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ENVI User's Guide: ENVI Map Projections |
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Use map projections to represent all or part of the three-dimensional surface of the earth in two-dimensions. Distortion always occurs when projecting a spherical surface onto a planar map and different projections cause different map characteristics to be distorted. You must choose the projection that shows the characteristics important to your goals accurately at the expense of other characteristics, which will be distorted. Map projections can have properties such as equal area, conformal, equidistant, or true azimuth (direction) characteristics. On an equal area map projection, circles of a fixed diameter drawn on any part of the map will encompass the same geographic area. This projection is useful for comparing land areas. However, the shapes, angles, and scales of equal area maps may be distorted. On a conformal map projection (e.g., UTM), the local angles are correct and the local scale in every direction is constant showing the true shape correctly. This projection is useful for measuring distance and direction between relatively near points. An equidistant map projection has a true accurate scale between one or two points and every other point on the map. Reference lines are called standard parallels or standard meridians. True direction map projections show the correct directions or azimuths among all points on the map. Many map projections use a compromise between these characteristics to reduce distortion yet provide accurate measurements for local areas. Several map projections are designed for specific uses, i.e., air and sea navigation, satellite mapping, etc.
Map projections are typically projected onto one of three types of surface: cylinder, cone, or plane. These surfaces wrap around or intersect with the globe and then are cut and laid flat to produce a map. Cylindrical projections are made by wrapping a cylinder around the globe and projecting the surface onto the cylinder. Often, the cylinder touches the globe at the equator so the meridians of longitude are projected as equidistant straight lines perpendicular to the equator and the parallels of latitude are projected parallel to the equator (mathematically spaced). The Mercator projection is an example of a cylindrical projection. A conical projection is made by placing a cone over the globe. Often, the apex of the cone is along the polar axis of the globe and the cone touches the globe at a parallel of latitude. In this case, the meridians are projected onto the cone as equidistant straight lines and the parallels are lines around the circumference of the cone that are circular when the map is laid flat. Planar or azimuthal map projections are produced when a plane is placed tangent to the globe's surface. In a polar azimuthal projection, the plane is tangent to one of the globe's poles so the meridians are projected as straight lines radiating from the pole and the parallels are circles centered on the pole. Modifications are often made to these types of projections and there are other projections that are not constructed in these ways.
You can add or modify map projections in ENVI. You can define your own parameters for the supported ENVI projection types and select the desired ellipsoid or datum (see Building Customized Map Projections). You can also define new projection types by providing the formula used to translate latitude and longitude coordinates into the new projection coordinates (see "ENVI_CONVERT_PROJECTION_COORDINATES" in the ENVI Reference Guide).
The map projections available in ENVI are listed in Table D-1. The ENVI projection number is used when creating map information structures when programming in ENVI.
The position of a point on a globe is often represented in spherical coordinates by degrees of latitude and longitude. The parallels of latitude run east-west and are formed by 90 equally spaced circles around the globe from the equator to each pole (north latitudes are positive and south latitudes are negative). The circle at the equator is at 0 degrees and the numbers increase to the north and south poles which are at 90 degrees each. The meridians of longitude are defined by north-south lines passing through each pole that intersect the equator at 360 equally spaced intervals. The meridian that passes through Greenwich, England is defined as 0 degrees and is called the prime meridian. Degrees of longitude are defined between 0 degrees and 180 degrees east (positive) or west (negative) of the prime meridian. Map projections are used to represent the latitude and longitude lines on a plan map.
The position of a point on a map is often represented in Cartesian (x, y) rectangular coordinates. The x-axis coordinates typically increase to the east and the y-axis coordinates increase to the north. The x and y coordinates are often called eastings and northings and the origin may be defined with a false easting and false northing. These coordinate grids are often divided into zones to reduce distortion. The Universal Transverse Mercator (UTM) and State Plane projections are examples of these type of coordinate systems.
The shape of the Earth is often represented by an oblate ellipsoid, which is an ellipse that is rotated about its shorter axis. The ellipsoids are described by two parameters, the semi-major and semi-minor axes. Reference ellipsoids are used to represent the shape of the Earth and many are based on surface measurements to give a regional best fit and not an entire Earth best fit. Therefore, different ellipsoids are often used for different regions of the Earth.
You can add an ellipsoid to ENVI by adding it to the ellipse.txt file in the map_proj directory of the ENVI distribution. To add an ellipsoid, enter the name, semi-major axis (a) in meters, and semi-minor axis (b) in meters to this file.
The ellipsoids available in ENVI are listed in Table D-2.
A datum is a smooth, mathematical surface that closely fits the mean sea level surface throughout the area of interest. It is created when an ellipsoid model is fixed to a base point on the Earth. Since the ellipsoid models are approximate, as you move away from the fixed point you get larger errors. Therefore, different datums exist for different regional areas to reduce error. Because different datums are defined by fixing an ellipsoid to different base points, changing datums changes the latitude and longitude of a point on the surface of the Earth. Therefore, it is necessary to know which datum is used when defining the coordinates of your points.
ENVI supports many datums which are listed in the datum.txt file in the map_proj directory of the ENVI distribution. Users can add a datum to ENVI by adding it to the datum.txt file. To add a datum, enter the name, associated ellipsoid name (must be in ENVI's ellipse.txt file), and the Molodensky x, y, and z shift values in meters from the WGS-84 datum. See the web pages listed under ENVI Map Projections for more information.
ENVI Online Help (August 12, 2005)