Airborne techniques are powerful research tools for a broad variety of applications, from resource assessment to exploratory geophysics to vegetation analysis. This chapter examines the enhanced scientific capabilities that precise GPS positioning provides to airborne geophysics. Three scientific techniques have been dramatically enhanced with precise positioning:
- accurate regional airborne mapping of topography;
- regional airborne gravity mapping; and
- high-resolution surveys of topography, gravity, magnetic, and electromagnetic properties.
Application of these techniques will result in numerous advances in basic and applied research. The scientific objectives that are now possible with precise positioning and airborne geophysics can be categorized in the following areas:
- interdisciplinary earth science studies;
- continental geodynamics;
- economic, environmental, and nuclear nonproliferation issues;
- geodetic studies; and
- global change monitoring.
These objectives were given high priority in the report Solid-Earth Science and Society (NRC, 1993).
Interdisciplinary Earth Science Studies and Airborne Geophysics
In recent years it has become clear that physical processes that had been studied in isolation are in reality linked. For example, geomorphic studies must incorporate an understanding of regional geology, as well as of climatic, tectonic, and hydrologic conditions. Similarly, studies of ice sheet stability must incorporate not only detailed understanding of ice properties, but also knowledge of the nature of surrounding oceans and underlying geology. Any facet of these problems may be studied in isolation, but complete understanding of the system will only come from examining the contribution of all of its components.
The research community, recognizing the importance of interdisciplinary studies and the need to share resources, has fostered the creation of cooperative research laboratories and oceanographic research vessels. The ocean science community in particular has witnessed fruitful cooperation between oceanographers studying hydrothermal mixing, marine biologists studying vent communities, and geophysicists studying magmatic processes. Airborne research platforms have a similar potential for fostering such research in a cost-effective way. As discussed below, interdisciplinary research topics that can be addressed by airborne geophysics integrated with precise positioning include the following:
- ice dynamics and sea level rise;
- erosional processes and landform development; and
- hydrologic cycle.
Ice Dynamics and Sea Level Rise
The melting or collapse of large ice sheets is believed to be responsible for rapid sea level rises documented in the geologic record. Our understanding of the collapse of ancient ice sheets is largely based on studies of the modern ice sheets that cover much of Greenland and Antarctica. The water stored in these ice sheets has vast potential for raising sea level globally. For example, the West Antarctic ice sheet, were it to melt completely, is capable of triggering a global sea level rise of 6 m (Mercer,
1978). When an ice sheet collapses, the ice is delivered to the oceans where it floats, causing a rise in sea level. Large rivers of ice (60 to 100 km wide) form; they transport the ice from the continent's interior to the ocean at speeds of up to 750 m/year. These ice rivers are critically dependent upon the presence of an oversaturated till at the ice-rock interface, which in turn is controlled by the underlying geology. Thus, understanding the ice dynamics and its potential contribution to global sea level rise is a problem requiring collaboration between glaciologists, oceanographers, and geologists.
Airborne geophysics can provide important constraints on this interdisciplinary problem. Accurate topographic measurements of 1-m resolution on the ice surface and 5-m resolution at its base are necessary to determine the stress state of an ice sheet. Simultaneous measurements of airborne gravity (accurate to 3 mGal) and magnetics (accurate to 1 nanotesla [nT]) can be used to locate volcanic sources of excess heat and the source and distribution of lubricating sediments. Incorporation of airborne electromagnetic methods would detect the thermal anomaly beneath the ice sheet.
Erosion Processes and Landform Evolution
Erosion is a complex process that depends on the nature of the bedrock being eroded, landform relief, steepness of slopes, climatic conditions, vegetation type and distribution, and hydrologic flow patterns. The relative importance of these variables has been difficult to determine because the variables are linked and because the rates and physical properties are difficult to measure. Yet, it is important to better quantify and understand erosion processes because of their importance in building construction, bridge and road stability, waste site evaluation, and even oil exploration.
