National Ocean Service Topographic Sheets
The oldest maps being utilized in the shoreline change analysis, produced by the US Coast & Geodetic Survey, date back to the 1850s. The oldest sets are called "Plane Table" maps. At the time these maps were produced, cartographers set up a table over the top of a benchmark and using an alidade, survey data was transferred to the map. This surveying technique was used as early as 1834, when the US Coast & Geodetic Survey completed the first shoreline survey. Even in 1928, "Plane Table" mapping techniques were considered the best for conducting topographic surveys. That year renowned surveyor O.W. Swainson wrote, "With it the map is actually drawn in the field when and where the features can be seen and where all the amount of detail to be mapped, and the accuracy required, can be judged to best advantage." To see examples of this surveying technique, visit the National Oceanic & Atmospheric Administration (NOAA) historic image collection's Historic Coast and Geodetic Survey Collection.
The 1950s map series was developed by the National Ocean Service (NOS) at NOAA as the basis for navigation charts. The maps are highly precise, created with aerial photography, depicting the coastal shoreline and inland waterways. As aerial photography techniques became more accurate and efficient, the technology eventually replaced older mapping techniques. Visit the aerial photography page for more information and to see how this data set is being used for shoreline change analysis.
Each of the T-sheet shoreline coverages consist of approximately 25 maps. The individual charts were originally drawn on cloth or paper, and are currently stored at the National Archives in Washington, D.C. The original historical maps were scanned by NOAA and provided to the Study on CD-ROM. The images are scanned at a resolution of 400 dots per inch. Upon receipt by Study staff, the images are prepared for inclusion in the Study GIS database.
The process of converting the digital images to GIS data is labor intensive. First, registration marks are identified on each of the images. These registration marks are to used to link the map to a real world coordinate system. What makes this process difficult is that the maps are drawn at a variety of scales based on different reference systems. Some of the oldest reference marks were established based on celestial observations. An average of 24 - 32 registration points are identified for each map. In some cases, as many as 60 registration points were used. Next the image is stretched and warped according to the registration points in a process called rectification. Further complicating the process is the fact that some of the maps have warped or shrunk with age. The features of the image files are traced to create vector coverages, and then they are merged to form a single map of the region.
The results from this process vary according to survey year. Error analysis results indicate that the 1950s survey is the most accurate of the T-sheet shorelines, followed by the 1920s and the 1870s surveys. Although less accurate than current aerial photography techniques, the T-sheets provide an invaluable look at the shoreline condition over a century ago and serve as a starting point for analyzing shoreline change in the Columbia River littoral cell.
For more information on the techniques the Study is using to convert these maps to digital files, an unpublished paper entitled NOAA Topographic Sheets: Error Assessment of Vectorized Lines Based on Coordinates from NGS Survey Markers (Daniels and Huxford, 1997) is available from the Study library.
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