MANAGEMENT APPLICATIONS. Dr John Louis, Dr David Lamb, Mr Gary Mc Kenzie, Dr Gordon Chapman,. Mr Asoka Edirisinghe, Mr Ivan Mc Leod* & Professor ...
OPERATIONAL USE AND CALIBRATION OF AIRBORNE VIDEO IMAGERY FOR AGRICULTURAL & ENVIRONMENTAL LAND MANAGEMENT APPLICATIONS Dr John Louis, Dr David Lamb, Mr Gary Mc Kenzie, Dr Gordon Chapman, Mr Asoka Edirisinghe, Mr Ivan Mc Leod* & Professor Jim Pratley Centre for Spatial Data Analysis Charles Sturt University Wagga Wagga, NSW, 2678, Australia ABSTRACT
This paper describes the design, calibration and operational use of the Multispectral Airborne Video System that is currently in use at Charles Sturt University, Wagga Wagga, Australia. The camera array consists of 4 high resolution CCD cameras mounted in an adjustable optically rigid frame. The data acquisition system is based on an industrial PC containing an AT Vista Board. The in-flight data acquisition software has been written in-house at CSU and provides for continuous real time viewing of image data, histogram analysis, aperture control and the simultaneous acquisition of 4 channels of image data. Links are also being developed to integrate GPS information with the multispectral image data acquisition system. Applications of the operational use of this system to acquire and calibrate imagery for agricultural landuse management and environmental monitoring are discussed. Results are given of calibration measurements using known ground radiometric reflectance standards to calibrate the air video imagery. In addition results are presented from extensive in-flight trials, designed to investigate the potential of air videography to assist in improved water scheduling of irrigated cotton crops. SYSTEM SPECIFICATIONS The Multispectral Airborne Video System (ABVS) comprises 4 high resolution monochrome Cohu CCD video cameras (CCIR model 4910) mounted in a rigid 2X2 optical frame together with a data acquisition system based on an industrial PC housing an AT Vista frame grabber. The camera array in use is a modification of the original DMSV prototype camera array built by SpecTerra Systems. The camera system is mounted on an equipment rack which contains the image acquisition hardware. The equipment rack is secured to the seat rails in place of the co-pilot’s seat in a modified Cessna 210 aircraft and the downward-looking camera protrudes into a protective bubble on a specially modified door.
*
Department of Agronomy and Soil Science University of New England Armidale, NSW, 2350, Australia -1-
The Camera Array The 4 video cameras are housed in a precision mounting frame which allows the 4 cameras to be precisely aligned parallel to each other. Each video camera acquires an image in a preset spectral band determined by an interchangeable filter. Currently the narrow band interference filters shown in Table 1 are in use. Table 1 ABVS spectral response characteristics Camera #1 #2 #3 #4
Band No. 1 2 3 4
Filter Colour Blue Green Red Near infrared
Central Wavelength 460 nm 550 nm 650 nm 770 nm
Bandwidth 25 nm 25 nm 25 nm 25 nm
A 12 mm focal length lens is used on each camera which gives approximately a 1.00 m x 0.97 m pixel at an altitude of 4600 ft (AGL)*. Each camera provides an image of 740 x 576 pixels resulting in a ground coverage of 744 x 559 m or 41.6 hectares. Increasing the aircraft altitude decreases the ground resolution but increases the ground coverage. At 7100 ft (AGL) the ground coverage is approximately 100 hectares. Adjustment of the camera aperture is controlled by a serial link to the 486 data acquisition computer. A camera integration time of 1 ms has been selected as a compromise between sufficient CCD element response and minimum pixel smearing due to the forward motion of the aircraft. Minimum camera gain is used to ensure linearity of the system response and to reduce noise. Data Acquisition Hardware & Software Central to the image acquisition hardware is an industrial IBM compatible 486 SX computer, housing a 4 MByte AT-Vista frame grabber. The frame grabber incorporates 740 x 576 x 8 bit x 4 channels and digitises the captured image from each camera before writing it to 110 Mb hard-disk which is capable of holding approximately 65 1.7 Mb images. Aperture adjustment, real time image display and image capture & recording is controlled through software (ABVCP) developed at Charles Sturt University’s Centre for Spatial Data Analysis. The video signal from the camera array conforms to PAL Television line scan standards with each image frame made up of 2 interlaced image fields, with a field scan rate of 50 Hz. The camera is mounted so that the aircraft flies in the same direction as the image horizontal scan direction. Due to the 1/50 second time interval between the interlaced image fields and the forward motion of the aircraft, the odd and even image fields are sheared relative to each other. The data acquisition software is designed to allow for easy adjustment of air speed and altitude to control pixel size and to ensure that the two interlaced fields are sheared only by an integral number of pixels. Image processing software (ABVIP) has been developed at the Centre for Spatial Data Analysis to unshear images to provide other basic image manipulation and display facilities, (McKenzie, et. al., 1992).