An airborne research platform provides an ideal facility to study erosion processes. Such a study would include accurate measurements of topography (accurate to better than 1 m) to quantify erosion rates, high-resolution airborne magnetic and gravity measurements to define the nature of the bedrock accurately, and remote sensing images to determine the effect of vegetation. Repeated airborne surveys would show how landforms evolve with time.
As the world's population expands, the distribution and allocation of water resources will become increasingly important. Water disputes in arid regions such as the western United States, the Middle East, and Africa illustrate the importance of water to society and politics. In Texas there are concerns about freshwater levels in the aquifers, as well as deterioration in water quality due to saltwater intrusion and oil contamination in heavily pumped regions. To ensure a clean water supply, it is necessary to adopt an approach that will determine the sources of contaminants and the processes that control their dispersion through the aquifer system.
Airborne platforms can provide additional information to constrain groundwater hydrology problems. Precisely navigated, high-resolution airborne magnetics can identify abandoned well heads and the source of fluid contaminants, and electromagnetic techniques can show the distribution of saline water. Airborne gravity and magnetic measurements, which are accurate to 1 mGal and 0.01 nT, respectively, can also be used to locate structures such as dikes, faults, and fractures that control the accumulation and movement of groundwater. For example, an airborne magnetic survey over Western Australia clearly delineates the subsurface trace of igneous dikes and fractures (Figure 2.1). In this region of Australia, the groundwater tends to dam up against the dikes and mix with salt-concentrated soil, causing agricultural losses. Similar magnetic surveys have also been flown to map the subsurface extent of igneous units that influence the flow of groundwater away from a weapons testing area (Grauch et al., 1993).
Airborne magnetic methods, combined with other geophysical data, can also be used to determine the thickness of strata above the magnetic basement. This information can be used to help assess the groundwater potential of a region (e.g., Babu et al., 1991). Finally, airborne topography measurements with accuracies of better than 10 cm can indicate the rate and form of subsidence caused by fluid withdrawal.
Continental Geodynamics and Airborne Geophysics
Plate tectonics provides a useful template for studying geodynamic processes both on continents and within ocean basins. Studies of seafloor magnetic anomalies, fracture zones, and earthquake focal mechanisms have revealed a record of the relative motions of the Earth's torsionally rigid plates over tens of millions of years. Our understanding of continental geodynamic processes, however, lags greatly behind our knowledge of oceanic lithospheric processes for the following reasons:
- In many places the continental lithosphere is deforming in complex ways and not simply by the rigid body rotation that characterizes the oceanic lithosphere;
- Continental lithosphere is difficult to subduct and preserves a much longer and more complex history of accretion and deformation than does oceanic lithosphere; and
- The rheological profile of continental lithosphere, principally its continental crust, allows for significant subsurface decoupling, making it more difficult to relate upper and lower lithospheric processes.
Airborne geophysics has the potential to advance our understanding of continental dynamics, particularly in the following areas:
- Active tectonics: complex deformation and nonrigid behavior near the Earth's surface;
- Volcanology: processes of inflation, eruption, and degradation of volcanic features; and
- Regional geodynamics: evolution and rheologic properties of the continental lithosphere.
The contributions that airborne geophysics can make to these research areas are described below.
