International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
RADARGRAMMETRIC DIGITAL SURFACE MODELS GENERATION 
FROM TERRASAR-X IMAGERY: 
CASE STUDIES, PROBLEMS AND POTENTIALITIES 
P. Capaldo, M. Crespi, F. Fratarcangeli, A. Nascetti, F. Pieralice 
Area di Geodesia e Geomatica — DICEA — Università di Roma “La Sapienza” 
via Eudossiana, 18 — 00184 — Rome, Italy 
Commission VII, WG VII/2 
KEY WORDS: DEM/DTM Generation, Radar, High resolution, Orientation, Modelling, Matching, Software 
ABSTRACT: 
The interest for the radargrammetric approach to Digital Surface Models (DSMs) generation has been growing in last years thanks to 
the availability of very high resolution imagery acquired by new SAR (Synthetic Aperture Radar) sensors, as COSMO-SkyMed, 
Radarsat-2 and TerraSAR-X, which are able to supply imagery up to 1 m ground resolution. 
DSMs radargrammetric generation approach consists of two basic steps, as for the standard photogrammetry applied to optical 
imagery: the imagery (at least a stereo pair) orientation and the image matching for the generation of the points cloud. The steps of 
the radargrammetric DSMs generation have been implemented into SISAR (Software per Immagini Satellitari ad Alta Risoluzione), 
a scientific software developed at Geodesy and Geomatics Institute of the University of Rome "La Sapienza". 
Moreover, starting from the radargrammetric orientation model, a tool for the Rational Polynomial Coefficients (RPCs) for SAR 
images have been implemented. The possibility to generate RPCs, re-parametrizing a rigorous orientation model through a 
standardized set of coefficients which can be managed by a Rational Polynomial Coefficients (RPFs) model (similarly to optical 
high resolution imagery) sounds of particular interest since, at present, the most part of SAR imagery (except from Radarsat-2) is not 
supplied with RPCs, although the corresponding RPFs model is available in several commercial software. In particular the RPCs 
model has been used in the matching process and in the stereo restitution for the DSMs generation, with the advantage of shorter 
computational time. 
This paper discusses the application and the results of the implemented algorithm for radargrammetric DSMs generation from 
TerraSAR-X SpotLight imagery, acquired in Spotlight mode over Trento (Northern Italy). Urban and extra-urban (forested, 
cultivated) areas were considered in two different tiles, and a final overall accuracy ranging from 4.5 to 6 meters was achieved as 
regards the point clouds, enough well distributed independently from the land cover; moreover, it was highlighted the benefit to filter 
the originally derived points cloud with a global DSM as SRTM DEM, what leads to an accuracy improvement of about 20% paying 
a loss of matched points of about 10%. 
1. INTRODUCTION technique (Hirschmiiller, 2008). Some of these approaches, 
already well established for the aerial and satellite optical 
Both optical and SAR imagery are characterized by proper ^ imagery, have been recently adapted also to SAR imagery for 
deformations due to the different acquisition geometries and the radargrammetric DSMs generation, and at present they are 
processes, which have to be duly taken into account during the implemented in several commercial software; nevertheless, 
two fundamental steps for DSMs generation (image orientation basically they were tuned over low resolution SAR imagery. 
and matching) if their potentialities have to be fully exploited. On the other hand, nowadays the interest for radargrammetry is 
At first, the correct orientation of remote sensing imagery is a now rapidly growing due to the new high resolution sensors in 
fundamental task for orthoimagery generation and 3D X-band as COSMO-SkyMed, Radarsat-2 and TerraSAR-X, 
feature/object extraction. which are able to supply imagery up to 1 m ground resolution; 
Optical and SAR imagery are orientated using rigorous or therefore, investigations were recently developed to exploit the 
Rational Polynomial Functions (RPFs) models with related whole high resolution SAR potentialities (Toutin, 2010; Perko 
coefficients (Rational Polynomial Coefficients - RPCs). The et al, 2011). In fact, radargrammetry represents a possible 
former approach tries to model the physical image acquisition alternative solution to avoid the classical decorrelation problem 
processes, the latter is a purely analytical approach based on the affecting the interferometric technique, using just a couple of 
RPCS supplied by the image providers. images with appropriate geometric configuration. Moreover, 
As regards the image matching, different approaches (mainly thanks to the high agility of the new sensors, it is possible to 
regarding optical imagery) have been developed primarily in acquire in a very short time images with a large range of 
the aerial photogrammetry, like the classical Area Based incidence angles, suitable for radargrammetric processing. 
Matching (ABM), or the Feature Based Matching (FBM) In this regards, the scientific software SISAR (Software per 
(Zhang and Gruen, 2006), or those based on cost functions and Immagini Satellitari ad Alta Risoluzione) developed at the 
dynamic programming techniques (Birchfield and Tomasi, Geodesy and Geomatic Institute of the University of Rome “La 
1999), or the recent and quite promising semi-global matching Sapienza”, encompassing both rigorous and RPFs orientation 
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