UTM - Universal Transverse Mercator Geographic Coordinate System The idea of the transverse mercator projection has its roots in the 18th century, but it did not come into common usage until after World War II.  It has become the most used because it allows precise measurements in meters to within 1 meter. A mercator projection is a ‘pseudocylindrical’ conformal projection (it preserves shape). What you often see on poster-size maps of the world is an equatorial mercator projection that has relatively little distortion along the equator, but quite a bit of distortion toward the poles. What a transverse mercator projection does, in effect, is orient the ‘equator’ north-south (through the poles), thus providing a north-south oriented swath of little distortion. By changing slightly the orientation of the cylinder onto which the map is projected, successive swaths of relatively undistorted regions can be created.  This is exactly what the UTM system does. Each of these swaths is called a UTM zone and is six degrees of longitude wide. The first zone begins at the International Date Line (180°, using the geographic coordinate system). The zones are numbered from west to east, so zone 2 begins at 174°W and extends to 168°W. The last zone (zone 60) begins at 174°E and extends to the International Date Line. The zones are then further subdivided into an eastern and western half by drawing a line, representing a transverse mercator projection, down the middle of the zone. This line is known as the ‘central meridian’ and is the only line within the zone that can be drawn between the poles and be perpendicular to the equator (in other words, it is the new ‘equator’ for the projection and suffers the least amount of distortion).  For this reason, vertical grid lines in the UTM system are oriented parallel to the central meridian.  The central meridian is also used in setting up the origin for the grid system. Any point can then be described by its distance east of the origin (its ‘easting’ value).  By definition the Central Meridian is assigned a false easting of 500,000 meters. Any easting value greater than 500,000 meters indicates a point east of the central meridian. Any easting value less than 500,000 meters indicates a point west of the central meridian. Distances (and locations) in the UTM system are measured in meters, and each UTM zone has its own origin for east-west measurements. To eliminate the necessity for using negative numbers to describe a location, the east-west origin is placed 500,000 meters west of the central meridian. This is referred to as the zone’s ‘false origin’.  The zone doesn't extend all the way to the false origin. The origin for north-south values depends on whether you are in the northern or southern hemisphere.  In the northern hemisphere, the origin is the equator and all distances north (or ‘northings’) are measured from the equator.  In the southern hemisphere the origin is the south pole and all northings are measured from there.  Once again, having separate origins for the northern and southern hemispheres eliminates the need for any negative values.  The average circumference of the earth is 40,030,173 meters, meaning that there are 10,007,543 meters of northing in each hemisphere.

http://www.tpub.com/content/USMC/mcr3161a/css/mcr3161a_60.htm

 FAQ: WHAT IS UTM?
(4 July 1998)

UTM, Universal Transverse Mercator, is a system of world coordinates like latitude and longitude, from 80 degrees south latitude to 84 degrees north latitude, except the measurements are in meters and UTM lines are orthogonal (always at right-angles to each other).  This projection makes all latitude and longitude lines (except the central meridians) curved.

Unlike the regular Mercator, "Transverse Mercator" is a projection with the cylinders rotated 90 Deg (to east-west) and touch the earth "universally" at 60 "central meridian" longitudes. Coordinates are "UTM northing" and "UTM easting."

UTM Northing is the distance north from the equator in meters and Easting is the distance east from 60 central meridians of 6-degree-wide zones starting at longitude 180 degrees.

UTM Northing is divided into 8-degree-high zones from 80 Deg. south latitude to 84 Deg. north latitude using the letters C thru X with the equator at M/N.  A,B and Y,Z are reserved for the UPS
coordinate system at the poles. For a map of UTM zones see (HERE).

All UTM coordinates are POSITIVE by incorporating a system of "false" easting and northing.  Adding an arbitrary 500,000m to the cerntal meridians and 10,000,000m to the distance south of the
equator in the southern hemisphere keeps all readings positive.

The UTM map scale increases to either side of the central meridan, so the scale is reduced to 0.9996.  This way there will be two meridians to either side of the central meridian where the
scale will be correct which improves the average map scale accuracy.

