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NMPlot User's Guide Geographic Coordinate Systems
NMPlot User's Guide, Chapter 20: Geographic Coordinate SystemsThe location of a point on the Earth's surface can be numerically described in a number of ways. Examples include:
the longitude and latitude of the point (100.5° west, 45.3° north)
the location with respect to another known point (500 feet northwest of the Empire State Building)
the Universal Transverse Mercator coordinates of the point (715,326.1 meters east, 5,037,295.1 meters north, UTM zone 15)
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.
An Album of Map Projections, U.S. Geological Survey Professional Paper 1453, by John P. Snyder and Philip M. Voxland, United States Government Printing Office, 1989.
Map Projections-A Working Manual, U.S. Geological Survey Professional Paper 1395, by John P. Snyder, United States Government Printing Office, 1987.
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.
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.
Choose Coordinate System Conversion Tool from the Tools menu. The Coordinate System Conversion Tool dialog box appears.
From Coordinate System - Select the geographic coordinate system that you are converting from. See Coordinate System Control for information on selecting a coordinate system.
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.
To Coordinate System - Select the geographic coordinate system that you are converting to. See Coordinate System Control for information on selecting a coordinate system.
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.
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.
Center longitude and latitude - Type the center longitude and latitude. To minimize distortion, the center latitude should be near the center of your area of interest.
False easting and northing - Type the false easting and northing, and select the units used to express them. Locations in this coordinate system are expressed using these units.
Datum - Select the coordinate system's datum. See Datum Control and Introduction to Datums for information on selecting datums.
The Lambert Azimuthal Equal Area coordinate system is commonly used for both small- and large-scale maps of regions that are roughly circular 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.
Center longitude and latitude - Type the center longitude and latitude. To minimize distortion, the center longitude and latitude should be near the center of your area of interest.
False easting and northing - Type the false easting and northing, and select the units used to express them. Locations in this coordinate system are expressed using these units.
Datum - Select the coordinate system's datum. See Datum Control and Introduction to Datums for information on selecting datums.
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.
Center longitude and latitude - Type the center longitude and latitude. To minimize distortion, the center latitude should be near the center of your area of interest.
Standard parallels - Type the first and second standard parallels. Typically, these will be near the southern and northern borders of your area of interest.
Important:
The standard parallels cannot be symmetrical spaced on opposite sides of the equator. For example, standard parallels of 30° south and 30° north are illegal.
False easting and northing - Type the false easting and northing, and select the units used to express them. Locations in this coordinate system are expressed using these units.
Datum - Select the coordinate system's datum. See Datum Control and Introduction to Datums for information on selecting datums.
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.
X-Y coordinates of a reference point - Specify the (X, Y) coordinates of the reference point.
Longitude and Latitude of a reference point - Specify the longitude and latitude of the reference point, in decimal degrees.
Units of the XY coordinate system axes - Select the units used to measure distances along the X-Y coordinate system axes.
Rotation of X axis in degrees counterclockwise from east - Typically, projections measure distances east and north from a reference point. However, rotated projections are also possible. Specify the direction that the X axis points, in degrees counterclockwise from east.
Datum - Select the coordinate system's datum. See Datum Control and Introduction to Datums for information on selecting datums.
As an example, assume a Local Flat-Earth XY coordinate system is defined as follows.
Reference Point, X-Y: 100, 200
Reference Point, Longitude and Latitude: 90° west, 45° north
Units of XY Axes: feet
Rotation of X Axis: 45°
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 is the most common and well-known geographic coordinate system. Locations are specified in degrees of east longitude and north latitude.
Express as - Select how the longitude and latitude should be presented. You have three choices.
Decimal Degrees (for example, 85.175000°)
Degrees, Minutes, and Seconds (for example, 85° 10' 30.00")
Degrees and Decimal Minutes (for example, 85° 10.5000' )
Note:
In some situations, the Express as choice will not be available.
Display Order - Select the order in which longitude and latitude are presented. You have two choices.
Longitude-Latitude (long: 85.175° W lat: 35.675° N)
Latitude-Longitude (lat: 35.675° N long: 85.175° W)
Note:
In some situations, the Display Order choice will not be available.
Datum - Select the coordinate system's datum. See Datum Control and Introduction to Datums for information on selecting datums.
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.
Center longitude and latitude - Type the center longitude and latitude. To minimize distortion, the center longitude and latitude should be near the center of your area of interest.
False easting and northing - Type the false easting and northing, and select the units used to express them. Locations in this coordinate system are expressed using these units.
Datum - Select the coordinate system's datum. See Datum Control and Introduction to Datums for information on selecting datums.
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.
Center longitude and latitude - Type the center longitude and latitude. To minimize distortion, the center longitude and latitude should be near the center of your area of interest.
False easting and northing - Type the false easting and northing, and select the units used to express them. Locations in this coordinate system are expressed using these units.
Datum - Select the coordinate system's datum. See Datum Control and Introduction to Datums for information on selecting datums.
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.
Always use zone - If you check this box, select the UTM zone number that will always be assumed. If you do not check this box, you will be asked for the zone whenever you enter coordinates.
Note:
In some situations, you will be required to specify the UTM zone number.
False easting - Select the false easting. Typically, this is 500,000, but in rare situations, a different value may be used. Unless you know otherwise, use the default of 500,000.
False northing - Select the false northing. This is usually 0 in the northern hemisphere. In the southern hemisphere, 10,000,000 is often used. Unless you know otherwise, use the recommended default of 0 or 10,000,000, depending on your hemisphere.
Datum - Select the coordinate system's datum. See Datum Control and Introduction to Datums for information on selecting datums.
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