INVESTIGATOR Investigator: Alan H. Strahler, Professor, Dept. of Geography and Center for Remote Sensing, Boston University, 675 Commonwealth Avenue, Boston, MA, 02215 Office Telephone: 617-353-5984; Fax: 617-353-3200 E-mail (internet): alan@bucrsb.bu.edu or astrahler@bucrsb.bu.edu Contacts: PI is most knowledgeable about data. Acknowledgement: Please cite PI as data provider. INTRODUCTION These measurements were made in the context of validation of several types of geometric-optical BRDF models that the PI and coworkers have derived over the years. These are based on the simple premise that we may regard the radiance (or reflectance) measurement made by a radiometer as a linear compound of four component radiances (reflectances) associated with sunlit tree crown, sunlit background, shaded tree crown, and shaded background. That is, R = ( Ac C + Ag G + At T + Az Z ) / A , where R is the radiance (reflectance) observed; Ac, Ag, At, and Az are the areas in the sensor's field of view associated with sunlit crown, sunlit background, shaded crown, and shaded background, respectively; C, G, T, and Z are the radiances (reflectances) of each component ("component signatures"); and A is the area of the radiometer's field of view. Our models concentrate on predicting the proportions (A's/A) acurately for different view and illumination positions, different canopy shapes and densities, etc. But we need information on the component radiances (reflectances) G-Z if we are to validate the geometric-optical models properly. Thus were these SE-590 measurements made as a part of the OTTER project. In addition to component signatures, directional measurements of radiance (reflectance) are also required. For these, we are using ASAS data, collected from the OTTER sites as part of the OTTER data collection effort. Since the component signatures will obviously vary according to the time of day, we tried to collect the ground signature data close to the time of the overpass of the C-130 aircraft that collects ASAS. THEORY OF MEASUREMENTS In collecting the radiance measurements, we simply tried to take a number of readings that we thought were representative of the types of surfaces within the ASAS field of view. Since we were also interested in directional effects, we tried to collect views of the target surfaces at +45, 0 and -45 degrees whenever possible. By this we mean collecting measurements in the principal plane (plane of the sun's position) at +45 degrees (near the hotspot direction), 0 degrees (at nadir), and -45 degrees (in the forward-scattering direction). These angles were determined using a clinometer held against the case of the radiometer head. In the +45 direction, it was often necessary to move away from the target in an azimuthal direction to avoid the shadow of the instrument and operator. In general, measurements were made from one-half to one meter away from the target. A single measurement was made for each target, although sometimes several spectra were collected from the same type of surface. EQUIPMENT Instrument Description: Platform: ground. Key Variables: surface radiance Principles of Operation: grating spectrometer Instrument Measurement Geometry: The sensor is mounted within a hand-held box on a two-meter cable from the recording device, a data logger with casette tape recorder. The box is pointed at the target by eye, aligning the top long edge of the box so that it points toward the target. Manufacturer: Spectron Engineering, Denver, Colorado, 303-733-1060. Calibration: There are two parts to the calibration problem. First is the instrument itself. Second is the "Lambertian" panel against which the radiances will be ratioed. In general, the approach was to standardize the BU SE-590 and Spectralon panel to the Ames-2 instrument and Ames panel, thus providing continuinty of calibration with the OTTER