Annex C - Coordinate Systems

Transcription

1 This annex provides some background information on coordinate systems. The intent of this annex is to provide enough detail to allow the IMSMA FM user to correctly work with coordinates. For more detail on the topics covered here refer to the references in Annex F - Further Reading. The IMSMA FM works with geographic coordinates (Latitude and Longitude), and two global projected coordinate systems (UTM and MGRS). Geographic coordinates are always given with reference to a datum, and projected coordinates are always given with respect to a projection and a datum. Projections and datums will be described after the coordinate system descriptions. Latitude and longitude. Latitude and longitude is the most common coordinate system used for navigation. It will allow you to pinpoint your location with a high degree of accuracy. Latitude is an angular distance measured north and south of the Equator. The Equator is 0 degrees. As you go north of the equator the, latitude increases all the way up to 90 degrees at the north pole. If you go south of the equator, the latitude increases all the way up to 90 degrees at the south pole. In the northern hemisphere the latitude is always given in degrees north and in the southern hemisphere it is given in degrees south. Longitude works the same way. It is an angular distance measured east and west of the Prime Meridian. The prime meridian is 0 degrees longitude. As you go east from the prime meridian, the longitude increases to 180 degrees. As you go west from the prime meridian longitude increases to 180 degrees. The 180 degree meridian is also known as the international date line. In the eastern hem- Coordinate systems C-1

2 isphere the longitude is given in degrees east (or positive degrees) and in the western hemisphere it is given in degrees west (or negative degrees). Figure C.1 : Prime meridian and equator (Source: Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder) At the equator, one degree of latitude or longitude represents approximately 70 statute miles. At higher latitudes the distance of one degree of longitude decreases. Latitude stays the same because they are always equally spaces apart. If you look on a globe you will see this to be the case. On the other hand, if you look on a globe you will notice that the lines of longitude get closer together as they approach the north and south poles. Degrees are not accurate enough to find a precise location. At best, one degree of latitude and longitude would define a 70 square mile area. To over come this problem, 1 degree is divided into 60'(minutes). So if 1 degree equals 70 miles and one degree can be divided into 60' then 1' equals 1.2 miles. Dividing 1 degree into 60' allows one to calculate their position with much better accuracy. In some instances even more accuracy is needed. To do this we can divide 1' into 60"(seconds). If 1' equals 1.2 miles and we can divide it into 60", then 1" equals 0.02 miles. It it is worth taking a few seconds to memorize the following numbers. It will help you to use latitude and longitude more effectively: 1 degree = 70 miles or 110 km 1' = 1.2 miles or 2 km 1" =.02 miles or 30 m Universal Transverse Mercator (UTM). UTM is a global projected coordinate system. It measures positions in meters from an origin defined by the coordinate system - the origin is defined such that UTM coordinates are always positive numbers. C-2

3 The UTM system divides the surface of the earth up into a grid of zones. Each grid is identified by a number across the top called the zone number (6 degrees of longitude wide) and a letter down the right hand side called the zone designator (8 degrees of latitude wide). Use of the zone number is mandatory, the zone designator can be omitted. Figure C.2 : UTM Zones (Source: Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder) Each zone has a central meridian. Zone 14, for example, has a central meridian of 99 degrees west longitude. The zone extends from 96 to 102 degrees west longitude. The zone number is mandatory because the easting (distance in meters east of the central meridian) is measured with reference to the central meridian. Zone designators can be omitted because northings are measured from the equator. Note that: eastings are measured from the central meridian, with a m false easting to ensure positive coordinates northings are measured from the equator, with a m false northing for positions south of the equator The zone number can be calculated as follows: Negative (west) longitude: zone = Integer value of ((180 - longitude) / 6) + 1 Positive (east) longitude: zone = Integer value of ((longitude - 180) / 6) + 1 Military Grid Reference System (MGRS). MGRS is an extension of the UTM system. A UTM zone number and designator are followed by 100 km square east- C-3

4 ing and northing identifiers. The system uses a set of alphabetic characters for the 100 km grid squares. Figure C.3 : MGRS 100km designators (Source: Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder) UTM zone number, UTM zone, and the two 100 km square characters are followed by an even number of numeric characters representing easting and northing values. If 10 numeric characters are used, a precision of 1 meter is assumed 2 characters imply a precision of 10 km From 2 to 10 numeric characters the precision changes from 10 km, 1 km, 100 m 10 m, to 1 m MGRS and UTM systems are often employed in products produced by the US National Imagery and Mapping Agency ( formerly the Defense Mapping Agency. Datums Coordinate systems are always referenced to a datum. The IMSMA FM only supports the use of one datum. The default is WGS-84. If a different datum is to be used, this should be configured after installation before the IMSMA FM is put into operational use. You will want to change the datum used in the MAC from WGS-84 if you are using map data in the IMSMA GIS that is referenced to a datum other than WGS-84. ArcView cannot transform map data from one datum to another. To transform coordinates between datums you must know the parameters of the reference ellipsoid that the datum uses, and the values of the origin shift (given as a shift in x, y, and z). Various software utilities are available to perform C-4

5 datum conversions, and will be required if positions have been collected with respect to a datum other than the one in use at the MAC. This will be the case if coordinates were calculated with a GPS configured to another datum. Figure C.4 : Ellipsoid parameters (Source: Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder) Note that in the IMSMA FM only the ellipsoid is defined. The ellipsoid definition is used to transform coordinates from the geographic to UTM/MGRS systems. The user must know what datum the coordinates are referenced to, and ensure that the map data used in the IMSMA GIS is referenced to the same datum. You will need to be concerned about projections if you are using map data stored with reference to a projection other than UTM (the UTM system defines a coordinate system and a projection). If you are working with raster map data, changing the projection is a complicated procedure. If you are working with vector map data, the projection can be changed using the IMSMA GIS unproject utility. The IMSMA GIS expects all shapefiles to contain geographic coordinates. This tool allows the unprojection of shapefiles which store the coordinates in a projected coordinate system (e.g., a specific UTM zone, or any other projection supported by the utility). The tool takes the shapefile of the active theme and creates an unprojected version in geographic coordinates (decimal degrees), after the user specifies the projection. The user must know the projection system of the theme being deprojected. No datum transformation is carried out. Exactly one feature theme must be active for the tool to be enabled. Projections C-5

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