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DEMSs Created from Airborne IFSAR — An Update 
Bryan Mercer 
Intermap Technologies Corp., 1000, 736 — 8" Ave, S. W., Calgary, AB, Canada, T2P 1H4 
bmercer@intermap.ca 
Commission II, WGIU2 
KEYWORDS: Interferometric SAR, LIDAR, Mapping, DEM 
ABSTRACT 
The factors affecting the wide-scale use of DEMs (Digital Elevation Models) and their associated ORRIs (Ortho-Rectified Radar 
Images) created from airborne IFSAR (Interferometric Synthetic Aperture Radar) have been evolving rapidly over the past few years. 
These factors include both technical and non-technical characteristics. In this paper we review several of these characteristics 
including vertical accuracy, sample spacing, image resolution, bald-earth extraction, vegetation penetration, cost and availability. 
These factors will be reviewed mainly in the context of the STAR-3i and TopoSAR systems, which are both commercial airborne 
IFSARs operated by Intermap Technologies. The objective is to provide a status report on what can be expected with current data 
sets and what might be expected in the near future. Key to an understanding of most of these factors is an appreciation of price vs. 
performance and how DEMs derived from airborne IFSAR relate to those created from satellite systems on the one hand and lidar or 
photogrammetric systems on the other. In particular we will focus on two major ‘events’ that illustrate what can now be considered 
status quo, on the one hand, and what is a very interesting developmental trend on the other. 
1. INTRODUCTION 
In recent years the appetite for three-dimensional geospatial 
data sets has been steadily increasing as diverse applications 
grow and the quality and availability of data sources expands. 
Users of DEMs (Digital Elevation Models) have the opportunity 
to match requirement — in terms of such metrics as vertical 
accuracy and horizontal sample spacing — with availability and 
with price, scaled over several orders of magnitude. At the low 
price end of the availability spectrum, satellite-based systems 
including both radar (SRTM, Radarsat, ERS) and optical 
(ASTER, SPOTS3), provide broad coverage — almost global in 
extent - with typical sample spacing of 30 to 100 meters and 
vertical accuracies ranging from 5-50 meters RMSE. At the 
higher price end (relatively speaking), airborne lidar typically 
provides DEMs with sample spacing from 0.5 to 2 meters and 
vertical accuracies in the 15 — 30 cm RMSE range, often of 
limited areas where the desired detail matches a particular need 
and justifies a higher unit cost. Airborne photogrammetry 
competes in the same arena with similar achievable accuracies 
but usually more coarsely sampled data. Airborne IFSAR 
(Interferometric Synthetic Aperture Radar) on the other hand 
finds itself in an intermediate niche where DEM products 
quoting vertical accuracies from 0.5 — 3 meters RMSE and 
sample spacing of 5 meters are now produced routinely, at costs 
that are also intermediate between the space borne and airborne 
optical products. Furthermore, the availability or accessibility, 
in an off-the-shelf context, is becoming an important factor, 
particularly for the development of new applications and 
markets. Although lacking the global acquisition capability of 
the aforementioned satellites, airborne IFSAR does have rapid, 
wide-area acquisition capability which has recently manifested 
itself in national DEM acquisition programs (NextMap Britain, 
for example, which will be described below). The DEMs from 
such programs are now available in a database for general 
access at relatively low cost and while they currently contain 
about 2 million kmsq of DEMs, they are growing rapidly. 
Among the problems that challenge IFSAR is the issue of 
foliage — in particular, closed forest. The DSM (Digital Surface 
841 
Model) that is acquired represents, in the case of forest canopy, 
a volumetric response which in the case of short wavelength (X- 
Band and C-Band) IFSAR is typically an effective height 
somewhat less the true canopy height (e.g. Andersen et. al., 
2003). Over the past few years there has been considerable 
research interest in the use of long wavelength IFSAR (L-Band 
and P-Band) IFSAR, supplemented by polarimetric information 
(POLInSAR) in order to extract bare ground elevation as well 
as canopy information (e.g. Cloude and Papanathassiou, 1998) 
These advances have also been introduced to commercial 
systems (Hofman et. al., 1999) and look very promising for the 
future. 
In this overview paper we will focus on the wide-area coverage 
capability demonstrated to date and note the potential of long 
wavelength POLInSAR for the future. As background, the 
technical characteristics of IFSAR will be presented and the 
features of two of Intermap's airborne IFSAR systems will be 
described. The results of the fore-mentioned NextMap Britain 
program will be presented with respect both to external and 
internal validation studies. The plans for NextMap USA and 
other similar programs will be briefly addressed. We will also 
summarize the results of a polarimetric P-Band project in which 
a ground elevation model was recovered beneath canopy with 
heights up to 50 meters. 
2. AIRBORNE IFSAR BACKGROUND 
2.1 General 
The interferometric process has been widely discussed in the 
literature, particularly for the case of repeat pass interferometry 
(e.g. Zebkor and Villsenor (1992), Goldstein et. al., (1988). 
Some of the general issues associated with airborne 
interferometry have been discussed, for example, in Gray and 
Farris-Manning (1993), Madsen et al. (1991). The geometry 
relevant to height extraction, ‘h’, is illustrated in Figure 1. If 
the two antennas, separated by baseline ‘B’, receive the back- 
scattered signal from the same ground pixel, there will be a 
path-difference ‘8’ between the two received wave-fronts. The