Measuring Earth: Maps, Coordinates & Topographic Profiles

Latitude and longitude, contour lines, gradient, and how to read a topographic map and build an elevation profile.

9 minNYS 2AEarth Science

Earth's shape and size

Earth is an oblate spheroid — very nearly a perfect sphere, but bulging slightly at the equator and flattened at the poles. The evidence is subtle: the force of gravity is a touch stronger at the poles, and Polaris sits slightly differently than a perfect sphere predicts. For nearly every Regents question you can treat Earth as a sphere.

Latitude and longitude

Any point on Earth has two coordinates:

Latitude — distance north or south of the equator, 0° to 90°. The equator is 0°; the poles are 90° N and 90° S. Lines of latitude (parallels) run east-west.
Longitude — distance east or west of the Prime Meridian, 0° to 180°. Lines of longitude (meridians) run north-south and meet at the poles.

A handy trick: in the Northern Hemisphere, the altitude of Polaris (the North Star) above the horizon equals your latitude. Stand at the equator and Polaris sits on the horizon (0°); stand at the North Pole and it's directly overhead (90°).

Quick check #1

On a topographic map, four points lie between the same two contour lines, but the contour lines are bunched tightly together at point C. What does that tell you about point C?

Contour lines

A topographic (contour) map shows elevation using contour lines — each line connects all points at the same elevation. Reading them:

The contour interval is the elevation change between neighboring lines.
Closely spaced contours mean a steep slope; widely spaced contours mean a gentle slope.
Closed loops are hills; the highest elevation is inside the innermost loop.
Hachured contours (with little tick marks) show a depression.
Where a contour crosses a stream it bends into a V that points upstream (uphill), because the stream sits in the low point of the valley.

Gradient

Gradient measures how quickly a field value (like elevation) changes over distance:

Gradient = change in field value ÷ distance

If a hillside rises from 200 m to 400 m over 4 km, the gradient is (400 − 200) ÷ 4 = 50 m/km. The same formula works for temperature or air-pressure fields — it's just "how steep is the change."

Building a profile

A topographic profile is a side-view of the land along a line. To build one, note the elevation wherever the line crosses a contour, plot those elevations against distance, and connect the points with a smooth curve. Hills become bumps; valleys become dips.

Quick check #2

A river valley shows contour lines bending into a V. Which way does the V point relative to the river's flow?