Hjdmlogical Sciences -Journal- des Sciences IIydro!ogiques,4l(4) August 1996 6 2 5

Remote sensing applications to hydrology: airborne laser altimeters

JERRY C. RITCHIE USDA, Agricultural Research Service, Hydrology Laboratory, Beltsville, Maryland 20705, USA

Abstract Shortly after the development of the first laser instrument in 1960, studies began on using laser distancing technology from airborne platforms to measure surface features on the landscape. Airborne laser altimeter studies in the 1960s and 1970s were used to measure terrain features and sea ice roughness. Research in the 1980s and 1990s has shown that airborne laser measurements can be used to measure directly topography, stream channel cross sections, gully cross sections, soil surface roughness, and vegetation canopy height, cover and distribution. These laser measurements can be used to estimate forest biomass and volume, aerodynamic roughness and leaf area indices. Airborne laser altimeters provide quick and accurate measurements for evaluating changes in land surface features and can be an additional tool in the arsenal of remote sensing equipment used to understand watershed properties and to develop plans to manage water resources.

Applications hydrologiques de la télédétection: altimètres laser aéroportés Résumé Peu après la mise au point du premier instrument laser en 1960, des études visant à utiliser la technologie laser de mesure des distances en vue de la caractérisation des paysages à partir de plates-formes aéroportées ont été entreprises. Au cours des années soixante et soixante-dix, les études d'altimétrie laser aéroportées ont été utilisées pour mesurer des caractéristiques des terrains et la rugosité de la glace de mer. Au cours des années quatre-vingts et quatre-vingt-dix les études entreprises ont montré que les mesures laser aéroportées pouvaient être utilisées pour mesurer directement la topographie, les sections du lit des rivières et des ravines, la rugosité de la surface des sols, l'épaisseur, la surface et la distribution de la végétation. Ces mesures laser peuvent être utilisées pour estimer le volume et la biomasse des forêts, sa rugosité aérodynamique et des indices de surface foliaire. Les altimètres laser aéroportés fournissent rapidement des mesures précises permettant d'apprécier les modifications des caractéristiques de la surface terrestre. Les altimètres laser peuvent devenir un nouvel outil dans la panoplie de la télédétection pour connaître les caractéristiques des bassins versants et pour définir des scénarios de gestion des ressources en eau.

INTRODUCTION

Shortly after the development of the first optical laser in 1960 at Hughes Aircraft Company Laboratories (Jenkins & White, 1976), instruments employing this new technology were developed to measure distance by timing

Open for discussion until 1 February 1997

626 Jerry C. Ritchie

the round-trip travel of a laser pulse between the laser transmitter, the surface being measured and the laser receiver. As early as 1964, Rempel & Parker (1964) published a paper proposing a laser terrain profiler for micro-relief studies based on laser distancing technology from airborne platforms.

Lasers and laser technology have been applied to measuring and monitoring a variety of other environmental parameters including chemical constituents in the atmosphere (Sharp, 1982; Measures, 1984; Quran, 1989), chlorophyll and oil in water (Hoge & Swift, 1983; 1987) and water depth (Hickman & Hogg, 1969; Harris et al, 1987; Penny et al., 1989) as well as for measuring terrain features. The need for accurate and rapid measurements and assessments of land surface terrain features to estimate land surface roughness, water movement, air movement, erosion, deposition, vegetation cover and distribution has led to the application of laser distancing technology for in situ measurements of soil surfaces (Rômkens et al., 1986; Bertuzzi et al., 1990; Huang & Bradford, 1990) and canopy properties (Walklate, 1989; Vanderbilt et al., 1990; Sinoquet et al., 1993) and for laser altimeter measure­ments from aircraft (Rempel & Parker, 1964; Jensen, 1967; Jepsky, 1986; Bufton 1989, Ritchie & Jackson, 1989; Button et al., 1991) and satellite platforms (Cohen et al, 1987; Bufton, 1989; Partington et al, 1991; Seshamani, 1993; Harding et al., 1994b). Airborne laser altimeters provide accurate measurements, ease of automation and cost effectiveness.

This paper is limited to a discussion of the application of airborne laser altimeter data for measuring and monitoring landscape surface features related to understanding and managing water resources.

