Mapping the Shoreline at Norfolk, Virginia

A student presentation by
Lori Bird

Spring, 2008
This webpage project was created in partial completion for the
ES 775 Advanced Image Processing course at Emporia State University.

Image of Norfolk, Virginia. Data obtained from EROS

Geological History

Changing Shoreline

DOQ Documentation

Image Analysis

Conclusion

References

Abstract

Norfolk, Virginia is located where the James River empties into Chesapeake Bay on the eastern coast of the United States. This city, like many coastal areas, is losing its shoreline. The Chesapeake Bay is ever encroaching upon the land, swallowing acres. The Chesapeake Bay area show rates of sea-level rise twice that of the worldwide average (Larsen, USGS, 1998). Though many causes have been attributed to the loss of shoreline, accurately determining how much land is lost has been difficult until the advent of advanced geographic imaging. Because rising sea levels is a relatively slow process - measured millimeters annually - quantifying the loss of land has been a dilemma. With the advent of Digital Orthophoto Quadrangles (DOQs), digital images with a resolution of 1 meter and enhanced imagery analysis software, accurate images can be acquired and analyzed to document these changes and establish a baseline for interpretation. This webpage is a case study of the Norfolk, Virginia area.

Geological History

The Chesapeake Bay area is a drowned ancient river valley of the Susquehanna River. During the last period of glaciation, ice covered Canada and parts of the northern United States. Much of the water contained in the oceans today fell as snow during this glaciation period. This caused the water levels in the oceans to fall. It is estimated that at the sea levels were approximately 100 meters lower than they are today. During the past 15,000 years, these glaciers retreated, causing the water levels again to rise, flooding the Susquehanna River and creating the Chesapeake Bay and its estuaries. It is thought that the weight of the ice sheets to the north elevated the areas along the coast south of the glacial limit. As the weight of the ice was lifted, the land on the eastern coast began to reverse this uplift and sink slowly into the water.

Several other historical and current factors may have attributed to the rising sea levels in the Chesapeake Bay area. The bay area was the site of impact crater 36 million years ago. The impact crater is the largest in the United States and the sixth largest in the world. The crater's effect can be seen on the James River, which abruptly jogs northeast from its southeastern flow when it encounters the crater rim. The resulting ground deformation and long term compaction of the crater fill may contribute to the rising sea levels

Site of the Chesapeake Bay impact crater. Image courtesy C. Wylie Poag, Geotimes, 2004. Chesapeake map courtesy of Larsen, USGS, 1998

top

Changing Shoreline

Another factor cited for the rise in the sea levels in the Chesapeake Bay area is land subsidence. Groundwater was pumped out to supply the increasing urban populations. As the water was removed, the ground elevation decreased, allowing the surface water to cover the sinking land. The NOAA has established stations along the bay that monitor sea level. Initially, these stations were set in place to aid navigation. But they have provided consistent data that documents the rising sea levels. Sewells Point is located east of Norfolk on the southern banks of the James River. Data from this station reveals the mean sea level trend is 4.42 millimeters/year (1.45 feet/century) with a standard error of 0.16 mm/yr based on monthly mean sea level data from 1927 to 1999. The data is relative to the 1983-2001 mean sea level datum recently established by Center for Operational Oceanographic Products and Services (CO-OPS).

Mean Sea Level Trend 8638610 Sewells Point, Virginia, NOAA, 2008.

top

Digital Orthophoto Quadrangles Images

While graphs of the rising sea levels document the increase, the effects of the rising sea levels are difficult to conceptualize. That is where imagery can supplement the research. Digital orthophoto quadrangles (DOQ) are computed generated images of aerial photographs in which the images are corrected to remove any distortion caused by camera angle or terrain. They are basically a georeferenced image. These images are produced by the USGS Earth Resource Observation and Science (EROS) Center located in South Dakota. These images cover the United States and are updated over a period of five years. They are produced either in black and white (B/W), native or color infrared (CIR)formats. They have a resolution of 1 meter. The 3.75-minute DOQ are available in a GeoTIFF format. The GeoTIFF format is referenced to the NAD83. Since these images are periodically updated, and are available for a relatively inexpensive costs, they provide an ideal platform for establishing a baseline image of the shoreline.

The image of Norfolk was downloaded via file transfer protocol. Once the image was obtained, it was imported into the IDRISI program using the GeoTIFF conversion module and into its three bands.

Bands 1,2,3 of converted GeoTIFF image displayed using the grey scale palette.

Upon inspecting the bands, the river in band 1 is flat black, indicating no reflectance. This is characteristic of the infrared band. Water absorbs the electromagnetic radiation in this frequency range and so it appears in the image as black.

top

Image Analysis with IDRISI

Band 1 was then modified to isolate the water from other land covers using the RECLASS module to convert all values of 0 (water) to 1 and all other values to 0.

The image was then converted to a byte/ binary image using the CONVERT module.

The GROUP module was the utilized to establish groups of water.

Once the water groups that made up the James River/Chesapeake Bay water bodies were identified, the ASSIGN module was used to reset the pixel values of the desired groups to 1, and all other groups to 0.

This allowed the main water bodies to be isolated. The AREA module was employed to determine the area of the water and land covers. The land cover 36.28 square kilometers. The water covered 11.79 square kilometers.

Steps to creating the shoreline image.
top

Conclusion

Because of its unusually high rising sea levels, the important estuaries for wildlife and the urban areas that occupy its shore, the Chesapeake Bay is the site of many research endeavors. The NOAA has created a study area in northern North Carolina for the Application of Water Level and Datum Information to Sea Level Rise Impact Studies. The USGS is working with scientists trying to understand the how the Chesapeake Bay impact crater effects the rising sea levels. Various local environmental groups are monitoring the rising water levels and their impact on the wetlands. Private and public property owners are interested in protecting their property from further encroachment

DOQ and other remote sensing platforms can provide a baseline image from which the various groups can correlate their data. It is important that the data are documented and the rising levels are compared to annual and seasonal fluctuations as well as tidal surges to ensure no data are distorted or used to support an invalid hypothesis.

Other remote sensing data can augment the images obtained from the DOQ. Some of these are quite expensive, are composites over time, do not have the infrared image or do not have adequate resolution. The DOQ images, while not very timely, will produce the resolution necessary at the right price. The images can also be correlated with the tides and seasonal fluctuations the characterized the tidal basin.

top

References

Earth Resources Observation and Science, EROS, accessed May, 2008.

IDRISI GIS Software, Andres Version, Clark University, IDRISI

Larsen,C. The Chesapeake Bay: Geologic Product of Rising Sea Level, USGS PUBS accessed May, 2008.

NOAA, Application of Water Level and Datum Information to Sea Level Rise Impact Studies Impact Studies, accessed May,2008.

NOAA, Coastal Oceanographic Applications and Services of Tides And Lakes, COASTAL, accessed May, 2008

NOAA Tides and Currents, accessed May, 2008.

Poag, C. W., Coring the Chesapeake Bay Impact Crater, Geotimes January, 2004.

The Rising Tide - Cause, Effects and Planning for Rising Sea Level, Wetlands Watch, accessed May, 2008.

top