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Integrated Program / Observational Stratgies/ Strategy 2: High-Resolution Topography

 

 

OBSERVATIONAL STRATEGIES:

HIGH-RESOLUTION TOPOGRAPHY

 

 

High-Res TopographyAccurate 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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