The original definition of the meter was 1/10,000,000th the distance from the equator to the north pole (which is no longer exact.)  Due to the oblate shape of the earth, UTM (WGS-84 datum)
northing for 45 Deg. N latitude is 4,986,272m instead of 5,000,000m. For UTM (NAD-27 datum) the northing value is 4,986,055m.

This 217m difference is related to the UTM northing errors of approximately 200m experienced by posters here on the NG with some DRG maps.  This is due the program, GPS, or user selecting the wrong datum.

NGA Coordinate Systems Analysis Team (CSAT)

August 2005

GRID COORDINATES

·        Universal Transverse Mercator (UTM)

·        Military Grid Reference System (MGRS)

·        Universal Polar Stereographic (UPS)

·        United States National Grid (USNG)

A simplified definition and explanation of UTM and related systems

THE UTM SYSTEM

UTM coordinates are based on a family of 120 Transverse Mercator map projections (two for each UTM zone, with one for each N/S hemisphere).

·        The earth is divided into 60 zones, each 6° wide in longitude (with the exception of a few non-standard-width zones for Svalbard and southwest Norway).  See Figure 1.

Figure 1. 

·        Numbering of zones begins at 180° and proceeds eastward. 

o       Zone 1 is from 180°W to 174°W,

o       Zone 2 is from 174°W to 168°W, and so on.

·        Each zone has a central meridian.

o       Zone 1 central meridian is 177°W,

o       Zone 2 central meridian is 171°W, and so on.

·        The X value, called the Easting, has a value of 500,000m at the central meridian of each zone (Figure 2).

·        The Y value, called the Northing, has a value of 0m at the equator for the northern hemisphere (Figure 3), 10,000,000m at the equator for the southern hemisphere.

             Figure 2.  Easting values.            Figure 3.  Northing values.

·        UTM is limited to the area between 84°N and 80°S.  Beyond that, Universal Polar Stereographic (UPS) coordinates are used.  See section on UPS.

REFERENCING / EXPRESSING A POSITION IN UTM COORDINATES

·        In the UTM system, positions are expressed as Easting / Northing, e.g. “580817mE, 4251205mN”.  In some cases, the letters are left off, e.g. “580817 4251205”.

·        If positions occur near UTM zone junctions, the UTM zone may also be specified, e.g. “580817mE, 4251205mN, Zone 15”.

·        Since the above expresses two possible positions on the earth, the hemisphere may also be specified, e.g. “580817mE, 4251205mN, Zone 15, Northern Hemisphere” (Figure 4).

Figure 4.   Example of position expressed in both Lat/Long and UTM coordinates.

·        Many systems abbreviate the above, representing the hemisphere as a single letter, “N” for northern hemisphere, and “S” for southern hemisphere, e.g. “15N 580817 4251205”.

·        CAUTION: IN MANY SYSTEMS, THE LETTER AFTER THE ZONE NUMBER DESIGNATES A LATITUDINAL BAND, NOT A HEMISPHERE.  MORE ON THIS BELOW.

MGRS

The Military Grid Reference System (MGRS) is an alpha-numeric system for expressing UTM / UPS coordinates.  A single alpha-numeric value references a position that is unique for the entire earth.  The components of MGRS values are as follows:

(Example: 15S WC 80817 51205)

·        The first two characters represent the 6° wide UTM zone.

o       Leading zeroes are included so that Zone 9 is “09”.

o       For polar areas outside the UTM area, these characters are omitted.

·        The third character is a letter designating a band of latitude.

o       Beginning at 80°S and proceeding northward, the 20 bands are lettered C through X, omitting I and O.

o       The bands are all 8° high except band X, which is 12° high.

o       Outside the UTM area, A and B are used near the south pole, Y and Z near the north pole.

·        The vertical UTM boundaries and horizontal latitudinal band boundaries form (generally) 6° X 8° Grid Zones.  Hence, the first three letters of the MGRS value, e.g. “15S”, are referred to as the Grid Zone Designation (GZD).

