USGS TerraWeb: Multibeam Backscatter and Bathymetry Data: Alcatraz Island Area

Project: Sediment and Pollutant Transport in Central San Francisco Bay

Multibeam Backscatter and Bathymetry Mosaics

SAN FRANCISCO BAY
Multibeam Backscatter and Bathymetry Data: Alcatraz Island Area

Geographic Reference
Image Map
The map shows the names and locations of various underwater and on-land geographic features in the Bay area. (212 kb)

In addition, the shaded relief(below) is labeled with features in the Alcatraz Island local area discussed in the text.

IRS-IC Panchromatic data used with permission, © Space Imaging

Backscatter Mosaic(left-hand image below)

Shown below on the left is the backscatter image mosaic. Like the larger West-Central Bay data, this data set is in a UTM projection, and when viewed at full resolution is comprised of pixels with two meter resolution (click inside the image area to get a compressed version with a grid, and then click inside the white grid to get that section at full resolution). The dynamic range of backscatter values/brightness in the image are modulated by the different bay floor material types, such as sand, mud, or rock, and by small-scale seafloor relief. In general, the darker/lower backscatter areas are those composed of thick sand, including sandwaves one to three meters high, whereas the brighter/higher backscatter areas are indicative of smaller sandwaves/ripples (less than one meter high), exposed bedrock or boulders.
Shaded Relief Image(center image below)

This is a shaded relief image generated from the bathymetry data, Utilizes digital image processing techniques. A shaded relief image can be generated with the sun positioned at any desired elevation and azimuth. This enhances the topographic component in the data, including surface fracture, linear features, faults, folds, drainages, and slopes. Several topographic landforms are easily seen in this image. Large sand waves, as well as the build up of dredged material, can be seen to the southwest and south of Alcatraz Island, respectively. Also, the three rock pinnacles to the northwest of Alcatraz Island - Harding Rock (northwest), Shag Rock (center), Arch Rock (southernmost) - can be seen in this image. The linear feature to the left of the pinnacles is a rock outcrop that is partially covered by sediment.
Color Coded Bathymetry plus Shaded Relief Image(right-hand image below)

This image was generated using both the bathymetry and shaded relief images. By digitally combining these two data sets, both the elevation and structural information can be shown in the same image. The bathymetry data were color coded to cover the depth range from 7 to 47 meters. The scale at the bottom of the image represents water depth in approximately 1.1 meter increments.

Backscatter Mosaic
(270 kb)

TEXT

Shaded Relief Image
(300 kb)

TEXT

Color Coded Bathymetry plus Shaded Relief Image
(350 kb)
TEXT
Perspective Views of the Color Coded Bathymetry plus Shaded Relief Image

One way to show the elevation relationships within an image is to generate a perspective view. Basically, this is a procedure that allows the 'viewer' to be positioned somewhere other than directly above the image. In the images shown below the viewer has been positioned to the west of the Golden Gate Bridge at elevation angles of five, ten, and fifteen degrees above the bay floor, and is looking toward Alcatraz Island. The topographic characteristics of the sandwaves, dredged material mound, and the three rock pinnacles around Alcatraz Island can easily be seen in these images. The scale at the bottom of each image represents water depth in approximately 1.1 meter increments.

Fifteen Degree View
(185 kb)

Ten Degree View
(142 kb)

Five Degree View
(99 kb)
Backscatter plus Shaded Relief Image(left-hand image below)

The backscatter image contains information about the bottom type at a given pixel, along with some topographic information, while the bathymetry image contains only topographic information (i.e., the cover material affects the backscatter image but not the bathymetry image). Therefore, to maximize the information content in a single image a digital combination of these two images can be generated. In this case, the backscatter and shaded relief images were digitally merged using the sum of the two images. The resulting image has both the bottom type and topographic information contained in the two images.
Shaded Relief Image in Stereo(center image below)

The stereo image is also the shaded relief image shown above, however, the bathymetry data were used to create a stereo pair. Parallax was introduced into the shaded relief image in the horizontal direction using the bathymetry data to generate the stereo image. The stereo/3-D image can be viewed by using red/blue or red/green (red on the left) glasses. This is an excellent way to study and analyze the topographic component of an area and show the local and regional slope information. Note how well the sandwaves, dredged material mound south of Alcatraz Island, and the three rock pinnacles northwest of Alcatraz Island, are expressed in the 3-D image.
Backscatter plus Shaded Relief in Stereo(right-hand image below)

This image is the same image shown on the far left, but with stereo introduced similar to the shaded relief image just to the left of this image. Depending on the characteristics of the site and the interest of the user, the shaded relief or the combined presentations may be the better of the two for stereo viewing. Both are shown here for comparison.

Backscatter plus Shaded Relief Image
(280 kb)
TEXT

Shaded Relief in Stereo
(420 kb)

TEXT

Backscatter plus Shaded Relief in Stereo
(430 kb)
TEXT


To the West-Central Bay Multibeam Bathymetry Mosaics	Back to 2x Zoom Regional View	Back to the Title Page

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Last Modified: Thu Oct 31 15:19:29 MST 2002