Wieke Heldens

Urban Footprint Processor—Fully Automated Processing Chain Generating Settlement Masks From Global Data of the TanDEM-X Mission

The German TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) mission (TDM) collects two global data sets of very high resolution (VHR) synthetic aperture radar (SAR) images between 2011 and 2013. Such imagery provides a unique information source for the identification of built-up areas in a so far unique spatial detail. This letter presents the novel implementation of a fully automated processing system for the delineation of human settlements worldwide based on the SAR data acquired in the context of the TDM. The proposed Urban Footprint Processor (UFP) includes three main processing stages dedicated to: i) the extraction of texture information suitable for highlighting regions characterized by highly structured and heterogeneous built-up areas; ii) the generation of a binary settlement layer (built-up, non-built-up) based on an unsupervised classification scheme accounting for both the original backscattering amplitude and the extracted texture; and iii) a final post-editing and mosaicking phase aimed at providing the final Urban Footprint (UF) product for arbitrary geographical regions. Experimental results assess the high potential of the TDM data and the proposed UFP to provide highly accurate geo-data for an improved global mapping of human settlements.

TanDEM-X mission—new perspectives for the inventory and monitoring of global settlement patterns

Abstract. TerraSAR-X add-on for digital elevation measurement (TanDEM-X) is a German Earth observation mission collecting a total of two global coverages of very high resolution (VHR) synthetic aperture radar (SAR) X-band data with a spatial resolution of around three meters in the years 2011 and 2012. With these, the TanDEM-X mission (TDM) will provide a unique data set which is complementary to existing global coverages based on medium (MR) or high resolution (HR) optical imagery. The capabilities of the TDM in terms of supporting the analysis and monitoring of global human settlement patterns are explored and demonstrated. The basic methodology for a fully-operational detection and delineation of built-up areas from VHR SAR data is presented along with a description of the resulting geo-information product—the urban footprint (UF) mask—and the operational processing environment for the UF production. Moreover, potential follow-on analyses based on the intermediate products generated in the context of the UF analysis are introduced and discussed. The results of the study indicate the high potential of the TDM with respect to an analysis of urbanization patterns, peri-urbanization, spatio-temporal dynamics of settlement development as well as population estimation, vulnerability assessment and modeling of global change.

Can the Future EnMAP Mission Contribute to Urban Applications? A Literature Survey

With urban populations and their footprints growing globally, the need to assess the dynamics of the urban environment increases. Remote sensing is one approach that can analyze these developments quantitatively with respect to spatially and temporally large scale changes. With the 2015 launch of the spaceborne EnMAP mission, a new hyperspectral sensor with high signal-to-noise ratio at medium spatial resolution, and a 21 day global revisit capability will become available. This paper presents the results of a literature survey on existing applications and image analysis techniques in the context of urban remote sensing in order to identify and outline potential contributions of the future EnMAP mission. Regarding urban applications, four frequently addressed topics have been identified: urban development and planning, urban growth assessment, risk and vulnerability assessment and urban climate. The requirements of four application fields and associated image processing techniques used to retrieve desired parameters and create geo-information products have been reviewed. As a result, we identified promising research directions enabling the use of EnMAP for urban studies. First and foremost, research is required to analyze the spectral information content of an EnMAP pixel used to support material-based land cover mapping approaches. This information can subsequently be used to improve urban indicators, such as imperviousness. Second, we identified the global monitoring of urban areas as a promising field of investigation taking advantage of EnMAP’s spatial coverage and revisit capability. However, owing to the limitations of EnMAPs spatial resolution for urban applications, research should also focus on hyperspectral resolution enhancement to enable retrieving material information on sub-pixel level.

Identification and characterization of urban structures using VHR SAR data

The global process of urbanization is associated with various ecological, social and economic changes in both the built-up area and the adjacent natural or cultivated landscape. To manage the effects and impacts of this development, effective urban and regional planning requires accurate and up to date information on the urban dynamics. This paper introduces a methodology to automatically detect human settlements and then further characterize the identified built-up areas in terms of the building density based on VHR SAR data. The SAR imagery is acquired by the German satellite system TerraSAR-X. Regarding the delineation of the built-up area in the region of Munich we achieved an overall accuracy of 94 % and a Kappa of 0.86. The estimation of building density showed a coefficient of determination (r2) of up to 0.74. The mean absolute error of the modeled building densities was 5 %.

