International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999
COMPARISON OF DEMS FROM STAR-3i INTERFEROMETRIC SAR AND SCANNING LASER
J. Bryan Mercer and Steven Schnick
Intermap Technologies Corp.
Calgary, Alberta, Canada
Tel: (403) 266-0900
Fax: (403) 265-0499
e-mail: bmercer@intermap.ca
KEY WORDS: bald-earth, DEM, Digital Elevation Model, IFSAR, interferometry, laser, lidar, SAR, STAR-3i
ABSTRACT
Within the past few years there has been a significant increase in the use of Digital Elevation Models (DEMs), for
applications as varied as telecommunications and precision agriculture, forestry and flood-risk assessment. Additionally,
one may expect that externally-supplied DEMs will be used increasingly for ortho-rectification of high-resolution satellite
imagery as it becomes available.
There are now available several technologies from which DEMs may be created, with varying degrees of detail, accuracy,
associated cost and availability. In both North America and Europe, government agencies have traditionally provided
mapping products, including derived DEMSs, to the public. However, the applications noted above are expected to require
increasing detail beyond what these government sources currently provide. Three technologies that are currently creating
DEMS at increased levels of detail include (1) soft-copy photogrammetry using stereo air-photo, (2) scanning airborne laser
or lidar and (3) interferometric airborne radar.
In this paper we examine the capabilities and deficiencies of two of these DEM sources — airborne laser scanning systems
and interferometric airborne radar. STAR-3i is an airborne interferometric SAR system carried in a Lear jet and operated by
Intermap Technologies Corporation. In the past two and a half years of commercial operation, DEMs have been acquired
for several hundred thousand km on several continents. At the same time, several companies have been creating DEMs
over smaller areas using airborne laser scanning systems, and comparative data sets are now becoming available.
In this paper, the comparative DEM performance of these two technologies will be demonstrated with respect to three
application areas of interest: (1) bald-earth performance for flood-plain risk analysis, (ii) building height extraction in urban
areas, and (iii) forested and agricultural areas with respect to vegetation issues. In these examples, the laser-derived DEMs
are treated as truth at the 15 cm (1 sigma) level. The radar-derived DEMs, created on a 5 meter grid, are shown to exhibit a
‘noise floor’ at the 35 cm (1 sigma) level for typical operational altitudes in non-urban bald-earth environments. While the
performance is not as strong as that of the laser systems, there are obvious cost and schedule advantages where large area
coverage is required. These will be addressed. It is concluded that the technologies tend to provide complementary rather
than competitive solutions for many applications.
1. INTRODUCTION STAR-3i system, an airborne interferometric SAR
(Synthetic Aperture Radar), owned and commercially
Digital Elevation Models (DEMs) are being used
increasingly at a variety of mapping scales, and for a
range of applications. The DEM price, irrespective of
technology, tends to vary with the level of detail sought
— that is, with the desired mapping scale. The
introduction of DEMs created from active airborne
sensors (specifically interferometric radar on the one
hand, and scanning laser or lidar on the other) has, in
recent years, expanded the circumstances under which
DEMs can be collected.
Radar is able to gather data rapidly over large areas in
day/night, cloud-covered conditions. This factor
positively impacts both the timeliness and the price of
DEMs. Interferometric radar (IFSAR) has received
much attention recently because of its potential to
improve the level of detail achievable in radar-derived
DEMs. Since early 1997, about 750,000 km? of DEMs
have been acquired over several continents by the new
operated by Intermap Technologies. With data collected
and presented at sample spacings down to 2.5 meters and
with vertical accuracies at the meter to sub-meter level,
this presents users with the possibility of obtaining wide
area DEM coverage at levels of detail and consistency
not previously available in general. On the other hand,
the STAR-3i system is an X-band radar and the DEM
derived from it references the scattering surface or
volume with which the radar beam interacts. This means
that scattering objects such as trees or buildings will
contribute to the radar-derived DEM. This problem is
similar to that of optically-derived DEMs, but of course
the radar has its own particularities.
Airborne scanning laser systems are being operated
widely with about 35 systems in operation at this time
(M. Flood, 1999). Vertical accuracies in the range of 15
to 30 cm (RMSE) are being generally claimed, with data
samples ranging from one to five meter spacing.