METHODOLOGY

Laser distancing technology is based on generating short duration laser pulses (nanosecond or less) which are transmitted toward a target. A target is defined as any object that reflects back the light pulse (or part of it) and triggers the receiver. The receiver measures the lapse time between pulse initiation and its return. Highly accurate timing (less than a nanosecond) is required. For example, one nanosecond timing allows an accuracy of approximately 30 cm for each measurement. Time is converted to distance using the velocity of light. Laser pulse width and timing electronics determine vertical resolution. Current timers allow the determination of surface topography with a vertical resolution of less than 5 cm.

Receivers are designed to measure either a single (first or last) pulse return (Jepsky, 1986) or to measure the wave form of the return signal (Aldred & Bonner, 1985; Harding et al., 1994a). By measuring a single pulse, a rapid cycling rate of the laser transmitter and receiver can be accomplished, allowing measurements of thousands of return pulses per second. Such measurements allow the determination of the surface of the landscape with a high density horizontal footprint along the flight line. Analysing the wave form of the

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reflected signal yields information on multiple reflecting layers within the vertical profile of the landscape elements. However, wave form measurements require additional data collection time, and therefore fewer measurements are made per second.

A profiling laser altimeter measures a two-dimensional strip across the landscape (Jepsky, 1986; Ackermann, 1988). The width of the strip is depen­dent on the altitude of the aircraft and the field-of-view of the laser. Such measurements are comparable to the line-transect, line-intercept and belt-transect methodologies that are commonly used for ground vegetation surveys (Canfield, 1941; Eberhardt, 1978). Scanning laser altimeter systems provide a raster or conical scan of the surface (Krabill et al., 1984; Bufton et al., 1993; Wagner, 1995) that can be used to make three-dimensional measurements of the landscape surface. The width of the laser scan depends on the aircraft altitude and the view angle of the laser.

The position of the laser platform must be known accurately to determine the flight line across the landscape (Krabill & Martin, 1987; Lindenberger, 1988). Aircraft position is tracked using the airplane's Inertial Navigation System (INS) or Global Position System (GPS). Differential GPS allows the flight path and altitude of the aircraft to be measured accurately (Krabill & Martin, 1987). A gyroscope and an accelerometer can be mounted on the base of the laser platform to determine positional movement at the laser transmitter and receiver. Aerial photography and video imaging have also been used to track the flight line.

APPLICATIONS

Landscape surface measurements presented in this paper were made with a pulsed gallium-arsenide diode laser profiler, transmitting and receiving up to 4000 pulses per second at a wavelength of 0.904 /mi. At a ground speed of 75 m s"1, a laser measurement of distance from the airplane to the landscape surface occurred at horizontal intervals of 1.875 cm along the flight line. The field-of-view of the laser was 0.6 milliradians giving a "footprint" on the ground that is approximately 0.06% of the altitude (or approximately 6 cm at 100 m altitude). The timing electronics of this laser receiver allowed a vertical resolution of 5 cm for a single measurement.

Digital data (distance from an airborne laser to the landscape surface) from the laser receiver and data from a gyroscope and an accelerometer mounted on the base of the laser platform were recorded simultaneously with a portable computer. A video camera, borehole-sighted with the laser, recorded an image of the flight line. Sixty video frames were recorded per second and each frame annotated with consecutive numbers, computer clock time and GPS derived coordinates. The computer recorded each video frame number with the digital laser data to allow precise location of the laser data on the landscape with the video.

628 Jerry C. Ritchie

Landscape surface elevation was calculated for each laser measurement based on known elevations along a flight line. Minimum elevations (maximum laser measurements between the aircraft and the landscape) along a flight line were assumed to be ground surface elevation with measurements above these minimums being due to vegetation or man made structures. This envelope of laser measurements provided data on vegetation properties.

TOPOGRAPHY

Accurate measurements of the surface features of the landscape are require­ments for most environmental and engineering studies. The first applications of the airborne laser altimeter were to measure topography (Link, 1969; Arp et al., 1982; Blair & McLellan, 1984; Krabill et al, 1984; Schreier et al, 1985) and sea ice roughness (Robin, 1966; Ketchum, 1971; Hibler, 1972, 1975; Tooma & Tucker, 1973). Laser altimeters can measure long topographic profiles quickly and efficiently. At an airplane ground speed of 75 m per second 4.5 km profiles are measured each minute. An example of a topo­graphic profile is shown in Fig. 1 using approximately 45 s of laser altimeter data. The data were reduced by block averaging 16 laser measurements giving an effective laser measurement rate of 250 measurements per second resulting in a "footprint" length of 42 cm for this profile. This profile (Fig. 1) shows the type of topographic data that can be collected with the laser altimeter. While the length of this profile was 3.5 km, profiles could be measured and analysed for any length. Greater spatial and vertical detail on these profiles can be

1400

1100. 1000 2000 metres — 3 0 0 0

Fig. 1 Topographic profile measured using an airborne laser altimeter. Profile was made by block averaging 16 laser measurements. Insert shows a 100 m section at foil resolution (no averaging).

