Final answer:
The thicker contour lines that are labeled and spaced by intervals of five contour lines on topographic maps are called index contours, representing elevations and helping to visualize the terrain.
Step-by-step explanation:
Thicker contour lines that are labeled and are typically spaced apart by intervals of five contour lines are called index contours. These lines are an essential feature on topographic maps as they represent the elevations on the Earth's surface, providing a sense of the terrain's shape and slope. When those index contours are labeled with their corresponding elevation values, they help map readers to quickly ascertain the elevation of different areas on the map, aiding in understanding the topographical layout of the mapped area. The spacing of the contour lines indicates the slope of the terrain; closely spaced lines depict a steep slope, while widely spaced lines indicate a more gradual slope.
Index contours play a crucial role on topographic maps, featuring thicker lines and labeled elevations at intervals of five contour lines. These lines are integral for conveying the elevation variations across the Earth's surface, offering insights into the topography's form and gradient. The labeled elevation values on index contours serve as reference points, enabling map readers to swiftly gauge the height of different locations on the map. This aids in comprehending the topographical features of the mapped area. Additionally, the spacing between contour lines is indicative of the terrain's slope—closely spaced lines denote steep slopes, while widely spaced lines signify more gradual inclines. In essence, index contours enhance the interpretability of topographic maps, facilitating a comprehensive understanding of the landscape's elevation and configuration.