images of the seabed. This allowed for quantitative morphometric analysis of submerged geomorphological features, among which the submarine canyons. We currently know very well the shape, morphologies and sizes of these fascinating features. Probably further advances in understanding the mechanisms of the canyon function can only come from multidisciplinary research that integrates geophysical, sedimentological, oceanographic and biological analyses. A multidisciplinary approach, such as the one followed in the ISLAND Project (ExplorIng SiciLian CAnyoN Dynamics) recently promoted by the European program EUROFLEETS (www.eurofleets.eu), is now essential not only to better understand the sedimentary dynamics and evolutionary mode of submarine canyons,  but also to assess the impact of canyon activity in generating natural hazards and controlling benthic ecosystems stability.