experiment. Instrument: The instrument was calibrated spectrally by Jennifer Dungan as part of the OTTER calibration plan. This was done by the use of didymium filters with narrow absorption bands. The BU instrument was found to be off by three bands with respect to the Ames-2 instrument. The range of the BU data are therefore 376.200 to 1113.700 micrometers for channels 1 to 249 (Dungan, memorandum of January 5, 1991). For radiometric calibration, the Ames panel was measured under the same conditions by both BU and Ames-2 radiometers, and after spectral adjustment, a set of ratios were obtained to be applied to BU measurements to bring the radiometric responses into conformity with Ames-2. Since measurements were made for a single broadband brightness only, only a simple ratio providing a single multiplicative factor was possible. To calibrate the BU panel, measurements were made of the BU panel by the Ames-2 spectrometer, and ratioed with the measurements made by the Ames-2 spectrometer of the Ames panel. As in the radiometric calibration above, only a single measurement was made, and thus only a single ratio correction was possible. In processing of the raw BU radiometric measurements, we combined both ratios into a single coefficient and simply adjusted every radiance by the multiplicative factor. The factors are listed at the end of this file. Other Calibration Information: Our SE-590 was retrofitted with the anti-aliasing filter in March, 1990. At that time, the instrument checked out and recalibrated. PROCEDURE Each site was visited near the time of the overpass of the C-130. The first activity was to record the radiance of the BU panel. The clinometer was used to measure the solar zenith angle, and the time of day was recorded. Then the radiances of the various cover types were measured. As the data were collected, the scan number was recorded and the cover type and viewing position was noted. After the measurements, were collected, the panel was measured once again, and the solar zenith angle and time of day were also recorded again. OBSERVATIONS No special conditions are noted. DATA MANIPULATIONS For analysis, the data were output from the SE-590 to a laptop PC micro and then converted to Macintosh format where they were manipulated in Excel. The data normalization carried out in two steps. First, the appropriate panel reference was established. Since the panel radiance varied between the start and end of the measurement period, we simply carried out a linear interpolation of panel radiances between the first and last measurement for as many measurements were made during the measurement period. This established a separate panel reference for each measurement, which was then divided into the observed radiance. The second step was to adjust this reflectance to that of the Ames-2 radiometer and the Ames panel, which was effected by multiplying the reflectance factor by the adjustment factor as described above and shown at the conclusion of this file. ERRORS Sources of Error: 1. Surface reflectances. Since the backgrounds were quite variable, radiances for the same surface type may also vary quite a lot. 2. Instrument performance. An inspection of the data shows a significant amount of noise in channels 4-17 and in channels above about 225 (1038 micrometers). (Note that channels 1-3 are not present, since these are channels that the Ames-2 records but the BU radiometer does not.) Measurements in channels in these wavelengths should be used only with caution. Quality Assessment: Validation: The data have been plotted and visually inspected for obvious problems. All scans included in the dataset seem reasonable. Confidence Level: In general, the quality of the data is good. There is considerable variation in measurements made of the same surface cover type, which is to be expected, since these are individual, hand-held