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 Accurate
measurements of topography and topographic change are fundamental
to most of the science themes addressed in this report. Topographic
measurement capabilities have advanced significantly in recent years,
such as from the Shuttle Radar Topography Mission (SRTM), but a
new generation of global digital elevation models with sub-meter-scale
accuracy is needed to provide more accurate and frequently measured
topography. Special attention must be paid to steep terrain (the
sources of landslides and some floods), because such terrain was
only partially imaged by SRTM.
Measurement
of transient topography, such as the surfaces of rivers in flood,
is also a key need. Airborne scanning laser altimeter and InSAR
techniques provide local to regional digital elevation models with
meter- to tens-of-meters-scale resolution at decimeter- to meter-level
vertical accuracy.
Because
direct remote sensing of seafloor topography is not feasible, our
historical understanding of the morphology of the 70% of Earth's
solid surface that lies beneath the oceans comes from observations
made with single- and multi-beam echo sounders mounted on oceanographic
research vessels. To date, only 0.1% of the oceans has been surveyed
at 100-m horizontal resolution, and large tracts of the seafloor
(such as most of the southern oceans) have not been mapped at all.
Seafloor topography can be inferred indirectly at lower (~10 km
horizontal) resolution, however, from satellite altimeter measurements
of the sea surface. Forthcoming developments in radar and laser
altimeters, constrained by selected higher-resolution shipborne
measurements, could improve the resolution of global seafloor topography
and morphology to a point that would substantially advance our understanding
of volcanism, faulting, sedimentation, and plate evolution in oceanic
regions.
A
specific new application of topographic measurements of the land
surface is to obtain landslide inventories. Landslides are primarily
associated with triggering events, such as rainfall, snowmelt, or
earthquakes. The statistics of these events, however, are poorly
documented. Since large landslide events are rare, it is essential
to obtain inventories on a worldwide basis. One goal is to automate
the measurement of landslide areas and volumes using differences
in topographic observations prior to and after each landslide event.
Because
topographic data and their temporal changes are fundamental to diverse
solid-Earth disciplines, the observational requirements are discipline
specific. Three classes of observations encompass the diversity
of requirements: improvements in vertical accuracy to 0.1 m in targeted
regions (with frequent repeats), one-time global mapping at 0.5-m
vertical accuracy to define the present topographic template which
surface processes and tectonics modify, and improved mapping of
ice sheets and glaciers.
Spaceborne
swath-mapping laser altimetry (imaging lidar) and dual-frequency
interferometric SAR technologies, potentially in combination, hold
the greatest promise for achieving these goals. Cross-track InSAR
with precision antenna spacing can map topography with centimeter-level
resolution. Lidar can measure topographic change over land, rivers,
and oceans. The combined use of lidar and InSAR can provide comprehensive
characterization of vegetation height and topography of high resolution
and accuracy for the “bald” Earth.
Suggested
mission phasing and requirements
Immediate
(1–5 years): Distribute all SRTM data, launch
ICESat, and demonstrate imaging lidar capabilities in Earth orbit
Near Term (5–10 years):
Global mapping to supercede the SRTM data set. One-time global mapping
of the ground surface at 2- to 5-m resolution and 0.5-m vertical
accuracy. Ice-sheet mapping at 1-km horizontal resolution, 1-cm
vertical accuracy for the ice or snow surface, and a repeat interval
of months (for annual changes) to years (for long-term changes).
Long term (10–25 years):
Continuously operating, targeted, high-resolution topographic mapping
and change detection capability. Targeted local to regional mapping,
with global access, at 1-m resolution, 0.1-m vertical accuracy for
the ground and water surfaces, and a repeat frequency of hours to
years depending on the rate of
topographic change.
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