Plate tectonics provides a framework for studying the evolution of geologic and geophysical processes through time. On global scales, the lithospheric plates are rigid and the plate boundaries are sharply defined within oceanic lithosphere but are broad and poorly defined within continental lithosphere. On regional scales, plate boundaries in both continental and oceanic settings are complex and can best be studied in regions of young and active tectonics. Within oceanic lithosphere, the structural complexity along plate boundaries may be tens of kilometers wide; within continental lithosphere, it may be 2,000 to 3,000 km wide (Argand, 1924; Molnar and Tapponnier, 1975). While deformation at plate boundaries is dominated by structures related to overall plate motion, it characteristically consists of many small fragments of crust that respond individually to global plate motions and that may be partially or wholly decoupled from deeper parts of the lithosphere (Figures 2.2(a) and 2.2(b)). For example, the convergent plate boundary between the subcontinent of India and Eurasia is marked by a zone of deformation more than 2,000 km wide extending from northern India into Mongolia (Baljinnyam et al., 1993). Not only is this deformation zone wide, but like all plate boundaries within continental crust, it contains a complex but integrated system of shortening, strike-slip, and extensional structures, even though the plates are converging. Regions undergoing principally extension may also be complex. Although extensional regions in East Africa are characterized by relatively narrow zones of deformation, extension of the 600-km-wide Basin and Range Province is diffuse. Moreover, while the region is dominated by extensional structures, strike-slip and compressional features are also found. Finally, transform boundaries, where the plates move horizontally past one another, such as along the San Andreas Fault in southern California, are often represented as thin lines on maps. Patterns of structures and seismicity, such as have occurred during the 1994 Northridge earthquake and other earthquakes in the southern California region, however, indicate that the transform boundary is characterized by a complex system of compressional, extensional, and strike-slip structures.
Much of the Earth's seismicity and volcanic activity is associated with these complex plate boundaries. At the same time, much of the Earth's population resides in cities likely to be affected by these natural hazards.
To improve our understanding of the nature of the plate boundaries and the mechanisms that create them, further studies are needed for the following purposes:
- to determine the spatial and temporal strain distribution and the structures that partition that strain;
- to document the relationship between the regional geology and the deformation within the boundary; and
- to relate the surface expression of deformation within the plate boundary to the motion and behavior of the underlying mantle in order to elucidate the driving mechanisms for the deformation.
To address these needs, research strategies need to include ground-based techniques, such as geologic mapping, paleomagnetic analysis, studies of seismicity and paleoseismicity, and geodetic measurements, as well as airborne geophysical methods. In particular, airborne geodetic techniques could complement these other research strategies. For example, accuracies of 1 to 10 millimeters per year are needed to map the strain across plate boundaries. The majority of these measurements are currently obtained either by periodic geodetic measurements or by permanently operating GPS arrays. Both of these approaches provide detailed insight into the motion of individual points along complex fault systems, but individual sites may be contaminated by local noise or may not be representative of the area. Airborne swath mapping techniques, such as SAR or scanning laser systems, would provide sufficiently accurate measurements of the Earth's surface between the geodetic monuments.
Documenting the structure of the crustal blocks that make up a deformation zone also requires detailed understanding of the local geology. Geologic mapping integrated with multichannel seismic reflection and refraction studies can be used to determine the local structure, but these studies can be limited by poorly exposed outcrops or by high cost. Precise airborne geophysical techniques, particularly airborne magnetics and gravity, can be used to help map the local geology efficiently and economically. These methods are capable of producing the required accuracies of several picotesla and 1 mGal over wavelengths of several hundred meters. Airborne techniques also offer the advantage of mapping larger areas than can be conveniently mapped using surface methods. This allows the high-resolution studies along plate boundaries to be placed in a regional
structural and tectonic framework. Such a framework is crucial for merging local observations with global plate motions and with the structure of the underlying mantle.
Volcanic eruptions destroy life, cause millions of dollars worth of property damage, and may even affect global climate patterns. Despite these adverse impacts on society, volcanic processes are still poorly understood. Airborne methods, particularly accurate topographic mapping, airborne gravity, magnetics, and gradiometry, can be used as follows:
- to develop models of volcano growth and degradation;
- to help determine the volume of erupted material; and
- to quantify inflation and eruption processes.
