Friday, July 27, 2007

ESRI's help pages: "An overview of map projections"

From ESRI's help pages: "An overview of map projections" ( Also of particular interest - How ArcGIS decides on the coordinate system to apply when a data-set without a spatial reference is added : Also remember A Geographic Coordinate System (GCS) has a 1. Spheroid (also called an ellipsoid) - that approximates the shape of the earth 2. A datum - that defines the placement of the spheroid relative to the earth. (there are 2 types of datums - GeoCentric and Local and these differ in the placement of the datum - Local places it such that the ellipsoid moves as closely to a local area, whereas geocentric places close to the center of the Earth). A Projected Coordinate System (PCS) has 1. A Geographic Coordinate System (which in turn has a spheroid and a datum) 2. Projection parameters - which defines how the GCS coordinates are transformed from and to the projected coordinate system. Although people use Spatial Reference and Coordinate Systems interchangeably, typically when one talks about a Spatial Reference it not only includes a coordinate system, but also * The precision with which coordinates are stored * and the spatial extent covered by the dataset So, to come to one of the comments in one of my previous posts and a common mistake (that even I made) is to use the term - geographic projection, which is in-correct. Usually the correct term to use in this case is "Geographic Coordinate System" - coordinate systems can either be geographic or projected (not both!). Though you do have the term "Geographic Transformation"! - which simply is a datum transformation and allows you to move between one datum and another.

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