·        The fourth and fifth characters are a pair of letters designating one of the 100,000-meter grid squares within the grid zone (or UPS area).  See Figure 6.

Figure 6. 

In this sample area, the Grid Zone Designators are shown in brown.  The smaller gray letters are the100,000-meter grid square designators.  The example point “15S WC 80817 51205” is located in square “WC” near the center of the figure.

·        The remaining characters consist of the numeric Easting and Northing values within the 100,000-meter grid square (Figure 7).

·        MGRS coordinates may be truncated to reflect lower-order precision.  For example:

15S WC 80817 51205

implies one-meter precision.  It may be truncated to express 100-meter precision:

15S WC 808 512

·        There are two lettering schemes for the 100,000-meter grid square designators.  Generally, one scheme is used for WGS-84, and the other is used for older ellipsoids associated with local datums.  Example:

15S WC 80817 51205

is on WGS-84.  When converted to NAD-27 datum, Clarke 1866 ellipsoid, its value is:

15S WN 80833 50993

The 100,000-meter grid square “WC” for WGS-84 generally coincides with the grid square “WN” for Clarke 1866.

See the section on USNG for further guidance.

Figure 7.

Magenta arrows show how MGRS easting and northing values are determined from within the 100,000-meter grid square.  The MGRS value of this position is 15S WC 80817 51205.

 THE LETTER AFTER THE UTM ZONE NUMBER: IS THAT A HEMISPHERE OR A LATITUDINAL BAND?

Since the creation of UTM, developers have interpreted the rules for expressing an earth-wide unique UTM position in one of two ways:

1.    By including an “N” or “S” after the zone number to specify a hemisphere.

2.    By including the 8° latitudinal band designator (see the section on MGRS) after the zone number.

Example: The position at 92°W, 38°N, expressed in UTM coordinates, is:

587798m E, 4206287m N, Zone 15.

This reference is valid for two positions on the earth.  In order to make it unique for only one position worldwide, i.e. 92°W, 38°N:

·        Developer #1 includes an “N” to specify northern hemisphere:

15N 587798 4206287

·        Developer #2 includes the 8° latitudinal band designator:

15S 587798 4206287

This situation is causing confusion among users and developers.  The two 8° latitudinal bands, from 0° to 8°N and from 32°N to 40°N, are assigned the designations “N” and “S”, respectively.  These designations are often mistaken for hemisphere designations.

Technical Manual 8358.1 contains the authoritative definition of UTM.  NGA will soon publish an updated version of TM 8358.1, which will provide clarification on this issue.

THE UNIVERSAL POLAR STEREOGRAPHIC (UPS) SYSTEM

UPS coordinates are based on a family of two Polar Stereographic map projections, one for each pole.

·        The origin of the UPS coordinate system is the pole (north or south), where X=2,000,000m and Y=2,000,000m.

·        The X-axis lies along the meridians 90°E and 90°W.

o       Moving from the pole (north or south), X-values (Eastings) increase along the 90°E meridian.

·        The Y-axis lies along the meridians 0° and 180°.

o       Moving from the north pole, Y-values (Northings) increase along the 180° meridian.

o       Moving from the south pole, Y-values (Northings) increase along the 0° meridian.

THE UNITED STATES NATIONAL GRID (USNG)

USNG is functionally equivalent to MGRS.  The difference between the two systems is in the method for specifying the datum.  In MGRS, an alternate lettering scheme is used for the 100,000-meter grid square designator when the position is referenced to an older datum (see section on MGRS).  The USNG does not use the alternate lettering scheme, but simply specifies the datum after the position reference.  For example, a position on the NAD 27 datum is reported in the two systems as follows:

MGRS: “15S WN 80833 50993”

USNG: “15S WC 80833 50993 (NAD 27)”

links

NGA http://earth-info.nga.mil/GandG/

USMC - Intergrated Publishing http://www.tpub.com/content/USMC/mcr3161a/css/mcr3161a_33.htm