result in a larger range of height errors across the swath, but this
effect will be less pronounced in multi-pass (e.g. NEXTMap)
since the imagery produced is averaged from multiple data takes
that are not aligned (Woodhouse et al., 2006). This hypothesis is
examined in this paper.
3. STUDY SITES
Three research sites are located in the United States (Minnesota
[2], Arizona [1]) and were selected because the bio-geophysical
characteristics of these sites provided a unique opportunity to
evaluate X-HH InSAR multi-pass data aggregated (called
NEXTMap) and single-pass (Intermap’s non-commercial data)
dataset as a source for high-resolution vegetation canopy height
estimates across a range of vegetation densities and structural
classes, as well as under a variety of topographic conditions and
environments (arid and temperate). A description of the three
sites is presented below.
3.1 Ely, Minnesota
The first site is situated in the temperate climate between
473130" N and 47/5230" N and 9137/30" W and 9152730"
W near the city of Ely, Minnesota. It is comprised of dense
homogenous coniferous and deciduous forests as well as mixed
forests with little understory. The common species are red pine
(Pinus resinosa), white pine (Pinus strobus), black spruce
(Picea mariana), and red maple (Acer rubrum). The site
covers an area of 169.8 km? and is dominated by rolling
topography with irregular slopes (0-18.7) and many craggy
outcrops of bedrock. The elevation range is 422 — 506 m.
3.2 International Falls, Minnesota
The International Falls, Minnesota, site is located between
483000" N and 4837730" N and 931500" W and 93 30700"
W. It represents more of a pure coniferous site than the Ely site.
Forests are dominated by coniferous species such as white pine,
white spruce (Picea glauca), and balsam fir (Abies balsamea)
with a mixture of white pine, red pine, and jack pine (Pinus
banksiana) more prominent in the eastern portion. The site was
16.35 km^ and situated on a lake plain with topographic
variation of less than 30 m and slopes less than 15.
3.3 Southern Arizona
The Arizona site is located near the Mexican border between
312250" N and 3145'00" N and 11014'53" W and
1113742" W. It represents a hot arid environment with a
diverse range of vegetation types on flat to steep terrain. The
vegetation classes are predominately grassland (e.g. Bouteloua
curtipendula and Schizachyrium scoparium), shrub / scrub (e.g.
thornscrub (Canotia holacantha)), and coniferous forests, with
minor coverage of wetlands, bare earth, and urban development.
Woody species dispersed throughout the area include various
species of oak (Quercus spp.), juniper (Juniperu spp.), desert
riparian cottonwoods (Populus fremontii), Goodding willow
(Salix gooddingi), Arizona ash (Fraxinus velutina), Arizona
walnut (Juglans major), Arizona sycamore (P/atanus wrightii),
Mexican elder (Sambucus mexicana), and velvet mesquite
(Prosopis velutina). The site is approximately 1,484 km? with a
range of elevations from 931 m in the plains to 1762 m in the
mountains and rolling topography of irregular slopes (0-28).
4. DATASETS
Digital elevation models (DEMs) are topographic models of the
earth's terrain representing either bare earth or surface
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
elevations. DEMs typically are offered as a continuous
elevation surface (Podobnikar, 2009). DEMs with elevations of
vegetation, buildings, and other cultural features digitally
removed are referred to as digital terrain models (DTMs),
whereas those that have maintained heights of features above
the ground are called digital surface models (DSMs). Intermap
has created a continental U.S. database of X-HH InSAR derived
DTM and DSM data under the NEXTMap mapping program.
NEXTMap DSM data are a compilation of multiple-data takes
aggregated together to reduce errors associated with side-
looking viewing geometry of SAR sensors (Figure 1). Multi-
pass data processing provides a stable dataset for which
vegetation canopy height can be modelled (Kellndorfer et al.,
2004; Andersen et al., 2008; Chen et al., 2010). NEXTMap
DTMs are derived from X-HH InSAR DSMs using a 3D
workflow based on an ISO-certified process (Intermap, 2011).
Incidence angle are not extractable from aggregated data.
Single-data take X-HH InSAR flight line strips used to create
the NEXTMap data were available and used to access incidence
angle variations.
Both the single- and multi-pass datasets were obtained as 32-bit
floating 5-meter posted elevation grids (also known as 5 m
ground sampling distance, or ground range pixel spacing) in
geometric coordinates for three sites. The NEXTMap DTM data
(multi-pass data aggregated have a 1 m, 1-3 m, and >3m linear
error (LE) 90% vertical accuracy in unobstructed terrain with
slopes less than 10°, 11° = 20, and greater than 20", respectively.
There are no published accuracies for the NEXTMap DSM or
the single-data take InSAR. Data of slopes less than 10” were
used in this research to isolate effects due to incidence angle
variations rather than changes in terrain slope.
Within a year of the INSAR data collection, field programs were
conducted to obtain tree and shrub vegetation heights using an
Abney hand spirit level or clinometer with an expected accuracy
of better than 0.5 m (e.g. 2.5% for a 20 m tree height) when the
observer has a clear view of the tree being measured. Mean
canopy height was taken as the average of the measured tree
heights and used as reference data to assess the vertical
accuracy of the InSAR derived vegetation canopy height of the
single-/multi-pass derived hg.
5. METHODS
The X-HH InSAR DTM data were subtracted from both the
single- and multi-pass datasets to derive vegetation canopy
height given by the hy, (Figure 2). To investigate the effect of
incidence angle on InSAR derived data, vegetation canopy
heights were extracted from the single- and multi-pass hg, for
each x-y in situ vegetation canopy height location, stratified by
three incidence angles (NR = 35, MR = 45’, FR = 55"), and by
vegetation cover type (shrub, deciduous, coniferous, mixed and
wetland). All height values were classified based on slope data
to ensure that only those values that represented terrain slopes
«10 were used; otherwise, the effects of incidence angle cannot
be evaluated independently. Root mean square and mean errors
were computed. To further investigate the effect of incidence
angle on InSAR derived hy, vegetation canopy heights
represented by h,, values along three transects located 1 km
(NR 2 35), 5 km (MR - 45), and 9 km (FR - 55) across
single-data take swaths in the range direction (Figure 5) were
extracted from the single and multi-pass datasets. These canopy
height values were compared to each other using the absolute
RMS difference (RMSD) and the absolute mean difference
(MD). RMSD was calculated since neither the single- nor multi-
