scattering level perceived by the radar - in this case 12.8
meters — and the standard deviation reflects the
variability of this level which is 1.7 meters in this
instance. These statistics, of course, are dependant on
canopy and radar viewing geometry and suggest
interesting topics for study, not to be pursued here.
STAR-3i and EarthData Laser
Data Set Mean (m) Std Dev (m)
A STAR-3i Radar 248.1 0.29
Bare EarthData Laser 247.4 0.29
Difference 0.7 0.33
8 STAR-3i Radar 261.8 1.79
Forest EarthData Laser 249.0 0.15
— Difference. 12.8 1.71
c STAR-3i Radar 249.6 0.27
Bare EarthData Laser 248.7 0.09
Difference 09 0.27
Table 2: DEM and difference surface statistics for
three regions of the Red River data set
It is interesting to compare a cross-section across the two
DEMs as presented in Figure 7. Because of the
previously noted offset of the laser data, the latter has
been incremented by +1.0 meter to facilitate comparison.
The radar data are somewhat noisier than the laser data,
as expected, but the profiles track quite well with some
small but real differences presumably caused by small
bushes in the dry river channel (meander scar) which are
observed in the radar but not in the laser bald earth
DEM.
6.2 Baden-Wurttemberg, Germany
The second example is for an area of mixed forest and
agriculture in Germany. Unlike the previous flood plain
example, the terrain consists of rolling hills and valleys.
The radar data were collected of the whole state of
Baden-Wurttemberg by the STAR-3i system in July,
1998. During this period, the vegetation was in full leaf
and crops were well developed so the radar DEM would,
of course, reflect the crops and forests as well as
buildings and other objects. The state mapping agency
(the LVA, or Landesvermessungsamt), had acquired
laser data for a sub-region of dimensions (10 km x 15
km) about 80 km NNW of Stuttgart. The data were
acquired by Topscan in January, 1996 during leaf-off
conditions. The residual vegetation and other objects
had been removed by Topscan to create a bald-earth
DEM.
The LVA kindly provided the laser data to Intermap for
test purposes. In return, the radar data were provided to
Karlsruhe University (Dr. Manfred Sties) for reciprocal
analysis on behalf of the LVA. Because the laser data
were referenced to a local datum and geoid, while the
STAR-3i data were referenced to the WGS-84 ellipsoid
(horizontally and vertically), it was necessary for each
party to transform the other's data into the preferred
reference system. This was done using common
transformation parameters provided by the University of
Karlsruhe. The independent analyses will be jointly
published in a forthcoming article. In this paper, we
present only a small subset of the results obtained by
Intermap in order to illustrate the theme of the paper.
The area presented here includes a strip about 0.8 km x
2.5 km in Northing and Easting respectively. The
colorized DEMs from the laser and radar are shown in
Figure 8 and Figure 9 respectively, while the difference
surface is presented in Figure 11. The ortho-rectified
image (ORI) from the radar is displayed in Figure 10.
The terrain heights range from about 257 meters in the
valley (blue) to about 303 meters on the highest ridge
(red). As noted earlier, the laser DEM represents a bald-
earth surface while the radar DEM includes the trees,
crops and other objects above the ground. An interesting
feature on the lower left side is a deep gravel quarry.
Areas depicted in white are due to under-sampling — that
is, the absence of data within the 15 meter threshold
placed on the surface interpolator. The difference
surface shows the forest (and some buildings) in green,
while the bald earth and low crops (< 2 meters) are in
shades of cream and brown.
The field conditions are quite evident in the ORI of
Figure 10. Forest and crop patterns as well as a village
(lower right) are evident. Some of these characteristics
are also evident in the difference surface of Figure 11. In
particular, the forest, buildings, and some crop types are
manifested by their height. It should be noted that the
ORI is a measurement of radar back-scatter and hence of
roughness. Therefore, some low crops (e.g., cabbage)
will appear rough and relatively bright in the ORI but
will not appear in the difference surface. On the other
hand, crops such as corn appear in both.
Difference Surface Statistics
STAR-3i minus Laser
Data Set
A Bald Earth -0.47 0.28
B Crops 0.66 0.34
C Forest 21.04 2.16
Table 3: DEM and difference surface statistics for
three regions of the Baden-Wurttemberg data set
Three polygons reference different surface conditions to
be sampled statistically. Polygon ‘A’ is interpreted as
bare-earth, 'B' is a crop (type unknown), and 'C' is
forest. Mean and standard deviation for the difference
surface is provided for each of them in Table 3.
The areas sampled are relatively small (- 100m x 100m)
and the resulting standard deviation for the bald earth
area is about 28 cm, similar to that described as the
'noise floor' for the Red River example (and constant
with more extensive sampling in this project area). The
variability is slightly larger in area 'B', as would be
expected in a crop covered region. The crop sample is
about 1.1 meters higher than the bald-earth, and probably
represents a scattering level lower than the visible
surface. Sampling of bald-earth areas over the whole test
area incorporates systematic errors of about 50 cm into
Figure 8: La
amnifiometers
Figure 9: Ra
-
ET
Figure 11: D
the radar DE
superposed. 1
with control.
The other no
shows an eff
variability of ;
relative unifor
6.3 Denver, C
The third exa
building heigl
center. Mode
the urban core
small percent
Because of t
characterized
structures, tt
successfully t
near-vertical x
However, urb:
to thousands
surrounded b:
recreation arez
For many a[
schedule-effec
However, one
to determine
apartment bui
outside the cc
related issues