*
AGL = above ground level
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A colour monitor is used by the camera operator to direct the pilot over the target site by providing a real-time colour composite image from three cameras. The monitor can be switched from real-time image viewing to camera status and control via the VGA/Vista switcher. A global positioning unit (GPS) is also interfaced to the imageacquisition software to record image location and assist in mission navigation. A schematic showing the configuration of hardware components and data links in the ABVS system is presented in Figure 1.
Figure 1 Schematic diagram of ABVS. Camera Alignment In order to ensure accurate spatial registration of the multispectral imagery, parallel alignment of each of the cameras is critical as this saves time-consuming image registration during the image pre-processing stage. A white alignment card with a central black square is used to align all four cameras to within 0.5 pixel. The edge of the black square is exactly twice the lens spacing. When viewed with the camera array, each camera provides an image of the central black square which is displaced by half of the side length of the square in a direction dependent on the relative position of each camera. The final composite image comprises the overlay of the four separate displaced images of the black square. Camera alignment to 0.5 pixel is achieved by adjusting the orientation of the cameras to provide a composite square made up of equal sized sub-squares. SYSTEM CALIBRATION Before the ABVS data can be used in a quantitative manner, the system must be accurately calibrated for the measurement of target reflectance and radiance. The calibration of the ABVS has been conducted in both the laboratory and in-flight. A -3-
calibrated hand-held radiometer, Spectral Devices Personal Spectrometer II (PS II), with a 10 degree field of view (FOV) is used to measure the radiance and reflectance of all targets used in the calibration process. Pressed Halon powder is also used as a near ideal, 100% reflector. The laboratory calibration involves a determination of the following system parameters; sensitivity, linearity, dark signal and spatial uniformity for each camera, as well as a calibration of the camera aperture setting. Complete details of the calibration procedure and results can be found in (Edirisinghe et. al., 1995). In-flight Calibration In-flight calibration of the multispectral airborne video system consists of three main steps. Firstly, airborne image acquisition, secondly, ground data collection such as reflectance and radiance and thirdly, collection of support data which includes sun position and environmental data such as atmospheric aerosols and water vapour content. The in-flight calibration gives the response of the system within the aircraft environment, taking into account aircraft vibration and other environmental factors. It is conducted to monitor the system performance in-flight and to verify the laboratory calibration. The in-flight calibration requires simultaneous measurement of standard ground targets with the ABVS and the ground based PS II Spectrometer. The ground based reflectance measurements data are then compared to the airborne video data. The use of an appropriate atmospheric model is required for precise calibration. Selection of suitable standardised ground reflectance targets is critical for in-flight calibration. Polyester (Holland) blind material samples were chosen for the standardised ground targets, because of their reliability, resistance to the Sun’s UV and commercial availability. The size of the target samples (compared to pixel size), and their radiometric brightness range, are important factors. The ground reflectance standards were chosen to produced reflectance values from 7% to 90% relative to pressed Halon powder. The size of the target was chosen to be at least be three times the ground pixel size, at maximum operational altitude, in both the vertical and horizontal video directions (Richter, R., 1992), to give the required number of pixels for statistical analysis over the range of flying heights. Five 6m × 6m test samples were selected from dark green (almost black) to white, each of which produce 36 pixels at a height of 4500 ft AGL. The airborne images were collected from carefully planned and executed flights under very clear sky conditions. The flight plan specifies the altitude, flying direction, and Sun azimuth and zenith angles at the time of image acquisition. The flight plan is designed to investigate the influence on the image of various height effects, to demonstrate various angular effects that the land surface and atmosphere create in the video data, and to demonstrate various directional effects of the ground reflected light, seen from different flying directions, as a result of the Sun-target-sensor direction change. Most of the images were acquired with an intermediate aperture, providing unsaturated video signals to be recorded for all four bands. At different flying heights, from 500 ft to 10,000 ft, the video signal was optimised by adjustment of the aperture. In order to obtain ground level (zero height) image data, a number of images were -4-
