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Coordinate Reference System

The Coordinate Reference System can be divided into projected coordinate reference systems (also called Cartesian or rectangular coordinate reference systems) and geographic coordinate reference systems.

The Geographic Coordinate Reference System

The geographic coordinate system is one of the most common coordinate systems in use. It uses degrees of latitude and longitude to describe a location on the earth’s surface.

Lines of latitude run parallel to the equator and divide the earth into 180 equal portions from north to south (or south to north). The reference latitude is the equator and each hemisphere is divided into ninety equal portions, each representing one degree of latitude.In the northern hemisphere degrees of latitude are measured from zero at the equator to ninety at the north pole. In the southern hemisphere degrees of latitude are measured from zero at the equator to ninety degrees at the south pole. To simplify the digitization of maps, degrees of latitude in the southern hemisphere are often assigned negative values (0 to -90°). Wherever you are on the earth’s surface, the distance between lines of latitude is the same (60 nautical miles,), so they conform to the uniform grid criterion assigned to a useful grid system.

Lines of longitude, however, do not stand up so well to the standard
of uniformity. Lines of longitude run perpendicular to the equator and converge at the poles. The reference line for longitude (the prime meridian) runs from the North pole to the South pole through Greenwich, England. Subsequent lines of longitude are measured from zero to 180 degrees East or West of the prime meridian. Note that values West of the prime meridian are assigned negative values for use in digital mapping applications.

Only at the equator,does the distance represented by one line of
longitude equal to the distance represented by one degree of latitude. As you
move towards the poles, the distance between lines of longitude becomes
progressively less, until, at the exact location of the pole, all 360° of longitude
are represented by a single point that you could put your finger on (you
probably would want to wear gloves though). Using the geographic coordinate
system, we have a grid of lines dividing the earth into squares that cover
approximately 12363.365 square kilometers at the equator.

To be truly useful, a map grid must be divided into small enough sections so
that they can be used to describe the location of a point on the map within an acceptable level of accuracy. To accomplish this, degrees are divided into
minutes (') and seconds ("). There are sixty minutes in a degree, and sixty
seconds in a minute (3600 seconds in a degree). So, at the equator, one
second of latitude or longitude = 30.87624 meters.

 

Projected Coordinate Reference System

A projected coordinate system is a flat, two-dimensional representation of the Earth. It is based on a sphere or spheroid geographic coordinate system, but it uses linear units of measure for coordinates, so that calculations of distance and area are easily done in terms of those same units.

The latitude and longitude coordinates are converted to x, y coordinates on the flat projection. The x coordinate is usually the eastward direction of a point, and the y coordinate is usually the northward direction of a point.The center line that runs east and west is referred to as the x axis, and the center line that runs north and south is referred to as the y axis.

The intersection of the x and y axes is the origin and usually has a coordinate of (0,0). The values above the x axis are positive, and the values below the x axis are negative. The lines parallel to the x axis are equidistant from each other. The values to the right of the y axis are positive, and the values to the left of the y axis are negative. The lines parallel to the y axis are equidistant.

In a three-dimensional coordinate reference system, another axis, normally labeled Z, is added. It is also at right angles to the X and Y axes. The Z axis provides the third dimension of space.

Examples of commonly used projected coordinate reference system are the Universal Transverse Mercator(UTM) and  WGS84 commonly seen on most maps.

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