measurements. Measurement Errors: Since the measurements are not replicated, no quantitative determination of error was possible. NOTES Beyond problems discussed above, there are no known further problems with the data. REFERENCES Strahler, A. H. and D. L. B. Jupp, 1990, Modeling bidirectional reflectance of forests and woodlands using Boolean models and geometric optics: Remote Sensing of Environment, vol. 34, pp. 153-166. Albers, B. J., A. H. Strahler, X. Li, S. Liang, and K. C. Clarke, 1990, Radiometric measurements of gap probability in conifer tree canopies: Remote Sensing of Environment, vol. 34, pp. 179-192. Franklin, J., S. D. Prince, A. H. Strahler, N. P. Hanan, and D. S. Simonett, 1990, Reflectance and transmission properties of West African savanna trees from ground radiometer measurements: Int. J. Remote Sensing, in press. Franklin, J. and A. H. Strahler, 1988, Invertible canopy reflectance modeling of vegetation structure in semiarid woodland: IEEE Trans. on Geosci. and Remote Sensing, vol. 26, pp. 809-825. Li, X. and A. H. Strahler, 1986, Geometric-optical bidirectional reflectance modeling of a conifer forest canopy: IEEE Trans. on Geosci. and Remote Sensing, vol. GE-24, no. 6, pp. 906-919. Li, X., and A. H. Strahler, 1985, Geometric-optical modeling of a conifer forest canopy: IEEE Trans. on Geosci. and Remote Sensing, vol. GE-23, no. 5, pp. 705-721. CALIBRATION FACTORS (Raw BU Channel Number + 3 gives Ames-2 Channel Number) Ratio, Ratio, Ames-2 Ames Panel/ Ames Instrument/ Multiplier Channel BU Panel BU Instrument Factor 1 2 3 4 0.7686 4.3923 3.3761 5 0.8370 2.9000 2.4273 6 0.8656 2.2327 1.9325 7 0.8899 1.9348 1.7218 8 0.8864 1.8034 1.5985 9 0.8927 1.6874 1.5063 10 0.8949 1.5762 1.4105 11 0.8983 1.5107 1.3570 12 0.9065 1.4299 1.2963 13 0.9050 1.3776 1.2467 14 0.9091 1.3286 1.2079 15 0.9041 1.2945 1.1703 16 0.9097 1.2674 1.1529 17 0.9122 1.2455 1.1362 18 0.9165 1.2140 1.1127 19 0.9126 1.2156 1.1094 20 0.9112 1.2150 1.1072 21 0.9130 1.2177 1.1117 22 0.9097 1.2012 1.0927 23 0.9100 1.1917 1.0844 24 0.9072 1.1994 1.0881 25 0.9100 1.1878 1.0809 26 0.9089 1.1705 1.0639 27 0.9088 1.1579 1.0523 28 0.9071 1.1448 1.0384 29 0.9080 1.1519 1.0459 30 0.9091 1.1554 1.0503 31 0.9095 1.1542 1.0498 32 0.9105 1.1751 1.0700 33 0.9121 1.1782 1.0746 34 0.9089 1.1906 1.0821 35 0.9076 1.1899 1.0800 36 0.9083 1.2021 1.0919 37 0.9064 1.2046 1.0919 38 0.9057 1.1931 1.0806 39 0.9040 1.1795 1.0663 40 0.9043 1.1846 1.0713 41 0.9036 1.1772 1.0637 42 0.9039 1.1597 1.0482 43 0.9034 1.1459 1.0353 44 0.9036 1.1565 1.0450 45 0.9027 1.1686 1.0549 46 0.9040 1.1698 1.0575 47 0.9044 1.1530 1.0428 48 0.9062 1.1708 1.0609 49 0.9062 1.1643 1.0551 50 0.9063 1.1439 1.0367 51 0.9066 1.1325 1.0267 52 0.9063 1.1308 1.0248 53 0.9060 1.1284 1.0223 54 0.9053 1.1190 1.0131 55 0.9051 1.1205 1.0142 56 0.9043 1.1422 1.0329 57 0.9032 1.1634 1.0508 58 0.9026 1.1754 1.0609 59 0.9017 1.1828 1.0664 60 0.9012 1.1925 1.0747 61 0.9012 1.1760 1.0598 62 0.9013 1.1642 1.0493 63 0.9010 1.1820 1.0650 64 0.9014 1.1753 1.0594 65 0.9010 1.1548 1.0405 66 0.9013 1.1370 1.0248 67 0.9020 1.1253 1.0150 68 0.9028 1.1236 1.0144 69 0.9036 1.1224 1.0142 70 0.9037 1.1191 1.0114 71 0.9050 1.1088 1.0035 72 0.9053 1.1169 1.0111 73 0.9058 1.1088 1.0043 74 0.9058 1.1084 1.0040 75 0.9055 1.1073 1.0027 76 0.9052 1.1196 1.0134 77 0.9043 1.1203 1.0131 78 0.9038 1.1246 1.0164 79 0.9030 1.1379 1.0275 80 0.9021 1.1592 1.0457 81 0.9012 1.1672 1.0520 82 0.9007 1.1651 1.0494 83 0.9002 1.1613 1.0455 84 0.8997 1.1585 1.0423 85 0.8999 1.1323 1.0189 86 0.9000 1.1069 0.9962 87 0.9001 1.0811 0.9731 88 0.9007 1.0719 0.9655 89 0.9010 1.0637 0.9583 90 0.9013 1.0590 0.9545 91 0.9017 1.0669 0.9621 92 0.9020 1.0775 0.9718 93 0.9022 1.0845 0.9784 94 0.9024 1.0903 0.9839 95 0.9031 1.0856 0.9804 96 0.9033 1.0935 0.9878 97 0.9031 1.0722 0.9683 98 0.9036 1.0696 0.9665 99 0.9041 1.0648 0.9627 100 0.9042 