GPS Explained

Our home here on earth is big. Really big. It's nearly impossible for our minds to grasp the true vastness of oceans, forests, deserts, mountains and valleys - over 510 million square kilometers! Fortunately, through the ingenuity and scientific rigor of a number of great minds, we can now determine the location of any point on the earth's surface using only two numbers called latitude and longitude.

Diagram showing latitude and longitude

Lines of Latitude

Earth has an axis, which is an imaginary line drawn through its centre about which it rotates. The two points at which this imaginary line intersects with the globe are known as the earth's poles, of which we refer to one as the "north" and one as the "south. Lines of latitude are circles drawn around the earth at a 90 degree angle to its axis, such that the circles of the smallest circumference are at the poles, and the one with the largest being equidistant between them. This largest circle is known as the equator, and it divides the planet into two halves, known as our northern and southern hemispheres. On most common maps (north/south being up/down and west/east being left/right, respectively), latitude is the measure of vertical distance between any two points.

Lines of longitude

Longitude offers us the second dimension we need to locate any position on the earth. Longitude is just latitude flipped on its side 90 degrees, such that we end up with a set of circles surrounding the planet that each intersect both the north and south poles. Longitude divides the earth into its eastern and western hemispheres and is the horizontal distance on most common maps.

Historically, it was not nearly as easy as it is today to determine those latitude and longitude coordinates but that’s where GPS comes in. GPS stands for Global Positioning System which is a series of satellites placed in orbit around our planet. These satellites transmit low power radio signals which GPS receivers can decode and use to determine their location. It takes at least three satellite signals to determine your GPS location (latitude and longitude) but most receivers can detect many more then that depending on where you are. With four or more signals a GPS receiver can also tell you your altitude.

Due to the role it played in developing the early methods for determining longitude, the Royal Observatory's location in Greenwich, London was chosen as the 0 degree line of longitude, or Prime Meridian. From this point, lines of longitude vary up to +180 degrees eastward and up to -180 degrees westward. Similarly, lines of latitude are defined from +90 degrees north of and -90 degrees south of the equator.

 

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