That is, we think of the map as a piece of paper wrapped around a globe with lines emanating from points on the globe and "projected" onto the paper. Map transformations are referred to as projections because they are commonly visualized as geometric constructs. Coordinates in the map plane ( x, y) are defined in the usual manner with x horizontal and increasing to the right and y vertical and increasing upward. There is only one definition of longitude-the angle between a ray from the center of the Earth to the point and the plane of the Greenwich Meridian (an arbitrary reference point). It's more common to use geodetic latitude. On a sphere, the geodetic latitude is equivalent to the geocentric latitude. Geocentric latitude is the angle between a ray from the center of the Earth to the point and the plane of the equator. Geodetic latitude is the angle between a vertical ray at the point and the plane of the equator. When mapping smaller areas it is not uncommon to choose an ellipsoid specifically for each area each area is mapped on an ellipsoid with a different radius and eccentricity.Ĭoordinates for points on the surface of the Earth are given in latitude and longitude. For maps of large areas (continents or bigger) using the sphere introduces no significant error. The ellipsoid is flattened at the poles, relative to the sphere, by about 1 part in 300. When defining map projections then, it is common to approximate the surface of the earth with a simpler surface such as an ellipsoid or sphere. The surface of constant geopotential referred to as mean sea level or the geoid is quite complicated and its mathematical form is correspondingly complex. The derivation of a map transformation requires a mathematical representation of the surface of the Earth. The curved surface is usually the surface of the Earth and the plane is what we call a map. Concepts MapsĪ map projection is a procedure or mathematical formula to transform a curved surface onto a plane. Along with comments in the source code, it also describes the format of the map and grid parameter files. The purpose of this document is to give a brief introduction to mapping, gridding, and the associated terminology used in the Mapx library at NSIDC. The location is also constant, which makes it easy to compare data from different sensors or different time periods. Grids are an efficient means of storing data because the location of a value within the grid is implicit-it is not explicitly stored in the grid. Satellite remote sensing has made possible the collection of data over large areas of the Earth.
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