An Error Assessment of Vector Data Derived front Scanned National Ocean Service Topographic Sheets

Abstract

The National Oceanic and Atmospheric Administration, National Ocean Service (NOS) has undertaken a data-rescue project to convert historical topographic sheets (T-sheets) from paper or cloth to a digital format. The original maps have been scanned and saved as raster images. These images have been registered and the line work vectorized using heads-up digitizing methods to obtain X, Y coordinate pairs that delineate the location of the shorelines depicted on the original maps. A methodology is described here for obtaining error estimates for the derived vector data. The error assessment methodology uses the coordinates obtained for survey stations digitized from the scanned T-sheets and compares these coordinates to those published by the National Geodetic Survey. Differences between published coordinates for survey stations and those measured from the T-sheets have been calculated and descriptive statistics obtained. In southwest Washington and northwest Oregon the mean error for 1926/27 and 1950-era T-sheets is ±3 m for 1:10,000 scale sheets and ±6 m for 1:20,000 scale sheets.