taken with the ABVS setup about one metre above the ground on the back of a utility truck, with the test samples layed-out on the ground. The acquired ground level video data is used as data for zero height AGL in the analysis conducted to investigate the sensor detected radiance variation at different heights. The ground based PS II spectrometer is used to measure the reflectance of each reflectance standard square referenced to pressed Halon powder, taken as a standard white reflector. The PS II reflectance data is processed and data tables and graphs generated for interpretation purposes. The absolute radiance reflected from the Halon is taken before and after each experiment in order to verify the consistency of the reflected radiance during the experiment. Calibration Analysis The acquired images are analysed interactively with the help of the ABVIP image processing software, to correct for aircraft motion, to select areas of interest and calculate the mean digital number and scene standard deviation, and to examine camera alignment and band fit. The basic data collected from the interactive image processing are further analysed using the software packages Splus and ER-Mapper in the SUN UNIX environment. Least square regression analysis has been used to relate video digital number (dn) to ground reflectance standards. Multivariate linear modelling has also been carried out to investigate the dependence of the video digital signal with aircraft height and aperture. Table 2. In-flight calibration statistics Band
Offset (dn)
Blue Green Red NIR
35.28 6.37 28.33 40.53
St. err. Slope offset (dn) (dn/reflec -tance) 1.34 526.68 3.68 1091.63 1.91 612.52 2.48 744.22
St. err. slope (dn/reflt) 5.31 26.21 10.22 17.71
Residu. std. err. (dn) 3.19 6.30 4.62 5.97
Coeff. of the regres. r2 0.99 0.99 0.99 0.99
Table 2 shows the system calibration statistics including slope and offset for the inflight calibration and Figure 2 depicts the linearity of the video response to the PS II ground reflectance for the four video data channels. The determination coefficient of the regression (r 2 ), standard error of the video response offset and slope are also given in Table 2. The ABVS system has a high degree of linearity, with an r 2 value of 0.99 in each video data channel. Data vectors and matrices have been created from the domains of video digital numbers, PS II detected ground reflectance, PS II detected radiance, flying height, flying directions, time(Sun zenith and azimuth) and aperture size. These data have been subjected to extensive statistical analysis. Figure 3 shows the result of fitting a linear model of Video Digital Number to the Reflectance, Wavelength, Height and Aperture variables. The modelling analysis show a high degree of linearity, with r 2 = 0.9786, even when variations in aircraft altitude and camera aperture are included in the model. The results of the analysis help in understanding the behaviour and the nature of the video signal acquired under different conditions. They are also used in planning future airborne video imaging missions. -5-
Figure 2 Video digital number vs ground radiometer reflectance with aperture 18 and height 5,000 ft AGL over Charles Sturt University Oval on the 9th February 1995
Figure 3 Linear model of DN~Reflectance*Wavelength*Height*Aperture for inflight trials flown over Charles Sturt University Oval on the 9th February 1995 AGRICULTURAL LAND MANAGEMENT APPLICATIONS In addition to the basic radiometric calibration work outlined above, the ABVS air video system is also being calibrated to measure a number of important agronomic and environmental variables, including water turbidity in streams and soil moisture content. In particular, extensive field trials have been carried out in conjunction with National Mutual Cotton Pty. Ltd. and Togo Station. These trials have been designed to investigate the potential of air videography to assist in improved water scheduling -6-
of irrigated cotton crops. Togo Station has previously trialed air videography techniques to monitor cotton growth, (Button & Cull, 1991). In addition, Togo Station has for some years been using direct soil moisture measurements from Neutron Probes located within their cotton fields to assist in irrigation scheduling. However, considerable doubt exists as to how accurately these limited point measurements reflect synoptic soil moisture conditions across the entire cotton field. During the 1992 / 1993 cotton growing season, Field 71 on Togo Station was extensively monitored using multispectral air videography. The aim of the experiment was to determine the feasibility of using airborne spectral measurements of the cotton canopy to map within field variations in soil moisture deficit. In addition to the air videography, simultaneous ground radiometric and soil moisture measurements were also collected. Field markers were placed in Field 71 to enable accurate registration of the ground truth measurements to the mutispectral video imagery.
