Smooth Sheet Bathymetry: How to Work With Them in a GIS to Derive Bathymetry, Features and Substrates
February 25, 2013
Physically, a paper smooth sheet with muslin backing was the final product of a hydrographic survey (Hawley 1931). According to the Hydrographic Manual published by the National Ocean Service (NOS), a smooth sheet "is ultimately archived as the official permanent record of the survey and is the principal source and authority for charted hydrographic data" (p. 1-5, Umbach 1981). The concept of creating a smooth sheet rather than a data file of the soundings dates back to 1837 (for the United States). Hydrographers on surveying vessels would collect soundings with lead lines while recording visually determined sextant angles to shore-based navigation stations (Hawley 1931), rather than recording positions with latitudes and longitudes. Thus, the angles needed to be translated into latitude and longitude coordinates. As part of this process, the soundings were drawn on a smooth sheet, along with the shoreline, geographic features (e.g., kelp beds, rocky reefs, islets, rocks), seafloor substrates (e.g., gravel, sand, mud), and the navigational signals, in order to provide a visual record of the hydrographic survey, which could be annotated as new information became available. After completion of several smooth sheets in an area, the information from the smooth sheets was used to create or update navigational charts, which typically cover a larger area and thus are drawn at a smaller scale. Though more detailed than navigational charts, smooth sheets are not intended for use in navigation. Instead the smooth sheets were used as internal documents by the hydrographic agency. Only after they were scanned, digitized, and posted to National Geophysical Data Center (NGDC: (http://www.ngdc.noaa.gov/) (Wong et al. 2007) did they become widely used by non-hydrographers.
The NOS navigational charts are the legal standard for safe navigation on the ocean (Title 33 Code of Federal Regulations 164). Most people using these navigational charts are probably unaware that the charts might only display a small fraction of the smooth sheet information. Detailed hydrographic data typically are recorded on smooth sheets at a scale of 1:20,000, but are reduced in complexity at smaller scale 1:100,000 or 1:1,000,000 navigational charts. All scientists who conduct research on the ocean have probably used the small scale navigational charts for a variety of cruise planning and data analysis tasks without knowing that perhaps ten times as much information was available from the precursor hydrographic surveys, represented by the smooth sheets. Others who are aware of the smooth sheet resource might not understand some of the details about successfully using this rich data resource. Therefore, now that electronic copies of the smooth sheets are readily available, it is worthwhile for non-hydrographers to understand how to use them.