Spatial Thinking in Planning Practice: An Introduction to GIS

19 A Cartesian coordinate system is created for each zone by establishing an origin some distance (usually 2,000,000 feet) to the west of the zone’s central meridian and some distance to the south of the zone’s southernmost point. "is ensures that all coordinates within the zone will be positive. "e X-axis running through this origin runs east-west, and the Y-axis runs north-south. Distances from the origin are generally measured in feet, but some- times are in meters. X distances are typically called eastings (because they measure distances east of the origin) and Y distances are typically called northings (because they measure distances north of the origin). Figure 2.8. Visual depiction of the State Plane project system as displayed over the United States. http://gis. depaul.edu/shwang/teaching/geog258/Grid_!les/image002.jpg DATUMS All coordinate systems are tied to a datum. A datum de!nes the starting point from which coordinates are mea- sured. Latitude and longitude coordinates, for example, are determined by their distance from the equator and the prime meridian that runs through Greenwich, England. But where exactly is the equator? And where exactly is the Prime Meridian? And how does the irregular shape of the Earth !gure into our measurements? All of these issues are de!ned by the datum. Many di#erent datums exist, but in the United States only three datums are commonly used. "e North Ameri- can Datum of 1927 (NAD27) uses a starting point at a base station in Meades Ranch, Kansas and the Clarke El- lipsoid to calculate the shape of the Earth. "anks to the advent of satellites, a better model later became available and resulted in the development of the North American Datum of 1983 (NAD83). Depending on one’s location, coordinates obtained using NAD83 could be hundreds of meters away from coordinates obtained using NAD27. A third datum, the World Geodetic System of 1984 (WGS84) is identical to NAD83 for most practical purposes within the United States. "e di#erences are only important when an extremely high degree of precision is need- ed. WGS84 is the default datum setting for almost all GPS devices. But most USGS topographic maps published up to 2009 use NAD27. "is chapter material has been collected from the following web links that holds information with CC copy- rights: use and share alike. http://giscommons.org/earth-and-map-preprocessing/ Chapter 2: Coordinate Systems and Projecting GIS Data