A contour line is used to show points of equal elevation on a map. The contour interval must be known, which is the difference in elevation between one line and the next. Lines drawn close together indicate steepness in slope and lines drawn further apart indicate a gradual slope. Two contour lines can never cross as each indicates a different elevation. Gradient can be determined by the change in elevation divided by distance. Contour lines can be used to show topography of an area, changes in air pressure and even temperature. A depression symbol is a contour line with little dashes along the line. It indicates that the elevation is decreasing as seen in a crater or depression.