Analysis of Surface Thermal Patterns in Relation to Urban Structure Types: A Case Study for the City of Munich

Scientists have reached to a large extent agreement on climate warming for the coming decades. This will especially have immense impact on cities which show in general a significantly higher temperature compared to rural surroundings, e.g. due to high percentage of impervious surfaces. This study shows capabilities of airborne and spaceborne thermal remotely sensed data to derive and analyze land surface temperatures (LST). Dependencies of LST to urban structure types (UST) with respect to their location within the city are analyzed. Results prove distinct correlations between LST and vegetation fraction as well as percentage of impervious surfaces. Beyond this, different USTs prove influences on LST. Last but not least, a general decrease of LST with increasing distance to the city center is confirmed for the city of Munich. However, the USTs superimpose this trend and have a significant influence on the local LST.

A novel approach for anthropogenic heat flux estimation from space

The recently launched H2020 project URBANFLUXES (URBan ANthrpogenic heat FLUX from Earth observation Satellites) investigates the potential of EO to retrieve anthropogenic heat flux, as a key component in the Urban Energy Budget (UEB). URBANFLUXES advances existing Earth Observation (EO) based methods for estimating spatial patterns of turbulent sensible and latent heat fluxes, as well as urban heat storage flux at city scale and local scale. Independent methods and models are engaged to evaluate the derived products and statistical analyses provide uncertainty measures. Optical, thermal and SAR data are exploited to improve the accuracy of the UEB components spatial distribution calculation. Synergistic use of different types and of various resolution EO data allows estimates in local and city scale. Ultimate goal of the URBANFLUXES is to develop a highly automated method for estimating UEB components to use with Copernicus Sentinel data, enabling its integration into applications and operational services.

The global urban footprint — Processing status and cross comparison to existing human settlement products

The main goal of the TanDEM-X mission (TDM) is the generation of a global digital elevation model (DEM). The global SAR dataset, which is made available in the context of the TDM, is also used to create a global human settlement layer, the Global Urban Footprint (GUF). This paper presents a first large area cross comparison between the Global Urban Footprint and existing human settlement products, which shows promising results with an achieved confidence of 95.86% Overall, 71.15% Producers and 85.22% Users accuracy.

Unsupervised high-resolution global monitoring of urban settlements

The TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement, TDM) German mission collects two global data sets of very high resolution (VHR) synthetic aperture radar (SAR) imagery between 2011 and 2013. With a spatial resolution of ~3 m, these images provide a unique data source for the delineation of built-up areas in a spatial detail that could not be addressed so far by currently existing global human settlements layers (GHSL). In this paper we present a novel unsupervised procedure that automatically detects built-up areas from TDM data at the unprecedented spatial resolution of 0.4 arcsec (~12 m). The proposed technique is being used to generate a world-wide inventory of human settlements referred to as Global Urban Footprint (GUF), which is also intended to be publicly provided at a spatial resolution of ~3.0 arcsec (i.e., ~50-75 m).

Monitoring of global urbanization-time series analyses for mega cities based on optical and SAR data

Mega cities and their development are a synonym for the steady and dynamic trend of global urbanization. The number of these extreme metropolises exploded from three mega cities in 1975 (Mexico City, New York, Tokyo) to actually 27 and it is expected that more than 100 new ones will emerge within the next three decades. However, particularly in developing countries the spatiotemporal characteristics of mega cities development are still not well known. Therefore, this study introduces the concept and results of a global monitoring of the spatiotemporal mega cities development based on Earth Observation (EO) data. We applied straightforward, semi-automated object-oriented and pixel-based image classification algorithms to high resolution (HR) optical (Landsat) and very high resolution (VHR) SAR (TerraSAR-X) imagery covering a time span of 40 years in an interval of 10 years. By mapping the extent of the urban area for each single decade, we could subsequently apply post-classification change detection methods to visualize the dimension, patterns and dynamics of urban sprawl for the current 27 mega cities. The results of this study demonstrate the capabilities and benefits of satellite-based EO to support the collection of data on global urbanization trends and patterns. The analyses also showed the high potential of VHR SAR data for a more detailed characterization of settlement patterns and urban morphology.

Supporting urban micro climate modelling with airborne hyperspectral data

Urban climate modeling is a means to increase the understanding of the urban climate. In order to model real world environments, area-wide spatial information is required. This paper demonstrates how hyperspectral remote sensing data and additional height data can provide a large part of this information efficiently, reducing the need for extensive field surveys. From the hyperspectral data the surface materials, the LAI and the surface albedo are estimated. The height data supplies the heights of buildings and trees. Using these maps as input for the urban micro climate model ENVI-met, simulations of temperature, wind and humidity among others can be carried out.