What are index contour lines?

What are index contour lines?

• A. Thin dotted lines representing minor features.
• B. Heavy lines that mark index contour lines and show elevation values.
• C. Color-coded areas for vegetation.
• D. Ridge lines only.

Answer: (B)

Index contour lines are the bold, heavier contour lines on a topographic map that carry an elevation value printed on them, usually every fifth line. They provide a quick reference to altitude across the map, so you can read elevation at a glance without needing to read every single contour line. The heavier line weight and the numeric label differentiate index contours from the lighter, unlabeled contours that fill in the rest of the terrain detail. This setup lets you quickly gauge how high you are and compare different areas, which is essential for estimating slope and planning routes. Other features like thin dotted lines indicate minor features, color-coded areas show vegetation or land cover, and ridge lines describe terrain shapes themselves but do not provide the labeled elevation reference that index contours do.

Index contour lines are the bold, heavier contour lines on a topographic map that carry an elevation value printed on them, usually every fifth line. They provide a quick reference to altitude across the map, so you can read elevation at a glance without needing to read every single contour line. The heavier line weight and the numeric label differentiate index contours from the lighter, unlabeled contours that fill in the rest of the terrain detail. This setup lets you quickly gauge how high you are and compare different areas, which is essential for estimating slope and planning routes. Other features like thin dotted lines indicate minor features, color-coded areas show vegetation or land cover, and ridge lines describe terrain shapes themselves but do not provide the labeled elevation reference that index contours do.