Models of terrestrial volcanic productivity, growth, and degradation depend on measurements of a number of parameters, including the morphometry of the volcano (area, altitude, volume, flank slope, profile, crater size), the composition, eruptive style, explosivity, and tectonic setting. Morphometric characteristics of individual volcanos can be derived from digital topographic data, but systematic studies of suites of volcanic landforms at scales larger than ~1 km are needed to provide model input (e.g., Pike and Clow, 1981). Recent work by Garvin and Williams (1990) has demonstrated that Digital Elevation Model (DEM) data, with ~100-m horizontal resolution and 10-to 20-m vertical resolution, are suitable for characterizing 1-to 20-km-scale volcanos.
Accurate topographic data are required to estimate eruption volumes. The volume of volcanic deposits, however, is difficult to measure because of uncertainties in the boundaries of the deposit and in the topography of the pre-eruption surface. High-accuracy digital topography, with 0.5-to 3.0-m vertical resolution and 30-to 100-m horizontal resolution, is required to resolve deposit boundaries.
To model volcano inflation, deformation, and pre-eruptive surface modifications, it is necessary to obtain sub-meter geodetic measurements (vertical) on topographic length scales of 30 to 100 m on historically active volcanos. The integration of airborne measurements of accurate topogra-
phy with high-resolution gravity or gravity gradiometry would greatly enhance our understanding of volcano dynamics.
Regional studies provide the framework for understanding the dynamics of the continental lithosphere. Surface gravity and regional topography data play a crucial role in constraining the mechanical and thermal properties of the lithosphere. For example, dense gravity surveys, with accuracies of 2 to 3 mGal at resolutions better than 10 km, integrated with topography data can characterize crustal thinning within continental rift zones. Detailed gravity data have also revealed the complex state of isostatic compensation, and, thus rheology of mountain chains formed by continental collision, although there are still unresolved questions on what kinds of compensation mechanisms are associated with the different types, locations, and histories of plate collision. Finally, high-resolution gravity data over sedimentary basins are needed for an understanding of the thermal and mechanical driving forces that cause basin subsidence. For example, gravity measurements have been used to interpret the internal structure of the Dead Sea basin and model its evolution (e.g., Ten Brink et al., 1993). Marine gravity measurements show that the Dead Sea graben is isostatically uncompensated, consistent with its formation by pure mechanical stretching within the crust.
In summary, gravity data with accuracies of 1 to 10 mGal and resolutions of 100 to 200 km or better provide a crucial component in the understanding of the dynamics of the Earth's lithosphere (NASA, 1987). Accurate topography data, however, are necessary for interpreting the gravity data and for studying internal density variations and elastic strength. A standard analysis method is to compute a linear transfer function (admittance function) that best maps topography into gravity for a particular tectonic region. The assumption is that the topography acts as a load on the elastic lithosphere, which responds linearly and in phase with the load, so that the relationship between topography and gravity is uniquely specified by a linear admittance function. The differences between gravity-topography admittance functions become apparent for wavelengths greater than about 20 km. For this application, the topography should reflect the average elevation in a 10 × 10 km grid cell. When
working with 1-mGal accuracy gravity data, the areal averaged topography needs to be accurate to about 10 m.
High-Resolution Applications and Airborne Geophysics
Exploration for natural resources has long depended on geophysical remote sensing techniques. With the advent of high-resolution gravity and magnetic mapping from aircraft, there is an opportunity to better meet the needs of the mineral and petroleum industries. Further advances in the resolution and accuracy of these techniques will serve to broaden their application by industry.
A major focus for hydrocarbon exploration is to establish which sedimentary basins will become major producers in the future (NRC, 1993). Although seismic techniques are the dominant exploration tool, high-resolution gravity and magnetic measurements collected rapidly and efficiently from airborne platforms will have increasing appeal in the petroleum industry. GPS and new sensor technologies now allow the following features to be identified in sedimentary basins using airborne methods:
- structures beneath thick sheets of salt that cannot be easily imaged with seismic techniques because of the velocity structure of salt;
- faults which may play an important role in oil migration; and
- structures within the sedimentary column that may trap oil.
