Bathymetry is the study of the underwater terrain of lakes or ocean floors. It is the same as topography, except that it looks at the underwater topography. It is a field of oceanography. The name comes from Greek ("bathus"), meaning "deep", and ("metron"), meaning "measure". Bathymetric (or hydrographic) charts are often made to support the safety of underwater navigation and exploration. Most of these underwater topographic maps use contour lines (called "depth contours" or "isobaths") to show the terrain of the sea floor, and how deep it is. They usually also give surface navigational information.