Application of airborne Hyperspectral imaging
in Wetland delineation
There are few companies that make calibration panels with large sizes and different well-reflected gray shades. Those calibration panels are very nice but sometimes beyond the budgets of research centers at university and local government levels. Therefore, some other alternative materials were used. We examined cloths of their spectral reflectance ratios using a hand-held spectral radiometer (Model GER-1500, Geophysical & Environmental Research, New York) in local textile stores and found out some black, white and white-meshy fabrics with the same reflectance ratios from blue waveband through near infrared wavebands. Then, we sewed those fabrics and attached them on black tarpaulins to make five durable calibration panels with the size of 3.7 meter by 3.7 meter and the relative reflectance ratios of 4%, 10%, 25%, 45%, and 56%, respectively.
2.2.2 Setup for Aerial Hyperspectral Imaging
The preferred imaging day would be on or near the days when the LandSat 7 satellite would visit the study area in order to seek the potential of applying the results of airborne hyperspectral images to satellite images. Usually, spring is less clouded in Florida. Thus, the imaging day was considered around April 25 or May 11, 2000, which days the LandSat 7 satellite would visit on. The calibration panels and some extra ground targets were placed on the dike of the Ft. Drum Marsh before aerial imaging on the imaging day. The imaging was desired to do two different directions in order to avoid the shadows of the airplane and some clumps. One imaging was from north toward south, the other was from west to east.
2.2.3 Ground Truthing
Another airboat trip was made to do the ground truthing. Based upon the results of the unsurprised classification of the 1999 airborne image, there were some classes of that the spectral characteristics were not able to identified. According to the classification of the 1999 image, the desired ground truth points were pre-selected. The actual ground truth points might be changed as the field situations. At each point, first, the geo locations was first measured by the GPS unit. Then, the plants and were identified as well as the growth situation. The spectral reflectance, leaf area indices (LAI) were also measured using the spectral radiometer and a LAI meter (Model SunScan canopy analysis system, Detla-T Devices, Cambridge, England). At and around the ground truth points, digital video images at four narrow wavebands of 550 nm, 698nm, 798 nm, and 850 nm were recorded by a multi-spectral imager (Model MultiSpec Agro-Imager, Optical Insights, Arizona).
3. Results and Discussion
Ten ground targets were placed in the marshes on April 29, 2000. The aerial imaging was performed on May 12, 2000, and the field ground truthing was completed on May 17, 2000. The hyperspectral images were rectified by a piecewise rectification procedure. By observing the straight dike along the flight direction, the hyperspectral image was subset into several images for rectifications using polynomial functions, and then mosaiced together afterwards. By this special procedure, the root mean of square errors was 4.7 meter.
The spectral reflectance ratios of cattail and sawgrass have a little bit but still distinguishable difference (Juan and Shih, 1997). The ground spectral radiometer measurements of several different species was normalized (Tan, 1998) and displayed in Figure 2. The hyperspectral image was spectrally calibrated using the measured spectral characteristics of the calibration panels. The pixels at the ground truthing locations with known vegetation species were exacted and shown in Figure 3. The spectral reflectances ratios of different species were obvious in both Figure 2 and Figure 3 except sawgrass and cattail. In Figure 2, the difference between sawgrass and cattail was noticeable but insignificant, where the difference between sawgrass and cattail was significant. When cattail dies off, the dead leaves fall down and layer above the water surface but not break down at once. Therefore, the aerial images viewing from top to down showed more spectral influence of dead leaves of cattail than the spectral radiometer measurements from the side view.
Figure 2. Spectral Refletance Ratios of Different Wetland Vegetation Species Measured by Ground Spectral Radiometer
Figure 3. Spectral Reflectance Ratios of Different Wetland Vegetation Species From the Spectrally Calibrated Hyperspectral Image
4. Conclussion and Continuing Work
The aerial hyperspectral imaging was shown to be a considerable technique to delineate wetland. More analysis will be done to evaluate the most sensitive wavebands to identify different plant species. In addition, subpixel classifications from satellite images from the results of the hyperstral images will be also attempted.
5. Ackowledgements
The authors would like to address their appreciations to the Institute of Technology Development, Stennis Space Center, NASA for their kindly offer of the hyperspectral imager and the aerial imaging flight. They'd also like to express their sincere thanks to St. John's River Water Management District the assistance in field trips. Furthermore, we'd like to show our gratitude to Mr. Orland Lanni in the Center for Remote Sensing for his efforts in making the calibration panels. At the end, they'd like to honor Dr. Sun-Fu Shih, the former director of Center for Remote Sensing, who passed away in June, 2000 during the ongoing of this research.
References
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