Structures that trap oil and gas within sedimentary basins can be located with gravity and magnetic surveys. For example, salt sheets and diapirs produce gravity lows and anticlines produce gravity highs. The gravity signature of these structures is on the order of 1 to 10 mGal with wavelengths of 1 to 10 km. Airborne gravimetry is ideally suited for regional gravity mapping at these wavelengths and levels of accuracy and can be used for rapid exploration of hydrocarbon prospects (e.g., Gumert, 1992). In addition, high-resolution gravity gradiometry measurements can help resolve the nature of the basin strata and high-resolution magnetics
(sub-nT) can provide critical information on the form of faults and the detailed structure of sedimentary rocks.
The minerals industry typically focuses on even smaller targets than those that interest the petroleum industry. The location of large ore deposits in the shallow crust can be discerned from gravity surveys, but their small size (10 to 100 m) and low amplitude (< 1 mGal) make them difficult to map in detail. The development of high-resolution airborne techniques that can map small features was partly driven by the minerals industry in Canada and Australia. These countries routinely use high-resolution magnetics and gravity and current studies are exploring the use of gravity gradiometry where a resolution of 2 eötvös (10-9s-2) could provide the level of detail necessary to a successful exploration project.
With the conclusion of the Cold War and rapid changes in the former Soviet Union, issues of nuclear verification and nonproliferation have taken a different form. The prime threat is no longer perceived to be Russia or Ukraine but the development of nuclear capabilities by less developed countries. The present strategy involves a tiered detection and monitoring effort that incorporates global, regional, and local seismic networks and on-site inspections (U.S. Congress, Office of Technology Assessment, 1988). Airborne techniques, particularly swath mapping techniques that can cover large areas rapidly and remotely, should be part of the strategy for identifying the location of subsurface tests and for monitoring postshot subsidence. Subsurface test sites could be located by identifying excavation areas and characteristic surficial expressions of an explosion site. In the United States, underground nuclear tests typically create large, circular surface depressions (Figure 2.3) that are 2 to 70 m deep, and 20 to 130 m in diameter (e.g., Houser, 1970). The subsurface cavities could be identified with airborne gravity, gradiometry, or electromagnetic measurements; the surface expression of a test site could be determined with accurate topographic mapping. Repeated topographic mapping of a region could also identify postshot subsidence. Work at U.S. test sites has documented continuing displacements on the order of tens of centimeters in the 18 months following an event. Airborne observations
could supplement seismic detection of the occurrence and timing of a nuclear test.
Geodesy and Airborne Geophysics
One of the principal objectives of geodesy is to determine the shape of the Earth's surface by determining land and sea heights relative to a reference surface. The traditional reference is idealized mean sea level, or the geoid. The geoid is an equipotential surface in the Earth's gravity field; it can be determined from surface gravimetry data, either as a surface integral of gravity or as a line integral of the deflection of the vertical (the slope of the geoid).
On land, the geoid is the principal reference for heights; it is approximately determined from mean sea level observations at coastal tide gauge stations. Discrepancies of up to a meter between tide gauge stations on different oceans, or even along longer coastlines of the same ocean, indicate that mean sea level does not coincide exactly with the geoid (a unique surface). Comparison of vertical reference systems (vertical datums) of different countries, therefore, requires a consistent global geoid model determined from gravity data (Rummel and Teunissen, 1988). As a rough rule of thumb, if the geoid is resolved over a distance of x kilometers, the error introduced by neglecting the unresolved part is x/300 meters. In order to connect vertical datums to 10 cm or to obtain a consistent reference for larger vertical networks at this level, the geoid must be resolved to a horizontal spatial scale of at least 30 km.
