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NMPlot User's Guide, Chapter 20: Geographic Coordinate Systems

The location of a point on the Earth's surface can be numerically described in a number of ways. Examples include:

Each of these methods of describing a location is known as a geographic coordinate system. When working with maps, the term projection is also commonly used. While there are technical differences, for most practical purposes, you can consider the terms "coordinate system" and "projection" as synonymous.

NMPlot allows you to use a number of common coordinate systems when working with geographic data. The supported coordinate systems are:

For additional information about coordinate systems, the following references are recommended.

Coordinate System Control

A Coordinate System Control is used to select a geographic coordinate system.

The current coordinate system is displayed in the box. To change it, either press the Space Bar, or press the Select Coordinate System button . The Select Coordinate System dialog box is displayed.

Use the drop-down list at the top of the outlined area to select the coordinate system. Then supply values for the selected system's parameters.

Some coordinate systems have numerous parameters, which can be tedious to enter. Therefore, NMPlot allows you to save coordinate systems to files for later reuse. Press the Save To File button to save the current coordinate system to a file. Press the Load From File button load a previously saved coordinate system.

Coordinate System Conversion Tool

The coordinate system conversion tool allows you to convert a point's coordinates from one geographic coordinate system to another. To use the tool, follow these steps.

  1. Choose Coordinate System Conversion Tool from the Tools menu. The Coordinate System Conversion Tool dialog box appears.

  2. From Coordinate System - Select the geographic coordinate system that you are converting from. See Coordinate System Control for information on selecting a coordinate system.

  3. From Point - Specify the geographic coordinates of the point in the coordinate system you are converting from. The controls available for entering the point will vary, depending on the From coordinate system.

  4. To Coordinate System - Select the geographic coordinate system that you are converting to. See Coordinate System Control for information on selecting a coordinate system.

  5. To Point - The geographic coordinates of the point, expressed in the To coordinate system, are displayed. Press the "Copy To Point to Clipboard" button to put this text onto the clipboard.

Tip:

You can use the coordinate system conversion tool to convert between datums. Choose "Longitude and Latitude" for both the From and To coordinate systems. Set the datums of the From and To coordinate systems to your From and To datums, respectively.

Tip:

You can use the coordinate system conversion tool to convert a longitude and latitude from decimal degrees to degrees, minutes, and seconds, or vice versa. Make sure that you use the same datum for both the From and To coordinate systems.

Albers Conical Equal Area

Description

The Albers Conical Equal Area coordinate system is equal-area, meaning that the areas of all regions are shown in the same proportion to their true areas. It is commonly used for maps of the contiguous United States, and is recommended for equal-area maps that are predominantly east-west in extent.

Parameters

Azimuthal Equidistant

Description

The Azimuthal Equidistant coordinate system is used to visualize the distance and direction of various features as seen from a given point of interest (denoted by the center longitude and latitude).

The United Nations' emblem is based upon an azimuthal equidistant map with a center latitude of 90 degrees north.

Parameters

Equirectangular

Description

The Equirectangular coordinate system is essentially a direct scaling of longitudes and latitudes to distances east and north, with the scaling selected such that distortion is minimized along the central latitude parallel. Distortion increases with distance from the central latitude: this increase becomes quite rapid near the poles.

The Equirectangular coordinate system is most useful for maps of regions that are predominantly east-west in extent. The Equirectangular coordinate system is often used to map bands that encircle the Earth and are enclosed by two fairly close parallels of latitude: for example, the region between latitudes 20° north and 30° north. It is also used in situations where ease of mapping is paramount.

This coordinate system is also know by the names Equidistant Cylindrical, Rectangular, and La Carte Parallelogrammatique.

Parameters

Lambert Azimuthal Equal Area

Description

The Lambert Azimuthal Equal Area coordinate system is commonly used for large-scale maps of regions that are predominantly east-west in extent. This is an equal-area coordinate system, meaning that the areas of all regions are shown in the same proportion to their true areas.

Parameters

Lambert Conformal Conic

Description

The Lambert Conformal Conic coordinate system is commonly used for large-scale maps of regions that are predominantly east-west in extent.

This coordinate system is also know by the name Conical Orthomorphic.

Parameters

Local Flat-Earth XY

Description

In the Local Flat-Earth XY coordinate system, locations are specified using a local Cartesian coordinate system: for example, meters east and north of a reference point. To specify this coordinate system, you must know both the X-Y and Longitude-Latitude coordinates of a reference point.

To minimize distortion, you should select a reference point near the center of your area of interest.

The Local Flat-Earth XY coordinate system is good general-purpose projection for maps of fairly small regions (tens of miles across) that have roughly the same east-west and north-south extent.

Technically, this projection is based upon a conical projection developed by the US Federal Aviation Administration for use by their Integrated Noise Model (INM). It is documented in an appendix of the INM User's Guide.

Parameters

As an example, assume a Local Flat-Earth XY coordinate system is defined as follows.

In this example coordinate system, the point (110, 200) would be located 10 feet northeast of longitude 90° west, latitude 45° north.

Longitude and Latitude

Description

Longitude and Latitude is the most common and well-known geographic coordinate system. Locations are specified in degrees of east longitude and north latitude.

Parameters

Orthographic

Description

The Orthographic coordinate system is most often used to display an entire hemisphere. Such a map will appear similar to the Earth as seen from a spacecraft located directly above the center longitude and latitude.

When the center latitude is 90 south or 90 north, this projection is also used to display the polar regions of the Earth.

Parameters

Stereographic

Description

The Stereographic coordinate system is commonly used for both small- and large-scale maps (showing one hemisphere or less) of regions that are roughly circular in extent.

When the center latitude is 90 south or 90 north, this projection is also used to display the polar regions of the Earth.

Parameters

Universal Transverse Mercator (UTM)

Description

Universal Transverse Mercator, or UTM, is a common coordinate system that specifies locations in meters east and north from a reference point.

The UTM system divides the globe into 60 zones, each 6 degrees of longitude wide and stretching from 80 degrees south latitude to 80 degrees north latitude.

UTM is suitable for mapping regions that are contained entirely within a single zone and its two adjacent zones. Beyond this, distortion increases rapidly.

Parameters

Web Mercator

Description

The Web Mercator coordinate system is based upon the projection used by most web-based mapping services, including Google Maps, Bing Maps, MapQuest, and OpenStreetMap.

It is similar to the Mercator projection, but assumes a spherical earth for computational efficiency. As such, it has been criticized by some members of the cartography community as poorly conceived. However, there is no doubt as to its utility as a projection which a) is efficient, and b) can represent the entire globe (apart from the poles) as a seamless whole.

When displaying maps of the entire earth, Web Mercator does distort distances and areas, making the polar regions appear too large relative to areas near the equator. The distortion is minimal, however, when displaying maps of country-sized and smaller areas in the low- and mid-latitudes.

Web Mercator is used when overlaying data upon a raster map created by one of the web-based mapping services.

Parameters

While the false easting, false northing, and datum can be modified, it would be unusual that you would need to do so. Typically, Web Mercator is used with the default parameters of (0, 0) for the false easting and northing, and WGS 84 for the datum. These are the parameters used by all of the web-based mapping services.


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