DISSEMINATION OF PS-INSAR RESULTS FOR IMPROVEDINTERPRETATION AND ANALYSIS

Swati Gehlot(1,2), Edward Verbree(2), and Ramon F. Hanssen(1)

(1) Delft Institute of Earth Observation and Space Systems; Delft University of TechnologyKluyverweg 1, 2629 HS Delft, The Netherlands. Email: (S.Gehlot, R.F.Hanssen)@tudelft.nl

(2) OTB Research Institute for Housing, Urban and Mobility Studies, Delft University of TechnologyJaffalaan 9,2628 BX Delft, The Netherlands. Email: E.Verbree@tudelft.nl

ABSTRACT

Persistent Scatterer Interferometry is currently recognized as an efficient method for detection and monitoringof surface deformation such as land subsidence. The abundance of observations, particularly in urban environ-ments, stresses the capabilities for unambiguous data interpretation. This is due to the fact that the behaviorof a single PS can be due to localized effects, such as building instability or local groundwater and geotechnicalcircumstances, as well as due to wide scale phenomena such as subsidence due to the extraction of hydrocarbonresources.

For these reasons, it is practically impossible for providers of PS results to unambiguously interpret allthe data points. On the other hand, for end-users the PS results are often too difficult to interpret as well,since significant knowledge of the technique is needed to prevent over- or under-interpretation of the data.Consequently, the analysis of PS results needs to be performed in close collaboration between the end-users andthe providers of the technique.

In this study, the options for optimal dissemination of PS results are considered, aiming at PS informationsharing with the user-community. We present a MapServer GIS approach for web publishing of PS-InSAR dataresults in combination with other geodata. The geo-information combination with PS-InSAR results is doneat the server side and the user (client) does not require the installation of a dedicated and (expensive) GISsoftware for visualizing overlays of information layers. The final interface is a simple web browser where theuser can visualize and query PS information. Here we use the functionality of an open source Mapserver WMS(Web Map Server) to link the GIS database(s) with PS results and serve the user with information about thePS data attributes and time series of the PS. Besides this possibility, freely available viewers and servers (eg.Google Earth) can be used in conjunction with the PS database.

1 INTRODUCTION

The recent development of time series data analysis in interferometric synthetic aperture radar has evolved PS-InSAR [1] as an efficient tool for monitoring subtle and slow deformations of the earth’s surface. For interpretingPS-InSAR data unambiguously, methods and techniques, such as (localized) scientific visualization with geo-information synthesis and analysis are becoming more and more important. The prime need of conveying thePS results to a multitude of users may be anticipated as a first step to improve the unambiguous interpretationof the PS data. Additionally, the World Wide Web has developed into a prominent medium to disseminategeospatial data, maps, and their (complex) combinations. In this research we propose the use of scientificvisualization and analysis tools in assisting the PS data interpretation by local geo-information. Scientificvisualization refers to the use of visual geospatial displays to explore data and through that data explorationto generate hypotheses. Leading eventually to improved understanding, these hypotheses intend to stimulatethe visual thought process by using alternative and interactive graphic representations of the data.

2 PROBLEMS IN PS-INSAR DATA INTERPETATION

The products of the PS processing chain [1] consists of various estimated parameters such as locations, relativeheight profiles, ensemble coherence, velocity rates, etc. The realization and use of this vast PS informationdatabase is a key factor in the geographical or geological interpretation of the signal. In the context of PS-InSAR research, we see the development of classification of PS reflections in various forms based on the PTA(Point Target Analysis). However, the options of using geo-information have not been explored to a recognizableextent and therefore, the PS visualization in terms of physical ground features (geo-information)remains ad-hocand unorganized [2].

The PS-InSAR community consists of two distinct groups of users namely the providers of the information(i.e, the radar scientists) and the users of the information (i.e, the decision making authorities, planners etc.).There exist different levels of understanding of the PS technique in these two groups and hence no direct exchangeof ideas related to it. The slow ground subsidence, as seen by PS analysis depends on a lot of local factors forthe behavior of individual scatterers. This localized behavior remains unnoticed by the providers of the resultsin most cases and hence leads to an ambiguous interpretation of the PS information, i.e, under-interpretationand over-interpretation, see Fig. 1, 2, 3.