Heights above mean sea level (orthometric heights) are traditionally determined using spirit leveling, which is accurate to a few millimeters over several kilometers of leveling line length. A less labor-intensive procedure for determining orthometric heights is based on satellite ranging using GPS. With an appropriate number of satellites and good geometry between the satellites and the ground station, it is possible to obtain heights above the geocenter, or equivalently, above a reference ellipsoid defined in the GPS satellite coordinate system. With this method, absolute heights can be determined to an accuracy of a meter or better, while relative heights determined by differential techniques are accurate to centimeters or millimeters (depending on line length). To determine the corresponding
orthometric heights, the geoid height must be obtained with commensurate accuracy. Some tests indicate that the current relative geoid accuracy in the United States ranges from 1 cm/10 km, to 10 cm/100 km, to several decimeters over longer distances (up to 1 ppm) (Milbert, 1991). For certain applications, such as photogrammetric topographic mapping, a uniform accuracy and resolution of about 2 cm/25 km is now needed in view of the potential of kinematic GPS aircraft positioning at the 2-to 3-cm level.
In summary, there continues to be a purely geodetic need for high-resolution global gravity to determine the geoid and to correlate the various vertical datums of the world. The ultimate goal is to provide a uniformly accurate and consistent vertical reference for navigation and positioning. The promise of relatively inexpensive, yet very accurate, height determinations using GPS drives a need to know the geoid to comparable accuracy and resolution. Airborne gravimetry offers a cost-effective means to map the geoid, particularly in geographically remote areas.
Global Change Monitoring and Airborne Geophysics
Monitoring Ice Sheets and Mountain Glaciers
Melting ice has been identified as the major cause of sea level rise. The majority of ice that has the potential to raise sea level is held in the East and West Antarctic ice sheets and the Greenland ice cap, but the mountain glaciers that cover much of Alaska and Patagonia may also contribute to sea level rise (Meier, 1984; Thomas, 1991). The volume of ice held in these reservoirs and its change through time are poorly understood. Existing estimates are derived from historical records and from space-based measurements. A new generation of space-based radars and lasers (e.g., ERS-1, ERS-2, RADARSAT, JERS-1, EOS-ALT) will provide topographic measurements of large areas of the ice sheets (U.S. Congress, Office of Technology Assessment, 1993). Current sensors are inadequate for most ice sheet applications (see Table 1-3), but by the end of the l990s, higher-precision lasers and radars will be in orbit. Airborne techniques are necessary to establish a baseline now and can be used to systematically map the highest-latitude regions of the ice sheets not
covered by the space missions. Moreover, aircraft provide a more flexible platform for more frequent systematic mapping of glaciers.
Monitoring Ocean Circulation
Global models of ocean circulation are determined in part by continuous observations of ocean currents by satellite or aircraft altimetry. Altimetry techniques measure the sea-surface height, and the difference between sea-surface height and the geoid height yields the sea-surface topography. The steady-state component of sea-surface topography is caused by deviations of the oceans from hydrostatic equilibrium (i.e., the equipotential and isobaric surfaces no longer coincide) because of variations in temperature (primarily a zonal, north-south effect) and salinity. Other regional effects include the trade winds and the effluent of major river systems. The buildup of water creates a horizontal pressure gradient that can only be sustained in the presence of currents under geostrophic equilibrium. The geostrophic currents extend to considerable depth, commensurate with their horizontal scale.
Because of the role of geostrophic currents in heat transfer and thus in global climatology, it is important to map their spatial and temporal distribution accurately (e.g., Fu, 1983). High-resolution altimetry and accurate geoid determination are needed as inputs to global change models as well as to other environmental and ecological applications, such as the dispersion of pollutants and the migration of fisheries. These applications require measurements on a broad spectrum of scales: ocean gyres are thousands of kilometers across, with sea-surface topography of up to a meter; boundary currents are hundreds of kilometers in extent, with topographies of tens of centimeters to a meter; and mesoscale eddies have scales of tens to hundreds of kilometers and topography on the order of a few decimeters. To determine their steady-state components, the geoid must be known at all of these scales to accuracies much better than a meter, and in many cases to accuracies better than 10 cm. Corresponding geoid ''slopes'' must be known to accuracies of about one part in ten to the seventh, yielding current velocities accurate to about I cm/s and requiring the equivalent in gravity accuracy of about 0.1 mGal. Airborne gravimetry fills the gap at the shorter wavelengths not measurable by satellite techniques, especially in coastal regions where ocean circulation dynamics
are complex, and where, as it happens, operational and logistical support is available for aircraft geoid mapping missions.
