HAND, a new terrain descriptor using SRTM-DEM - DPI - Inpe

ear all topological components, extractable from the SRTM-DEM, which
allowed for the development of the new HAND terrain descriptor. The
results of the test of the HAND descriptor reported here demonstrate its
capacity to resolve meaningfully the delineation of local environments.
There are a host of generic terrain descriptors, e.g. topographic
indexes, which could be compared to the HAND, but an exhaustive
comparison is beyond the aim of the present work. The closest
descriptor to the vertical distance along a flow path is the non-
Euclidian horizontal distance along the same flow path (Tucker et al.,
2001). To test the similarity between the two descriptors, 1000
independent points (pixels) were randomly drawn from the test area
SRTM-DEM. According to the linear regression, the horizontal distance
can only explain 55% of the total variance that can be explained by the
HAND (Fig. 14).
There is an obvious relation between horizontal and vertical
distances because it is expected that as one moves away from the
drainage, the terrain will get higher. However, the other 45% of the
variance is new information that only the HAND descriptor can give.
Points with large horizontal distances but low HAND are indicative of
great flat areas connected to the drainage (swampy areas). The innate
swampy characteristic of these terrains, for example, would never be
seen with horizontal distances alone. With a large HAND value
(vertical flow path distance) and a small horizontal flow path distance,
the terrain is quickly rising away from the stream, i.e. a well etched
drainage valley. But, although the HAND has this evident advantage,
the horizontal flow path distance is a great advance over plain
Euclidian distances, because the latter may connect and relate points
on the terrain that do not belong to the same catchment. Furthermore,
horizontal flow path distances to the nearest drainage might have a
bearing in terms of span of time spent in draining flows, determined
by hydraulic conductivity and slope along the flow path. Height above
the nearest drainage correlates directly with gravitational potential,
and this fact alone sets this descriptor aside as unique.
5. Conclusions
The height above the nearest drainage algorithm was developed on
top of the local drain directions and drainage networks, two well
established and basic topographic descriptors. The HAND has added
the height difference along flow paths, or draining potential, as a
significant and unique terrain descriptor. The HAND terrain descriptor
produces a normalized digital elevation model (HAND grid) that can
be applied to classify terrain in a manner that is related to local soil
water conditions. The HAND grid, a DEM where all pixels have been
mapped according to their draining potential, has a wide range of
potential applications. The application of the HAND descriptor in
classifying the terrain within a monitored hydrological catchment in
Amazonia revealed strong correlations between soil water conditions,
like classes of water table depth, and topography. This correlation
obeys the physical principle of soil draining potential, or relative
vertical distance to drainage, which can be detected remotely through
the topography of the vegetation canopy found in the SRTM-DEM
data. To our knowledge no previous study has noticed or reported the
height above the nearest drainage as likely being a good terrain
descriptor. It increases usability of the SRTM-DEM data and provides a
new quantitative view on the steady state landscape, one that was
missing in the repertoire of terrain descriptors.
Acknowledgements
This work was developed within the Environmental Physics Group
of GEOMA modeling network (Brazil's Ministry of Science and
Technology funding), with invaluable collaboration and co-funding
from the LBA project (Igarapé Asu instrumented catchment study).
This work would not have been possible without full support of INPE
and INPA. The Igarapé Asu catchment study was also funded by the
ARTICLE IN PRESS
12 C.D. Rennó et al. / Remote Sensing of Environment xxx (2008) xxx-xxx
PPG7/FINEP Ecocarbon project and by CTEnerg and CThidro (Energy
and Water Resources Sectorial Funds). Brazilian Science Funding
Agency, CNPq and Fapeam, also contributed with grants and scholarships
of personnel involved in the project. Thanks to Gerard Banon
(DPI/INPE), two anonymous reviewers and the editor for their valuable
comments and questions. We thank dearly Antonio Huxley for support
in the field work. We thank also Evlyn Novo, Adriana Affonso and John
Melack for the cession of the JERS-1 based flood land numerical mask.
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Please cite this article as: Rennó, C. D., et al., HAND, a new terrain descriptor using SRTM-DEM: Mapping terra-firme rainforest environments
in Amazonia, Remote Sensing of Environment (2008), doi:10.1016/j.rse.2008.03.018