This circle has to be centred over the centre of the Earth. Note the following:. Why the fuss about Great Circles? The answer is that for any two points on the surface of the Earth, the shortest distance between those points is always along a Great Circle. When you look at a map a Great Circle is usually not a straight line. In , Canadian Sir Sanford Fleming, proposed a system of worldwide time zones based on lines of longitude.
To the east time progressively became later in a day and to the west time progressively became earlier in a day. Enter your Keywords. Earth's Coordinate System. Breadcrumb Home Fundamentals of Mapping. Latitude The first convention to be developed was latitude.
These observations also developed the conventions that the sun: rises in the east and sets in the west is in the south during a European winter and in the north in a European summer. A line of latitude is also known as a parallel. A line of longitude is called a meridian. Lines of longitude are not parallel, but converge toward the poles.
Thus, while a degree of longitude at the equator is equal to a distance of about kilometers, that distance decreases to zero at the poles.
How well do you understand the geographic coordinate system, really? Our experience is that while everyone who enters this class has heard of latitude and longitude, only about half can point to the location on a map that is specified by a pair of geographic coordinates. The websites linked below let you test your knowledge. We have discussed the fact that both latitude and longitude are measured in degrees, but what about when we need a finer granularity measurement?
To record geographic coordinates, we can further divide degrees into minutes, and seconds. The degree is equal to sixty minutes, and each minute equal to sixty seconds. Geographic coordinates often need to be converted in order to geo-register one data layer onto another. Geographic coordinates may be expressed in decimal degrees , or in degrees, minutes, and seconds. Sometimes, you need to convert from one form to another.
The minus sign is used in the decimal degree format only to indicate that the value is a west longitude or a south latitude. In this example, the minus sign indicates South, so keep track of that. Multiply the remainder by 60 seconds. Round off to the nearest second in this case. If the starting point had been the Longitude of Registered Penn State students should return now take the self-assessment quiz about Geographic Coordinates.
You may take practice quizzes as many times as you wish. They are not scored and do not affect your grade in any way. So far, you have read about Cartesian Coordinate Systems, but that is not the only kind of 2D coordinate system. A plane coordinate system can be thought of as the juxtaposition of any two measurement scales. In other words, if you were to place two rulers at right angles, such that the "0" marks of the rulers aligned, you would define a plane coordinate system.
The rulers are called "axes. A position defined by the coordinates 1,1 is located one unit to the right, and one unit up from the origin 0,0. The Universal Transverse Mercator UTM grid is a widely-used type of geographic plane coordinate system in which positions are specified as eastings distances, in meters, east of an origin and northings distances north of the origin. Some coordinate transformations are simple.
The transformation from non-georeferenced plane coordinates to non-georeferenced polar coordinates, described in further detail later in the chapter, shown below involves nothing more than the replacement of one kind of coordinates with another. The geographic coordinate system grid of latitudes and longitudes consists of two curved measurement scales to fit the nearly-spherical shape of the Earth. As discussed above, geographic coordinates can be specified in degrees, minutes, and seconds of arc.
Curved grids are inconvenient to use for plotting positions on flat maps. Furthermore, calculating distances, directions, and areas with spherical coordinates is cumbersome in comparison to doing so with plane coordinates. For these reasons, cartographers and military officials in Europe and the U. UTM grids are now standard not only on printed topographic maps but also for the geographic referencing of the digital data that comprise the emerging U.
The act of mathematically transforming geographic spherical coordinates to plane coordinates necessarily displaces most but not all of the transformed coordinates to some extent. Because of this, map scale varies within projected plane UTM coordinate system grids. Thus, UTM coordinates provide locations specifications that are precise, but have known amounts of positional error depending on where the place is.
Shown below is the southwest corner of a ,scale for which 1 inch on the map represents ft. A PCS is necessary to draw the data on a flat map. To learn more, see Geographic vs Projected Coordinate Systems.
A GCS is the full definition of how to tie coordinate values to real locations on the earth. The datum includes a spheroid, which is defined by its semimajor axis, semiminor axis, and inverse flattening values. The projection is the mathematical algorithm that defines how to present the round earth on a flat map. A PCS is the full definition of how a specific round earth model is projected onto a flat map. In addition to a projection, a PCS includes a geographic coordinate system which defines the earth model , a unit often meters , and a set of parameter values which vary depending on the projection false easting, central meridian, standard parallel, and so on.
These can be used to center the PCS on different parts of the world. As its name implies, a PCS is a coordinate system. These are both means of identifying coordinate systems, so you can be sure that you are using exactly the same parameters as someone else.
Save a projection file. The Define Projection tool is used mainly when data has an unknown coordinate system. In the geographic coordinate system, the sphere is divided into equal parts usually called degrees; some countries use grads.
Each degree is subdivided into 60 minutes, with each minute composed of 60 seconds. The geographic coordinate system consists of latitude and longitude lines. Each line of longitude runs north—south and measures the number of degrees east or west of the prime meridian.
Lines of latitude run east—west and measure the number of degrees north or south of the equator.
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