Topography exerts an important control on surface hydrology through its influence on intercepted radiation, precipitation, runoff, evaporation, snow ablation, soil moisture, and vegetation patterns. Topographic parameters also indicate the exposure of a landscape to weather and sunlight at a given latitude and thus are an indirect measure of its microclimate. Through feedback mechanisms, vegetation itself influences energy and mass fluxes, affecting not only the local environment but also the regional and global climate. Topography is therefore a key element in the study of complex ecosystems.
Quantitative hydrologic and ecosystem models require digital elevation data, but even high-resolution digital elevation data have serious deficiencies. These deficiencies are particularly evident when determining the topographic effects on solar radiation, which requires accurate slope and aspect information. A digital elevation grid is used to calculate the local gradient and angle to the horizon; the grid is combined with the solar geometry to determine the incidence angle of solar radiation on the slope. Noise in the digital elevation data of mountainous areas, which are often derived by digitizing contour lines from topographic maps, is magnified by the differencing operations used to calculate gradients. High-accuracy airborne topography measurements, with a horizontal resolution of 30 m or better and a vertical accuracy of 1 to 3 m or better, are needed to improve the solar radiation calculations described above.
Altimeters with the appropriate design and resolution can be used to assess several important vegetation parameters. In areas of low to moderate vegetation density, pulse shape analysis with laser profiling data allows estimates of the difference between the ground elevation and the canopy top. The pulse is scattered by the various vegetation layers as it penetrates, and, in principle, it is possible to discriminate the waveform of the various returns. Relative "brightness" determinations of each layer (related to projected leaf area) may permit calculations of extinction cross-section and Leaf Area Index, which are important in many ecological
studies. Seasonal and longer-range variations in this index can also be observed with sufficient repeat coverage and ground calibration studies.
It may also be possible to estimate canopy thickness and total biomass for determining total carbon storage. The repeated measurements and complete coverage offered by airborne radar interferometry can even yield estimates of deforestation rates. Other techniques, such as digital elevation maps, can be used to define barriers and corridors affecting species dispersal and to predict biomass, timber site quality, and burn patterns over large areas. The need for accurate slope and aspect data is especially great in semiarid, midlatitude regions where minor slope changes greatly affect local water availability, soil moisture, and vegetation cover over large areas.
Topography plays an important role in the distribution and flux of soil moisture, runoff generation, and discharge pattern. Digital topographic data are used to calculate watershed structure and, to some extent, runoff (e.g., Band and Wood, 1988; O'Loughlin, 1986). To model the surface and subsurface fluxes of water quantitatively, derivative quantities based upon elevation data (e.g., local gradient and upslope area) are required. Error propagation dictates that the original elevation data be of very high quality for use in the quantitative models described above. Because the data are derived from contour maps, which have large uncertainties in the vertical component, most digital data bases have proved inadequate for such calculations.
The influence of topography on terrestrial hydrologic processes is strongest in mountainous regions. High horizontal resolution is critical; the high topographic gradients and changing aspect in mountainous terrain render even 100-m data (the current resolution of many digital data bases) inadequate. Surprisingly, subtle topographic relief in regions of low slope (less than about 0.5) can also have significant hydrologic consequences, particularly for soil water dynamics and subsequent ecosystem processes. High vertical accuracy is required in these areas to identify subtle ridges or depressions that cause water flow to diverge or converge.