9. ANCILLARY DATASETS

The ocean bottom topography is also very significant in oceanography. The continental slopes and shelves that adjoin the continents form barriers for oceanic currents. Some eastward flowing ocean currents develop into strong western boundary currents (e.g., the Gulf Stream, and Kuroshio Current) along the continental slopes. Mid-ocean ridges form the largest relief features on earth and strongly influence deep water circulation and the distribution of deep ocean water properties. At shallow levels there are sills and constrictions that isolate deeper water in fjords and enclosed seas (e.g., the Mediterranean Sea, Gulf of Mexico, and Sea of Japan). Topographic features like these greatly control the water exchanged with the open ocean and determine many characteristics of the enclosed sea. In some cases, the water that spills over the sills is traceable over much of the open ocean. For example, the Straits of Gibraltar are very shallow (the Gibraltar Sill) and restrict the flow of the very saline waters of the Mediterranean Sea into the Atlantic. The saline water is more dense and would normally flow out along the bottom, but the sill retards this flow to the extent that when the water can finally spill over the sill, it is traceable over much of the Atlantic Ocean because of its relatively high temperature and salinity.

Topographic datasets are available in a range of grid resolutions. Some are coarse (e.g., 2.5 degree) while others are of high resolution (e.g., 30 seconds). Users should be aware that those with coarser resolutions generally represent a spatial average while the high resolution grids represent point values. In some cases, however, ``high-resolution'' datasets are actually interpolated from those with lower resolutions. The highest resolution datasets are generally available for specific regions of the globe (e.g., North America) because of the large volume of data.

Land-surface datasets contain much information. Some examples include: vegetation classifications and indexes, major world ecosystems, biomass, hydrological information and soil characteristics. These datasets are difficult to evaluate and each has strengths and weaknesses. Some are derived from atlas data and, given the rapidly changing world in which we live, may not be applicable on a local basis. Some may have detailed soil types while others use very broad definitions.