Figure 1: Under-interpretation1 Figure 2: Over-interpretation2 Figure 3: Trade-off

Fig.1 shows an example of under-interpretation1 of a PS signal (displacement profile) where dynamic behaviorof the measurements is largely ignored and the displacement is considered in accordance with a linear deformationmodel. This implies more trust on the deformation model and less considerations of the observation stochastics.The opposite extreme of the under-interpretation situation is shown in Fig. 2 as over-interpretation2, where atoo optimistic stochastic model is used and no physical assumptions are made on the deformation model. Herethe model is given less importance in comparison to the observations. The above described scenarios cover mostof the PS data interpretation, but what is still unexplored is the balancing between these two extremes. Forinstance, if the deformation regimes can be separated based on a local event (an infrastructure construction,water level variation due to extraction etc.), a substantial tradeoff can be realized between the under and overinterpretation of the PS data. As shown by Fig 3, different displacement behavior trend can be seen in twoperiods of time. In principle, the challenge is to apply Ockham’s razor, i.e, find a model with a minimumnumber of parameters to estimate which acknowledges the stochasticity of the data.

3 CONCEPT OF SCIENTIFIC VISUALIZATION

The term scientific visualization or Geovisualization is related to cartography, image analysis, information visu-alization, Exploratory Data Analysis (EDA), and provides theory, methods and tools for the visual exploration,analysis, synthesis and presentation of geo-information data [3].

In geo-visualization, the map is used as a fundamental element. In a web-based environment with basic GISfunctionalities, the map becomes dynamic, interactive, and accessible to many users as a visual communicationtool. The user can interact with the maps and the data behind it, which adds a different perspective, as thenature of the geo-spatial data under study can be explored interactively. The map should be seen as an interfaceto PS-InSAR data and geo-databases to support information access and exploratory activities. There is alsoa clear need for this capability since the magnitude and complexity of the available data pose a challenge asto how the PS data can be transformed into information and ultimately to knowledge. This reaserch assumesthat if one is able to look at the data from different perspectives, for instance via alternative map views, or incombination with other graphics such as diagrams, graphs or even photographs and videos, interpretation ofthe data will improve.

1the modeling of the behavior of the point uses insufficient parameters2The modeling of the behavior of the point uses too many parameters

4 WEB GIS REPRESENTATION OF PS-INSAR DATA

The recent integration of the internet and GIS technology has produced an expanding area of research referredto as ’Web-based GIS’ or ’Internet GIS’. Internet distributed GI services are capable to interact with multipleand heterogeneous systems and data servers. Via GIS, the different user groups (researchers, policy makersand authorities, NGOs, etc) can have access to large and useful databases and graphic tools to support theirinvestigations and decision making. Internet based GIS could play a key role in the collection and disseminationof information in a fast, relatively inexpensive, and straightforward manner during the various stages of PS-InSAR data analysis. That is, by assimilating geo-datasets from various online spatial data networks intostandard(s) defined by Open Geospatial Consortium (OGC) [4], vital spatial information is readily available tonumerous users. These users do not require high-level technical skills on the hardware, software, data collection,data fusion, and data transformation side. This advantage is harnessed in the present work of PS data andresults visualization using a server based geographic information combination.

Anther substantial benefit of web GIS is that it is independent of the computer platform (windows, unixetc.) and also the information database type (CAD, oracle, ESRI shapefiles, Postgres, Microstation etc.). Thepossibility of soliciting local expertise information also exists by updating the database via the internet. Theopen source utilities that are used in the work are free and can be easily customized to meet a variety of enduser requirements.

