What is an isoline used to represent on a map?

An isoline is used to represent lines of equal value on a map, which include measures such as elevation, temperature, pressure, or other continuous data. When isolines are drawn, they connect points that have the same value, helping to visualize gradients and patterns across a geographic area. For example, contour lines on a topographic map represent elevation, allowing users to understand the terrain's shape and relief.

The other options involve different representations. Populated areas are typically indicated using various symbols or shading rather than isolines. Air quality measurements may be represented using thematic maps or gradients but are not limited to isolines alone. Transportation routes are depicted using lines denoting roads, railways, or pathways, which do not convey equal values like isolines do. Hence, the correct understanding of isolines relates specifically to their function in mapping equal values, making the second choice the only appropriate answer.