## Visualizing GPS Satellites

Figure 2.16 GPS receivers detect signals from the 27 GPS satellites orbiting Earth. Using signals from at least three satellites, the receiver can calculate location within 10 m.

First, a GPS receiver, located in New York City, receives a signal from one satellite. The distance from the satellite to the receiver is calculated. Suppose the distance is 20,000 km. This limits the possible location of the receiver to anywhere on a sphere 20,000 km from the satellite.

Next, the receiver measures the distance to a second satellite. Suppose this distance is calculated to be 21,000 km away. The location of the receiver has to be somewhere on the area where the two spheres intersect, shown here in yellow.

Finally, the distance to a third satellite is calculated. Using this information, the location of the receiver can be narrowed even further. By adding a third sphere, the location can be calculated to be one of two points as shown. Often one of these points can be rejected as an improbable or impossible location.

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Figure 2.17 GIS mapping involves layering one map on top of another. In this image, you can see how one layer builds on the next.

Figure 2.17 GIS mapping involves layering one map on top of another. In this image, you can see how one layer builds on the next.

GIS maps might contain many layers of information compiled from several different types of maps, such as a geologic map and a topographic map. As shown in Figure 2.17, layers such as rivers, topography, roads, and landforms from the same geographic area can be placed on top of each other to create a comprehensive map.

One major difference between GIS mapping and traditional mapping is that a GIS map can be updated as new information is loaded into the database. Once a map is created, the layers are still linked to the original information. If this information changes, the GIS layers also change. The result is a map that is always up-to-date—a valuable resource for people who rely on current information.