Slide 1

Prof. G. Robert Brakenridge March 12, 2011 Director, Dartmouth Flood Observatory http://floodobservatory.colorado.edu/ CSDMS, INSTAAR, University of Colorado Campus Box 450 Boulder, CO 80309-0450 USA Office: 303-735-5485 Cell: 603-252-0659 Email: Robert.Brakenridge@Colorado.edu Slide 2 1. Image Data Acquisition 2. Identification of water pixels 3. Create vector GIS (water boundary) polygon around water pixels 4. Import GIS file into Surface Water Record 5. Sort GIS files, within workspace; create map 6. Publish map Slide 3 Web browsers: the usual suspects (final product should be viewable by all) Remote sensing: Envi GIS: MapInfo Web publisher: Dreamweaver Slide 4 http://rapidfire.sci.gsfc.nasa.gov/subsets/ http://rapidfire.sci.gsfc.nasa.gov/subsets/ These and other subsets are available from this website in Geotiff format; choose 250 m images and the 721 band combination Slide 5 Geotiffs are in byte format (range of 0-255 shades of gray or numbers, per band). Thus, original radiometric resolution of the MODIS sensors is reduced in this format. The files are geocoded at full spatial resolution (bands 1 and 2 at 250 m). Band 7, originally at 500 m resolution, has been resampled to 250 m). Band 2 provides the most information for water/land discrimination. Simple thresholding of the band 2 images can separate water (dark) from land (relatively light) pixels, however cloud shadows will be misclassified as water if simple thresholding is employed. Slide 6 To solve the cloud shadow problem: acquire six images (preferably, Terra and Aqua, today and two days prior). The next four slides, as examples, show dark water, and (also dark) shifting cloud shadows. Images from May 18 and 17, 2009, G (green band, MODIS band 2) from the RGB tiff. Slide 7 Slide 8 Slide 9 Slide 10 Slide 11 DFO finds the following approach works well: 1.Use ENVI band math tool. After reading in all four geotiffs, follow (two) steps below. a. Bandmath step 1: (float(b2) gt 150 ) or (((float(b1)+1)/(float(b2)+80)) gt.7) or float (b3) >50 where b1 is green band of the 721 (red,green,blue) geotiff, b2 is the blue band, and b3 is the red band. (this is calculating an adjusted ratio of MODIS band 2/band1, after removing clouds and cloud shadow on cloud: failed thresholds assign a 1 value) Slide 12 b.Band math step 2: float(b1)+float(b2)+float(b3)+float(b4)+float(b5)+flo at(b6) where b1 through b6 are the results of the step 1 math, applied to six images (commonly, three consecutive days, Terra and Aqua) (float sets the band math operations into floating point notation, so values may range from +/ 1e38) 2.Apply threshold to the result. Pixels < 4 are water, so all pixels with values of 0, 1,2, or 3. Slide 13 For each single image, clouds are masked: they receive a value of 1. Where corrected ratio of the two bands is > 0.7, a 1 also assigned. Ratio is corrected in order to avoid dividing by 0, and, empirically, to provide best threshold for water/land in most scenes. The band ratio approach also improves scene-scene calibration The result for each scene is binary: all pixels are assigned 1 (cloud or land) or 0 (water or cloud shadow) Adding the values for four such scenes and thresholding water at