4.1 WEB MAP SERVER (WMS)

A Web Map Service (WMS) [4] is a standardized term for creating maps of georeferenced data. This data consistsof different attributes and the WMS aids to display it in form of a ’map’— defined as a visual representationof geo-data. The data itself may not be open to the users but as a visual representative instead. These mapsare generally rendered in a pictorial format such as PNG, GIF or JPEG etc. A standard web browser can aska WMS to perform these operations simply by submitting requests in the form of Uniform Resource Locators(URLs) [4]. The content of such url’s depends on which of the tasks is requested. When requesting a map, aclient may specify the information to be shown on the map (one or more Data Layers), what portion of theEarth is to be mapped (a Bounding Box ), the projected or geographic coordinate reference system to be used(the Spatial Reference System, or SRS), the desired output format, the output size (Width and Height), andbackground transparency and color. When two or more maps are produced with the same bounding box, spatialreference system, and output size, the results can be layered to produce a composite map. The use of imageformats that support transparent backgrounds allows the lower Layers to be visible. Furthermore, individualmap Layers can be requested from different Servers.

The Open Geospatial Consortium(OGC), has standardized three basic WMS operations in url requests [4].Firstly, the command GetCapabilities returns a description of the service’s information content and acceptablerequest parameters, or simply, the details of what services this particular WMS can perform. Second specificationas GetMap returns a map image whose geo-spatial and dimensional parameters are well defined in the requestedurl. Finally, an optional request of GetFeatureInfo returns information about particular features shown on amap. The WMS specification enables the creation of a network of distributed Map Servers from which clientscan build customized maps. A particular WMS provider in a distributed WMS network need only be theprovider of its own data collection. This stands in contrast to vertically-integrated web mapping sites thatgather in one place all of the data to be made accessible by their own private interface [5].

5 WEB-GIS USER INTERFACE USING MAPSERVER

In the present work, a MapServer [5] web-GIS tool was selected as the CGI engine for building spatially enabledinternet applications (Web-GIS map user interface). Originally developed by the University of Minnesota,MapServer is a core mapping engine with an open source development environment. Since MapServer is designedand developed to support the evolving OGC standards [4], it can be used to incorporate any remote data sourcesthat publish data consistent with these standards. Usually the dedicated GIS and mapping applications arecomputationally intensive tasks, and by using a dedicated server, less load is put on the user’s computer.Probably the main advantage of a web based system is the ability to provide mapping capability to anyone whocan run a Web-browser, even in low bandwidth conditions. Gigabytes of spatial data can be manipulated on theserver side and only small compressed images (usually less than 100 KB) are sent to the client. Via Mapserver,the dedicated GIS operations on the server can be called with simple buttons on the client side. The HTMLclient (browser) with a query string sends variable values to the server, and the Mapserver CGI parses/renders

the variables, reads the digital geodata (in form of shapefiles) described in the mapfile, draws the requestedmap, prepares an HTML page (according to template definition) for publishing and sends this HTML page tothe client browser. This process continues each time the browser sends in a variable request to the server. Fig.4 shows a sequential flow of Mapserver WMS operations.

Figure 4: Sequential flow of operations in a MapServer WMS

5.1 Interface design with a simple web browser

Since our system is intended to be used by users with different levels of understanding of PS-InSAR data andresults, it is important that the design interface is simple and understandable for a variety of geovisualizationtasks. Examples of a web browser based mapping application can be seen in Fig.5 and Fig.6.

The visualization interface is initialized by the request of server url (http://enterprise.lr.tudelft.nl/ swati/)from the browser. In the present work, the city of Rotterdam in the south of the Netherlands is selected asthe test area. The basic geoinformation data included for the study are road boundaries [7], infrastructureboundary [7], water areas [7], building boundaries [6], houses or constructed areas [6], open land [7], and alsothe PS-InSAR data points on top of all these layers. All the data layers are used in form of shapefiles. Thelayers included in the map have a on/off option for display. The WebGIS interface also supports ancillarymap graphics showing a legend, scale bar, display control buttons etc. The interface provides basic optionsfor querying the PS points database for details of individual PS point records i.e., their location, topographicheights, ensemble coherence, yearly linear deformation, and time series of linear deformation. Fig.5 and Fig.6show some examples of the simple web client with PZI (Pan Zoom Identify) functionality.

