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>> Pubs Warehouse > DS 780 Natural-Color and Color-Infrared Image Mosaics of the Colorado River Corridor in Arizona Derived from the May 2009 Airborne Image CollectionBy Philip A. Davis 
 Index map stored in Adobe Acrobat (PDF)       format displaying the extent of the Colorado River within Arizona in the       geographic coordinate system. Index map is also provided in jpeg format.       Click on the figure for a PDF (100 kB). 
Abstract The Grand Canyon Monitoring and Research Center (GCMRC) of the U.S.       Geological Survey (USGS) periodically collects airborne image data for the       Colorado River corridor within Arizona (fig. 1) to allow scientists to study the impacts of Glen       Canyon Dam water release on the corridor’s natural and cultural resources.       These data are collected from just above Glen Canyon Dam (in Lake Powell)       down to the entrance of Lake Mead, for a total distance of 450 kilometers       (km) and within a 500-meter (m) swath centered on the river’s mainstem and       its seven main tributaries (fig. 1). The most recent airborne data collection in       2009 acquired image data in four wavelength bands (blue, green, red, and       near infrared) at a spatial resolution of 20 centimeters (cm). The image       collection used the latest model of the Leica ADS40 airborne digital       sensor (the SH52), which uses a single optic for all four bands and       collects and stores band radiance in 12-bits. Davis (2012) reported on the       performance of the SH52 sensor and on the processing steps required to       produce the nearly flawless four-band image mosaic (sectioned into map       tiles) for the river corridor. The final image mosaic has a total of only       3 km of surface defects in addition to some areas of cloud shadow because       of persistent inclement weather during data collection. The 2009 four-band       image mosaic is perhaps the best image dataset that exists for the entire       Arizona part of the Colorado River.       
Some analyses of these image mosaics do not require the full 12-bit       dynamic range or all four bands of the calibrated image database, in which       atmospheric scattering (or haze) had not been removed from the four bands.       To provide scientists and the general public with image products that are       more useful for visual interpretation, the 12-bit image data were       converted to 8-bit natural-color and color-infrared images, which also       removed atmospheric scattering within each wavelength-band image. The       conversion required an evaluation of the histograms of each band’s       digital-number population within each map tile throughout the corridor and       the determination of the digital numbers corresponding to the lower and       upper one percent of the picture-element population within each map tile.       Visual examination of the image tiles that were given a 1-percent stretch       (whereby the lower 1- percent 12-bit digital number is assigned an 8-bit       value of zero and the upper 1-percent 12-bit digital number is assigned an       8-bit value of 255) indicated that this stretch sufficiently removed       atmospheric scattering, which provided improved image clarity and true       natural colors for all surface materials. 

The lower and upper 1-percent, 12-bit digital numbers for each       wavelength-band image in the image tiles exhibit erratic variations along       the river corridor; the variations exhibited similar trends in both the       lower and upper 1-percent digital numbers for all four wavelength-band       images (figs.       2–5). The erratic variations are attributed to (1) daily variations in       atmospheric water-vapor content due to monsoonal storms, (2) variations in       channel water color due to variable sediment input from tributaries, and       (3) variations in the amount of topographic shadows within each image       tile, in which reflectance is dominated by atmospheric scattering. 

To make the surface colors of the stretched, 8-bit images consistent       among adjacent image tiles, it was necessary to average both the lower and       upper 1-percent digital values for each wavelength-band image over 20       river miles to subdue the erratic variations. The average lower and upper       1-percent digital numbers for each image tile (figs. 2–5)       were used to convert the 12-bit image values to 8-bit values and the       resulting 8-bit four-band images were stored as natural-color (red, green,       and blue wavelength bands) and color-infrared (near-infrared, red, and       green wavelength bands) images in embedded geotiff format, which can be       read and used by most geographic information system (GIS) and       image-processing software. The tiff world files (tfw) are provided, even       though they are generally not needed for most software to read an embedded       geotiff image. 

All image data are projected in the State Plane (SP) map projection       using the central Arizona zone (202) and the North American Datum of 1983       (NAD83). The map-tile scheme used to segment the corridor image mosaic       followed the standard USGS quarter-quadrangle (QQ) map borders, but the       high resolution (20 cm) of the images required further quarter       segmentation (QQQ) of the standard QQ tiles, where the image mosaic       covered a large fraction of a QQ map tile (segmentation shown in (figure 6), where QQ_1 to QQ_4 shows the number       convention used to designate a quarter of a QQ tile). To minimize the size       of each image tile, each image or map tile was subset to only include that       part of the tile that had image data. In addition, some QQQ image tiles       within a QQ tile were combined when adjacent QQQ map tiles were small.       Thus, some image tiles consist of combinations of QQQ map tiles, some       consist of an entire QQ map tile, and some consist of two adjoining QQ map       tiles. The final image tiles number 143, which is a large number of files       to list on the Internet for both the natural-color and color-infrared       images. Thus, the image tiles were placed in seven file folders based on       the one-half-degree geographic boundaries within the study area (fig. 7). The map tiles in each file folder were       compressed to minimize folder size for more efficient downloading. The       file folders are sequentially referred to as zone 1 through zone 7,       proceeding down river (fig. 7). The QQ designations of the image tiles       contained within each folder or zone are shown on the index map for each       respective zone (figs. 8–14). 
Reference Cited
Davis, P.A., 2012, Airborne digital-image data for       monitoring the Colorado River corridor below Glen Canyon Dam, Arizona,       2009—Image-mosaic production and comparison with 2002 and 2005 image       mosaics: U.S. Geological Survey Open-File Report 2012–1139, 82 p., http://pubs.usgs.gov/of/2012/1139/. 

First posted July 8, 2013
- Readme PDF  (2.9 MB) 
- Readme folder . This folder contains a         TXT file and 14 JPG images (864 kB total) 
- Index maps  (16 files; 1.1 MB total)         
- Image files  (14 files; 19.7 GB total)         
- Metadata  (14 files; 1.6 MB total)         
- Shapefiles  (1 file; 0.1 MB) 

For additional information contact:GCMRC       Staff, Southwest Biological Science CenterU.S. Geological       SurveyGrand Canyon Monitoring and Research Center2255 N. Gemini       DriveFlagstaff, AZ 86001http://www.gcmrc.gov/

Part of this report is presented in Portable Document       Format (PDF); the latest version of Adobe Reader or similar software is       required to view it. Download the latest version of Adobe Reader, free of       charge.
Suggested citation:
Davis, P.A., 2013, Natural-color and color-infrared image mosaics of the Colorado River corridor in Arizona derived from the May 2009 airborne image collection: U.S. Geological Survey Data Series 780, http://pubs.usgs.gov/ds/780/. 

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