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obtained by using smaller block averages or by using all data points and a moving average filter. Ease and speed of data collection would allow measure­ment of several profiles with a minimum of extra survey cost. Krabill et al. (1995) used a scanning laser altimeter to measure the surface terrain of the Greenland ice sheet and estimated accuracies of -20 cm for several 1000 km of flight lines. These measurements were used to study the dynamics of the Greenland ice sheet. Such measurements of topography provided data for understanding the dynamics of water and wind flow across the landscape.

SOIL SURFACE ROUGHNESS

In situ laser instruments have been used to make detailed measurements of microtopography (Huang & Bradford, 1990; Bertuzzi et al, 1990; Rômkens et al, 1988). Similar measurements can be made with an airborne laser altimeter. The surface of a bare agricultural field (Fig. 2, upper profile) shows the surface roughness superimposed on the overall topography as measured with a laser altimeter. Better information on land surface roughness features can be obtained by using a 11-measurement moving average filter to remove random and system noise (McCuen & Snyder, 1986) as shown with the lower profile in Fig. 2. Micro-roughness of soil and vegetation have been shown to influence seed germination, water retention, infiltration, evaporation, runoff and soil erosion by water and wind (Zobeck & Onstad, 1987). Laser altimeter measurements (Fig. 2) of micro-roughness of the landscape surface can be used

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Fig. 2 Bare soil profile measured in an agricultural field. The lower profile was derived from the upper profile (raw data) using an 11-measurement moving average filter.

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to understand more accurately and calculate evaporation, soil moisture, runoff and soil erosion at field and landscape scales.

GULLY AND STREAM CROSS SECTIONS

Entrenched erosional features need to be quantified to estimate their effects on water movement and soil loss across the landscape. Measurements of these features can be difficult and time consuming using ground-based techniques. Measurement of a large erosional landscape feature (Fig. 3) was made using approximately 0.5 s of laser data measured from an altitude of 180 m. An 11-measurement moving average filter was used to define the shape and roughness within the gully. If the original land surface is represented using a straight line (dotted line, Fig. 3) between the edges of the gully, the area (insert lower left, Fig. 3) of the gully is 16.40 m2. Other shaped lines (i.e. concave, convex) or placement of lines to define the original land surface could have been used to define the original shape of the landscape surface. However, the principle for determining the cross section and area of a gully with the laser data would be the same no matter the shape or placement of the original land surface.

Similarly, the shape and roughness of a small stream channel were de­fined using an 11-measurement moving average (Fig. 4). The dotted line (Fig. 4) represents the maximum stage of this stream channel cross section, but other stages could be represented and used to calculate the carrying capacity at different stages. Data on bottom roughness can also be used to estimate resis­tance to flow of the stream. The same technique can be applied to measure and

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Fig. 3 Laser altimeter measurement of a gully. The original data were analysed using an 11-measurement moving average filter. Insert (lower left) shows the area of the gully.

Airborne laser altimeters 631

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metres Fig. 4 Laser altimeter measured stream cross section. Lower dashed line represents the stream cross section and upper dashed line represents the flood plain cross section.

delineate the cross section and roughness of the associated flood plain (Fig. 4, dashed line). Channel and flood plain cross sections and roughness would allow better estimates of channel and flood plain carrying capacity and resistance to flow.

Using these techniques, cross sections of gullies and stream channels have been measured at other sites (Link, 1969; Krabill et al, 1984; Jackson et al, 1988; Ritchie & Jackson, 1989; Ritchie et al, 1993b; Ritchie et al, 1994). These measurements can be used to quantify gully and stream channel cross sections and roughness, gully and stream bank erosion and channel degradation, to estimate soil loss from gully or channel banks and to measure channel and flood plain roughness and cross sections for estimating flow rates. Data on channel, gully and flood plain size, roughness, and degradation can help in design, development and placement of physical structures to control flow and to calculate flows.