Figure 5: PZI functnality on a simple web browser Figure 6: Enlarged view of the integrated visualization

5.2 Use of freely available viewers and data servers

There are a number of freely available viewers (Gaia viewer, JUMP workbench, ArcExplorer, etc.) [9] that allhave the basic data view (PZI) and combination functionalities. These viewers can be used in accessing differentdatabases and combining views of their data in a common spatial extent. The PS information database servercan be added as a separate layer and hence PS points can be viewed in conjunction with other geo-informationof the same area. Some examples of freely available data servers are NASA and Globe webservers which providethe global mosaic images, landslide images, SRTM reflectance with various arc lengths, arial photographs etc.The use of these viewers in combination with these data servers is particularly advantageous in integrated viewsof large areas in PS visualization, for example, analyzing a wide subsidence pattern. Fig.7 and Fig.8 show somescreenshots of PS combination views with various global layers using Gaia viewer [9].

Figure 7: Integrated visualization of PS with Globe pansharpened mosaic

Figure 8: Integrated visualization of PS with SRTM re-flectance map

5.3 Employing projected geo-data client Google Earth

Google EarthTM [8] is a free geo-data client with 3D view functionalities. The PS information data server isconnected to Google Earth server by the use of XML (eXtensible Markup Language) configuration to the GoogleEarth initialization files. A NetworkLink to the PS data WMS is defined in the Google configuration (.kml file),and this Networklink accesses the WMS by a ’.php script’. In principle, the .php script script results in an imageof PS data, displayed by Google Earth along with the Google server data. The PS database is included andstored as temporary information in the Google Earth view. The legends of PS or any other included databaseinformation can be shown as information tags in the side panel of the viewer. There is also a possibility ofsharing this PS database with the Google Earth community or within a network of research groups.

This state of information sharing via a free and popular viewer such as GoogleEarth could be very beneficialin showing the local PS results to the general people (authorities, residents, NGOs etc.). Some screenshots ofintegrated view of GoogleEarth with PS are shown in Fig. 9 and Fig. 10.

6 CONCLUSIONS

With the increasing availability of free data servers, the amount of local geo-information is increasing manifolds.In this research we propose the consideration of local geo-information to assist the interpretation of the PS.The open source web GIS utilities are used to provide a relatively powerful and straightforward platform fordata sharing. The supplementary geographical information databases may add as an independent validationsource as well as a connection to forward modelling of PS processing. Some approaches of combining the PSinformation with geo-information databases using web GIS are shown. The data combination facilitates tobridge the communication gap between the PS result providers and the actual users by providing the possi-bility of joint interpretation of the PS data. The joint interpretation may help in finding a trade-off betweenunder-interpretation and over-interpretation of PS behavior. Local expertise may reveal some very localized

Figure 9: PS and local geo-data combination with GoogleEarth

Figure 10: Zoom over PS points with high subsidence peryear (red dots)

phenomena (for instance, infrastructure construction, subsurface water extraction etc.) which may be helpfulto the deformation model considerations in the PS processing chain. However, the data sensitivity issues needto be considered and therefore an exchange of PS information between the various research groups and the localauthorities could be a first step in implementing the web GIS data sharing approach.[4]

References

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[2] Gehlot S., Ketelaar V.B.H., Verbree E., and Hanssen R.F. (2005). Conceptual Framework for PS-InSARInterpretation Assisted by Geo-information Technology. In ISPRS Workshop on ”High Resolution EarthImaging for Geospatial Information”, Hannover, Germany, 17-20 May 2005, cdrom pp.

[3] Dykes J., MacEachren A.M., and Kraak M.J. (2005). Exploring Geovisualization. Elsevier Publishers,Amsterdam

[4] The Open Geospatial Consortium Inc.(OGC)(2005). www.opengeospatial.org

[5] University of Minnesota MapServer Documentation (2005). http://mapserver.gis.umn.edu/

[6] The Large Scale Standard Map of The Netherlands (GBKN) (2005). http://www.gbkn.nl/

[7] Top10 Vector Map, Topographical Department of the Dutch Land Registary Office (2005).http://www.kadaster.nl

[8] Google Earth Homepage (2005). http://earth.google.com/

[9] Gaia Viewer Homepage (2005). http://www.thecarbonportal.net/