Coast & Geodetic Survey Topographic Sheets (T-sheets)
The U. S. Coast Survey, renamed as the U. S. Coast & Geodetic Survey in 1878, mapped the Puget Sound nearshore at a reconnaissance level in the 1840s, and then at a more detailed scale (1:10,000 and 1:20,000) in the following decades. The agency created two map series: topographic sheets, commonly referred to as "T-sheets, concentrated on intertidal and supratidal areas, and hydrographic sheets, or "H-sheets," showed soundings. The maps available from this site are the earliest detailed (1:10,000 or 1:20,000 scale) T-sheets for the Puget Sound nearshore.
These early T-sheets are the most comprehensive and detailed early map representations of nearshore conditions in the second half of the 19th century, and are generally very accurate, with some limitations. By the time of the survey some diking and land conversion had occurred, obscuring pre-settlement conditions; additionally the T-sheets tended to lose accuracy with distance inland. In these instances, other sources of information, including the field notes and plat maps of the government land survey, provide important cross-referencing. For more information on interpreting Puget Sound T-sheets, see Collins and Sheikh (2005a).
Familiarity with the instructions given to surveyors aids in the interpretation of T-sheets; Shore and Sea Boundaries, Volume 2, by A. L. Shalowitz (1964), is the standard reference on early Coast Survey mapping. Understanding the nuances specific to Puget Sound T-sheets, including the cartographic skills and habits of individual surveyors, is aided by a comparative examination of the region's T-sheets, and by consulting the descriptive reports for individual T-sheets (available for download on this site for those T-sheets where digital copies were available).
Download Puget Sound T-Sheets, metadata, and descriptive reports.
How we Georeferenced T-sheets
The early T-sheets were surveyed relative to a an obsolete datum; typically, the sheets show latitude and longitude graticules in the early, local, Puget Sound Datum (PSD), and a later update to the North American Datum (NAD). We calculated a datum shift to convert to current datums (e.g., NAD27, NAD83).
"Datum Difference" tables for survey stations around most of Puget Sound (Patton, 1999) provide X and Y values for calculating the shift from NAD to NAD27. An overall datum shift is determined for a T-sheet by averaging the shift from survey stations within or nearby the area represented on the T-sheet, and making a first order polynomial transformation to rectify the T-sheet into UTM Zone 10N NAD27 projection.
For a measure of the accuracy of the registration for each T-sheet, National Geodetic Survey (NGS) benchmarks
located on the T-sheet were compared to published NGS benchmarks that were retraceable to the time of the surveyed T-sheet. This accuracy assessment only used those benchmarks that could be established with a high degree of certainty to have existed at the time of the original T-sheet survey, and that were not later remounted.
The surveyors recorded these benchmarks on most T-sheets. However, in a number of cases only one benchmark could be retraced to the time of the original T-sheet survey, and there are some cases where no benchmarks could be found that correlated with the NGS published benchmarks.
How we Digitized T-sheets
We digitized on-screen at an average scale of 1:1,500, generally digitizing everything on a sheet, except for a few sheets where information extended farther inland; in those cases we digitized information within 500m of the nearshore. We did not digitize topographic lines, which topographers sketched by eye. We attempted to keep as closely as possible to each topographer's intent, using each surveyor's map categories to attribute our digitized coverages, and with a minimum of translation into modern terms.
DataDownload a vector geodatabase of edge-mapped T-sheets for the Puget Sound region (includes metadata).
View T-sheet geodatabase metadata.
Acknowledgments and References
We benefited immensely from discussions with Jennifer Burke (NOAA Fisheries and University of Washington), Alan Carter Mortimer (Point No Point Treaty Council), and David Doyle (NGS). David Doyle generously provided transformation information for areas where data from Datum Differences were insufficient.
Daniels, R. C. and R. H. Huxford. 2001. An error assessment of vector data derived from scanned National Ocean Service topographic sheets. Journal of Coastal Research 17: 611-619.
Patton, R. S., 1999. Datum Differences: Atlantic, Gulf, and Pacific Coast of the United States. U.S. Department of Commerce, U.S. Coast and Geodetic Survey, Washington D.C. (Original publication: 1936).
T-sheet Data for Other Regions
- San Francisco Bay area (georeferenced T-sheets and vector data)
- Florida Everglades area (georeferenced T-sheets)
- Hawaii Coastal Geology Group (georeferenced T-sheets)
- Coastal United States (T-sheets and nautical charts, not georeferenced)
- Coastal United States (vector coastline from T-sheets)