Figure 4 Cotton canopy NDVI vs soil moisture deficit in Field 71 on Togo Station over January and February 1993 Figure 4 shows the relationship between cotton canopy NDVI calculated from ground radiometer measurements and soil moisture deficit (SMD). Early in the irrigation cycle, for water deficit less than 60mm, there is positive correlation between NDVI and SMD. This reflects the increase in plant vigour following irrigation as the soil becomes less saturated. Late in the irrigation cycle, for water deficit greater than 60mm, the correlation between NDVI and SMD is negative, reflecting the decrease in plant vigour as the soil profile dries out. Figure 4 also shows a least squares quadratic fit of NDVI to SMD. Although the correlation coefficient is low, r 2 =0.1423, reflecting the noise in the data, the statistical regression is highly significant with an F ratio of 9.371 on 2 and 113 degrees of freedom. Work is currently in progress to extend the soil moisture calibration from the ground radiometric values to the airborne video imagery.
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CONCLUSION Multispectral airborne imaging techniques have been developed and the potential for their use in Australian agriculture and for environmental monitoring investigated. The system has been radiometrically calibrated to known ground reflectance targets, both in the laboratory and in airborne trials. In both cases, the system exhibits a high degree of linearity. Statistical linear models have been devised to quantify the relationships between the airborne video response and ground reflectance. Further work is in progress to calibrate the airborne video response using standard atmospheric remote sensing models. In addition, work is also underway to calibrate the system to measure environmental parameters such as water turbidity and agronomic variables such as crop response to field soil moisture variations. The results obtained to date indicate that low-cost multispectral airborne video systems can be successfully calibrated to measure useful agronomic and environmental variables. The next challenge is to integrate accurate GPS positioning data and to develop automatic moasicing techniques to allow imagery to be easily acquired on a larger, regional or catchment, scale. ACKNOWLEDGMENTS The authors would like to acknowledge the support, advice and encouragement provided to the project by Dr Frank Honey, SpecTerra Systems Pty. Ltd., Perth, Dr Peter Cull, Neutron Probe Services Pty. Ltd., Narrabri and Mr Hugh Holland, National Mutual Cotton Pty. Ltd., Togo Station. REFERENCES Button, B.J. and Cull, P.O. 1991, Airborne video sensing: a new approach to monitoring cotton: The Australian Cottongrower, vol12, No2: pp. 36-47. Edirisinghe, A., Chapman, G. E. and Louis, J.P. 1995, Radiometric Corrections and Calibration of Imagery from a Multispectral Airborne Video System: manuscript in preparation. McKenzie, G., Dare-Edwards, A., Louis, J., Van der Rijt, V. and Pratley, J. 1992, Application of Low-cost Airborne Video Technology to Australian Agriculture: 6th Australasian Remote Sensing Conference, Wellington, New Zealand. Richter, R. 1992, Atmospheric Correction of Imaging Spectrometer Data, In Remote Sensing Volume 2, Imaging Spectroscopy : Fundamentals and Prospective Applications, edited by F. Toselli and J. Bondechtel, Kluwer Academic Publishers, Dordrecht, The Netherlands.
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