Except for pre-1800 charts, and those of the Pacific Coast of North America issued prior to 1875, foreign government charts are collected only when no U.S. chart of an area is available. For example, some areas in Mex ico and Canada are covered only by Mexican and Canadian charts. NDI produces their digital products India. Naval Hydrographic Office. The collection contains many other type of oceanographic maps.
Bathymetric maps show ocean depths by contours and gradient tints.
Ocean bottom maps show the seafloor topography by shading or landform drawing. Still other maps show a variety of thematic su bjects such as marine climatology, ocean currents and temperature, fishing areas, offshore petroleum leases, marine ecological resources, and exploration routes. Many early facsimile maps, such as Jose de Canizares' Plano del puerto de Sn. Francisco 1766, shown here, are in essence nautical charts. All rights reserved.