VEGETATION CANOPY

Canopy properties on the landscape can also be estimated using a laser alti­meter. The ground surface under a vegetation canopy is estimated by assuming that minimum elevation measurements along transects represent laser measure­ments that penetrated the canopy and reached the ground surface. By calcula­ting the difference between the estimated ground surface (minimum elevation measurements) and the actual laser measurements, canopy height, canopy cover and distribution can be calculated (Fig. 5). Studies have shown that laser

632 Jerry C. Ritchie

0 200 400 800 1000 600 metres

Fig. 5 Forest canopy measured using an airborne laser altimeter.

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altimeter measurements provided accurate measurements of vegetation heights and cover (Arp et al, 1982; Nelson et al., 1984; Aldred & Bonner, 1985; Schreier et al, 1985; Ritchie et al, 1992, Ritchie et al, 1993a; Harding, et al, 1994a; Weltz ef al, 1994).

1 8 2 3 4 5 6 7 Monsoon'90 Met Flux site number

Fig. 6 Distribution of canopy cover by vegetation height at eight study sites on the Walnut Gulch Watershed in Arizona USA measured using airborne laser data.

Airborne laser altimeters 633

Similar measurements of vegetation properties were made at eight loca­tions in the Walnut Gulch watershed in Arizona, USA (Weltz et al, 1994). Altimeter data were used to estimate the distribution of vegetation cover by height of vegetation (Fig. 6). Sites 1-4 increased in elevation while sites 5-8 on a parallel flight line decreased in elevation. This sample showed little vege­tation taller than 0.5 m on the watershed and that cover and height of vegeta­tion decreased as elevation increased in the watershed (Weltz et al, 199A). Combining this vegetation cover data with canopy height and topographic data, estimates of effective aerodynamic roughness (ZQ) were calculated (Fig. 7) that agreed with estimates of aerodynamic roughness calculated from standard ground-based measurements (Menenti & Ritchie, 1994).

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Fig. 7 Comparison of aerodynamic roughness (Z0) estimated using laser altimeter data and calculated at eight study sites on the Walnut Gulch Watershed, Arizona USA.

Laser altimeter measurements of canopy properties have been used to estimate vegetationbiomass (Aldred & Bonner, 1985; Maclean & Krabill, 1986; Nelson et al, 1988; Hyyppà et al, 1993; Nilsson, 1994), infiltration, évapo­transpiration (Menenti & Ritchie, 1994) and erosion. Landscape scale measure­ments of canopy properties will improve understanding of spatial variability of landscape features and the functions that can be implied from those features. Such estimates at landscape scales will improve understanding of the effect of these factors on the water resources of natural and agricultural landscapes.

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CONCLUSIONS

Airborne laser altimeters are another tool in the remote sensing arsenal that can be used to quantify landscape topography, gully and stream cross sections and roughness and vegetation canopy properties. These properties and features are integral parts of the landscape and have to be measured and evaluated to under­stand the water resources of natural and agricultural landscapes. Measurements of these terrain features can be used to quantify water retention, infiltration, evaporation and water movement from upland surfaces and in channels and across flood plains. Channel and gully development, degradation and roughness can be measured and used to estimate soil loss and explain water quality and flow patterns. Measurements of canopy properties and their distribution across the landscape and their effect on water movement and aerodynamic roughness allow better understanding of evaporative loss, infiltration and surface water movement. Airborne laser altimeters offer the potential to measure landscape properties over large areas quickly and easily. Such measurements will improve understanding of water resources on natural and agricultural landscapes.

Acknowledgments The author expresses appreciation to the USDA-ARS Remote Sensing Research Unit, Subtropical Agricultural Research Laboratory, Weslaco, Texas, USA, for the use of their Aerocommander as the platform for flying the laser altimeter. Special appreciation goes to M. R. Davis, pilot, ARS, Weslaco, who flew the Aerocommander. Sincere appreciation is due to the staffs of the USDA-ARS Hydrology Laboratory, Beltsville, Maryland, USDA-ARS Northwest Watershed Research Center, Boise, Idaho, USDA-ARS Remote Sensing Research Unit, Subtropical Agricultural Research Laboratory, Weslaco, Texas, USDA-ARS National Agricultural Water Quality Laboratory, Chickasha and Durant, Oklahoma, and USDA-ARS Southwest Watershed Research Center, Tucson, Arizona, who provided logistical and ground support during the studies.

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