Maps from space

Science World, April 8, 2002

What's the height of the ocean surface? And how level is it? Scientists can determine the actual elevation within 4.5 centimeters (1.8 inches) thanks to satellites carrying radar altimeters. An altimeter works by bouncing microwave (invisible light) beams off the ocean surface and receiving them back at the spacecraft. The amount of time it takes the signal to return determines the distance between the spacecraft and ocean surface--the farther the distance, the longer it takes for the pulse to return. The data is then compared to a reference mathematical model of Earth's ellipsoidal (circular at all planes) shape. Scientists also consider variations in the data caused by natural effects such as atmospheric conditions, currents, and tides.

[ILLUSTRATION OMITTED]

The following exercise uses data collected from TOPEX/Poseidon, an earlier mission, to study the relationship between ocean-floor topography and ocean-surface topography.

1. FIGURE A is a map of the North Atlantic Ocean. The curve represents the path of TOPEX/Poseidon as it orbits above. The satellite is measuring a 1,800 km (1,119 mi)-long segment between latitudes 26 [degrees] N and 40 [degrees] N. Mark an "X" at the beginning and end of this segment on the map.

2. FIGURE B is filled with the satellite's collected data on sea-surface height. The data is in meters above or below the reference ellipsoidal shape of Earth's sea surface. Where is the highest and lowest sea surface located in this segment of the North Atlantic Ocean?

3. Convert the data from FIGURE B into a line graph in the space provided in FIGURE C.

ANSWERS

Diagram-Reading/Chart-Reading Skills: Maps From Space

2. Lowest: 26 [degrees] N, Highest: 36 [degrees] N

4. Lowest: near 26 [degrees] N, Highest: near 36 [degrees] N

5. Research shows that without factors such as seasonal changes, tides, or currents, the topography of the sea surface generally mimics the shape of the underlying ocean topography. This activity shows that features like sea-floor ridges might result in variations of sea-surface height.