Topographic Maps

A topographic map shows the shape of the land using contour lines of equal elevation. The contour interval is the elevation difference between neighboring lines. Every fifth line is usually darker and labeled with its elevation, and that one is called an index contour. Closely spaced contours mean a steep slope, and widely spaced contours mean a gentle one. A closed loop is a hilltop. A depression is marked with small tick marks called hachures, which point into the hole.

A few rules make every contour map readable. Contour lines never cross, because a single point cannot sit at two elevations at once. They close on themselves into loops, though a loop may run off the edge of the map before it closes. Every fifth line is usually drawn heavier and labeled with its elevation. Those are index contours, and you count the thinner lines between them using the contour interval. Loops inside loops mean a hill, with the smallest, highest loop at the top. Short tick marks pointing inward, called hachures, mark a depression such as a crater or a sinkhole, where the land goes down instead of up.

The most tested contour rule involves water. Where a contour line crosses a stream, it bends into a V. The V always points upstream, toward higher elevation, because the stream sits in a valley cut below the land beside it. That means you can read flow direction straight off the map. Water flows the opposite way from the direction the Vs point. As a check, streams always flow from higher contour values toward lower ones, which is downhill.

The reason is simple once you see it. A stream cuts a valley, so the land dips down toward the stream. A contour line following that dip has to reach upstream to stay at its own elevation, then come back. That traces a V pointing toward higher ground, which is upstream. Read a whole drainage this way and you can tell which way every stream flows without a single elevation label, just from the direction the Vs point.

Gradient puts a number on steepness. It is the change in a field value divided by the distance over which it changes. Written out, gradient equals change in field value over distance. If elevation drops 300 meters over 6 kilometers, the gradient is 50 meters per kilometer.

Gradient works for any field, not just elevation. A steep temperature gradient on a weather map means temperature changes fast over a short distance, which usually marks a front. On a contour map, high gradient and closely spaced contours are the same fact stated two ways.

Set how much the elevation changes and the distance it changes over. The lab computes the gradient from that formula and describes the terrain. Watch what happens when the same elevation change is squeezed into a shorter distance.

A profile is a side view of the land along a line drawn across a contour map. It is the shape you would see if you sliced the landscape like a cake. To build one, mark where each contour crosses the line. Transfer those elevations to a graph, then connect the points with a smooth curve.

Reading profiles and maps together is the payoff skill of this unit. The map view and the side view are two pictures of the same surface, and the Regents loves asking you to translate between them. Steep on the map, where contours crowd, is steep on the profile. A closed loop on the map is a bump on the profile.

A topographic map shows the elevation field using contour lines. They are equal-elevation isolines drawn at a fixed contour interval, with darker labeled index contours to keep you oriented. Crowded contours mean steep slopes. Closed loops mean hilltops. Hachured loops mean depressions. Contour Vs point upstream at every river crossing, so streams flow against the Vs and toward lower values. Gradient turns steepness into a number, using the change in elevation divided by the distance. A profile turns the map into the side view you would see standing on the ground. Learn these and every other field map in the course, including weather, pollution, and gravity, reads the same way.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.