1.0495 0.9490 101 0.9043 1.0362 0.9371 102 0.9046 1.0284 0.9302 103 0.9045 1.0092 0.9128 104 0.9043 1.0111 0.9143 105 0.9037 1.0088 0.9117 106 0.9031 1.0053 0.9079 107 0.9024 1.0062 0.9080 108 0.9016 1.0170 0.9170 109 0.9010 1.0053 0.9058 110 0.9006 1.0035 0.9037 111 0.8999 0.9855 0.8869 112 0.8993 0.9912 0.8914 113 0.8989 1.0158 0.9131 114 0.8985 1.0248 0.9209 115 0.8984 1.0360 0.9308 116 0.8980 1.0402 0.9342 117 0.8982 1.0456 0.9391 118 0.8980 1.0531 0.9457 119 0.8983 1.0488 0.9421 120 0.8980 1.0519 0.9446 121 0.8982 1.0397 0.9339 122 0.8982 1.0406 0.9347 123 0.8986 1.0352 0.9302 124 0.8988 1.0417 0.9362 125 0.8988 1.0394 0.9342 126 0.8988 1.0510 0.9446 127 0.8991 1.0771 0.9684 128 0.8994 1.0984 0.9879 129 0.8993 1.0906 0.9807 130 0.8996 1.0900 0.9805 131 0.8996 1.0940 0.9842 132 0.8997 1.0933 0.9836 133 0.8999 1.0927 0.9833 134 0.8999 1.0807 0.9725 135 0.9007 1.0808 0.9735 136 0.9010 1.0809 0.9739 137 0.9017 1.0726 0.9672 138 0.9020 1.0580 0.9543 139 0.9021 1.0427 0.9406 140 0.9022 1.0365 0.9351 141 0.9023 1.0234 0.9235 142 0.9023 1.0267 0.9264 143 0.9021 1.0306 0.9297 144 0.9018 1.0429 0.9405 145 0.9011 1.0503 0.9464 146 0.9005 1.0521 0.9474 147 0.9000 1.0489 0.9440 148 0.8992 1.0643 0.9571 149 0.8985 1.0759 0.9667 150 0.8980 1.0714 0.9621 151 0.8973 1.0834 0.9721 152 0.8968 1.0849 0.9730 153 0.8958 1.0869 0.9737 154 0.8959 1.0852 0.9723 155 0.8961 1.0735 0.9620 156 0.8964 1.0733 0.9621 157 0.8960 1.0690 0.9578 158 0.8958 1.0799 0.9674 159 0.8960 1.0818 0.9693 160 0.8964 1.0787 0.9669 161 0.8962 1.0729 0.9615 162 0.8964 1.0702 0.9593 163 0.8970 1.0644 0.9548 164 0.8973 1.0619 0.9528 165 0.8975 1.0587 0.9502 166 0.8978 1.0588 0.9506 167 0.8977 1.0585 0.9502 168 0.8975 1.0702 0.9605 169 0.8977 1.0750 0.9651 170 0.8978 1.0922 0.9805 171 0.8976 1.1107 0.9969 172 0.8979 1.1206 1.0062 173 0.8976 1.1133 0.9993 174 0.8978 1.0891 0.9777 175 0.8974 1.0622 0.9532 176 0.8980 1.0391 0.9331 177 0.8975 1.0215 0.9169 178 0.8976 1.0070 0.9039 179 0.8980 0.9981 0.8963 180 0.8979 0.9949 0.8933 181 0.8974 0.9932 0.8913 182 0.8976 0.9889 0.8876 183 0.8976 0.9871 0.8861 184 0.8976 0.9829 0.8822 185 0.8978 0.9682 0.8692 186 0.8978 0.9621 0.8638 187 0.8986 0.9518 0.8553 188 0.8990 0.9455 0.8500 189 0.8996 0.9352 0.8413 190 0.8992 0.9355 0.8412 191 0.8988 0.9470 0.8512 192 0.8989 0.9476 0.8518 193 0.8990 0.9292 0.8353 194 0.8990 0.9321 0.8379 195 0.8991 0.9387 0.8440 196 0.8981 0.9421 0.8461 197 0.8980 0.9366 0.8410 198 0.8976 0.9342 0.8385 199 0.8971 0.9413 0.8444 200 0.8974 0.9395 0.8431 201 0.8969 0.9350 0.8386 202 0.8962 0.9381 0.8408 203 0.8956 0.9459 0.8471 204 0.8954 0.9532 0.8534 205 0.8945 0.9566 0.8557 206 0.8951 0.9594 0.8588 207 0.8944 0.9612 0.8597 208 0.8949 0.9569 0.8564 209 0.8952 0.9429 0.8441 210 0.8945 0.9396 0.8405 211 0.8935 0.9352 0.8356 212 0.8955 0.9230 0.8266 213 0.8957 0.9258 0.8292 214 0.8943 0.9369 0.8378 215 0.8971 0.9590 0.8603 216 0.8966 0.9937 0.8909 217 0.8964 1.0096 0.9051 218 0.8971 1.0119 0.9078 219 0.8968 0.9876 0.8857 220 0.8950 0.9432 0.8441 221 0.8976 0.8921 0.8007 222 0.8976 0.8651 0.7766 223 0.8995 0.8493 0.7640 224 0.8997 0.8419 0.7575 225 0.9004 0.8216 0.7398 226 0.9007 0.8315 0.7490 227 0.9020 0.8473 0.7643 228 0.9021 0.8449 0.7622 229 0.9016 0.8329 0.7509 230 0.8974 0.8396 0.7534 231 0.9033 0.8436 0.7620 232 0.9034 0.8477 0.7658 233 0.9011 0.8352 0.7526 234 0.9000 0.8468 0.7621 235 0.9033 0.8649 0.7812 236 0.9014 0.8695 0.7838 237 0.9016 0.8504 0.7667 238 0.9057 0.8847 0.8013 239 0.9020 0.9375 0.8456 240 0.9023 0.9283 0.8376 241 0.9031 0.8838 0.7981 242 0.9009 0.8986 0.8096 243 0.9003 0.9388 0.8451 244 0.9038 0.9319 0.8423 245 0.9050 0.9118 0.8252 246 0.9088 0.9241 0.8398 247 0.9050 0.9418 0.8523 248 0.9066 0.9312 0.8442 249 0.9015 0.8934 0.8054 250 0.8996 0.9150 0.8231 251 0.9135 0.9207 0.8411 252 0.9056 0.9143 0.8280 253 0.8991 0.9143 0.8221 254 0.8965 0.9143 0.8197 255 0.8970 0.9143 0.8202