Location services  including GPSbased navigation systems and map sites such as Google Maps and Yahoo! Maps  have become popular among consumers. Many organizations are already making use of locationaware services, and many more will do so as they realize the benefits and potential that these services hold. In 2006, Gartner noted that "locationaware applications will hit mainstream adoption in the next two to five years" and that already an "increasing number of organizations have deployed locationaware mobile business applications." (See Resources for a link to this report.)
When an organization decides to implement a locationaware application, it is eventually a developer's task to write such an application. Building a locationaware service involves many tasks, big and small, and one (relatively small) task may be to convert coordinates from one system to another. This article presents code that performs such conversions, which could save you many hours of work.
Two different coordinate systems
Before we dive into this article's code, we need to discuss the coordinate systems that the code is designed to handle: the familiar system of longitude and latitude and the Universal Transverse Mercator (UTM) system. We'll also touch on the Military Grid Reference System (MGRS), which is based on UTM.
Latitude and longitude
The latitude and longitude system is probably the best known way to designate geographic coordinates. It presents a location as two numbers. Latitude is the angle from the center of the Earth to some eastwest line on the Earth's surface. Longitude is the angle from the center of the Earth to a northsouth line on the Earth's surface. Latitude and longitude may be expressed as decimal degrees (DD) or as degrees, minutes, and seconds (DMS); the latter gives numbers in a format such as 49Â°30'00" S 12Â°30'00" E. This is the format typically used in GPS devices.
Earth is divided by the equator (0Â° latitude) into Northern and Southern Hemispheres and by 0Â° longitude (an imaginary line from the North Pole to the South Pole that goes through the city of Greenwich in the UK) into Eastern and Western Hemispheres. The Northern Hemisphere has latitudes between 0 and 90 degrees, and the Southern Hemisphere has latitudes between 0 and 90 degrees. The Eastern Hemisphere is between 0 and 180 degrees, and the Western Hemisphere between 0 and 180 degrees.
For example, the coordinates 61.44, 25.40 (in DD) or 61Â°26'24''N, 25Â°23'60''E (in DMS) are located in southern Finland. The coordinates 47.04, 73.48 (in DD) or 47Â°02'24''S, 73Â°28'48''W (in DMS) are located in southern Chile. Figure 1 shows the Earth overlaid with latitude and longitude lines:
Figure 1. The Earth, with latitude and longitude lines displayed
See Resources for more detailed information.
Universal Transverse Mercator
The UTM coordinate system is a gridbased method for specifying coordinates. The UTM system divides the Earth into 60 zones, each based on the Transverse Mercator projection. Map projection in cartography is a way to present a twodimensional curved surface on a plane, such as a normal map. Figure 2 shows a Traverse Mercator projection:
Figure 2. A Transverse Mercator projection
The UTM longitude zones are numbered 1 through 60; all but two zones (more on which in a moment) are 6Â° wide from to east to west. The longitude zones cover the whole surface of the Earth between latitudes 80Â°S and 84Â°N.
There are 20 UTM latitude zones, each 8Â° high; the zones are lettered from C to X (with letters I and O omitted). Zones A, B, Y, and Z exist outside of this system; they cover the Antarctic and Arctic regions. Figure 3 shows UTM zones in Europe. The two nonstandard longitude zones are visible in Figure 3: zone 32V is extended to cover all of southern Norway, while zone 31V is shrunk so that it covers only open water.
Figure 3. UTM zones in Europe
UTM coordinates are presented in the format longitude zone latitude zone easting northing, where easting is the projected distance from longitude zone's central meridian and northing is the projected distance from the equator. The values of both easting and northing are given in meters. For example, the latitude/longitude coordinates 61.44, 25.40 are presented in UTM as 35 V 414668 6812844; the latitude/longitude coordinates 47.04, 73.48 are 18 G 615471 4789269 in UTM.
See Resources for more information about UTM and Traverse Mercator projection.
Military Grid Reference System
The MGRS is the standard used by NATO militaries. MGRS is based on UTM and further divides each zone to 100 km by 100 km squares. These squares are identified by twoletter digraphs: the first letter is the eastwest position within the longitude zone and the second letter is the northsouth position.
For example, the UTM point 35 V 414668 6812844 is equivalent to the MGRS point 35VMJ1466812844. This MGRS point is accurate within one meter and is presented using 15 characters, where the last 10 characters are the easting and northing values within the specified grid. MGRS may be presented using 15 characters (as in previous example), or 13, 11, 9, or 7 characters; the values so expressed would be accurate within 1, 10, 100, 1,000, or 10,000 meters, respectively.
See Resources for more information.
Converting coordinates
To define latitude and longitude coordinates for a location on the Earth, at minimum, you must be able to see the stars or the Sun and have a sextant and clock that displays time in GMT. You can determine latitude from the angle between a celestial object and the horizon, and you can calculate longitude from the Earth's rotation. This article does not go into these details (see Resources if you want to know more). Instead, we'll assume that we already have coordinates in DD, DMS, or UTM format.
Converting decimal degrees to degrees/minutes/seconds, and vice versa
It is easy to convert coordinates between DD and DMS. Here's the formula for converting from DD to DMS:
DD: dd.ff DMS: dd mm ss dd=dd mm.gg=60*ff ss=60*gg
Here, gg
is the fractional part of the calculation. Negative latitude denotes a location in the
Southern Hemisphere (S) and negative longitude is a location in the Western Hemisphere (W).
For example, imagine that you have the coordinates (in DD format) of 61.44, 25.40. You'd convert them as follows:
lat dd=61 lat mm.gg=60*0.44=26.4 lat ss=60*0.4=24
And:
lon dd=25 lon mm.gg=60*0.40=24.0 lon ss=60*0.0=0
Thus, in DMS format, the coordinates are 61Â°26'24''N 25Â°24'00''E.
The formula to go from DMS to DD is as follows:
DD: dd.ff DMS: dd mm ss dd.ff=dd + mm/60 + ss/3600
Remember, locations in the Southern Hemisphere (S) are at negative latitudes, and locations in the Western Hemisphere (W) are at negative longitudes.
Let's convert the DMS coordinates 47Â°02'24''S, 73Â°28'48''W to DD notation:
lat dd.ff=  (47 + 2/60 + 24/3600 )=47.04 lon dd.ff=  (73 + 28/60 + 48/3600)=73.48
Thus, the coordinates are 47.04, 73.48 in DD.
Converting from latitude/longitude to UTM and vice versa
Unlike decimal coordinates, which you can determine using a sextant and a chronometer, you cannot determine UTM coordinates without calculations. Although these calculations are nothing more than basic trigonometry and algebra, the formulas are very complicated. If you look at "The Universal Grids: Universal Transverse Mercator (UTM) and Universal Polar Stereographic (UPS)" (see Resources for a link), you'll see what I mean.
The UTM conversion formulas are not presented here, but the source code in the following section provides some light, and Resources has links to provide more information.
Converting coordinates with Java code
This section introduces the source code of a library class that performs coordinate conversion
between decimal degrees and UTM. This Java class is named com.ibm.util.CoordinateConversion
; my idea was to make a single class that has methods for conversions. The class includes inner classes that
actually do the conversion; if necessary, the inner classes can be refactored out from the CoordinateConversion
class to create a library package or add classes to existing package. The conversions that
the class performs are accurate to within 1 meter.
The source for CoordinateConversion
has about 750 lines of code, so it is not presented in this text of this article in its entirety. The relevant methods
are described in the following sections, and the full source code is included in the Download section.
CoordinateConversion
CoordinateConversion
is the main class and it is instantiated to perform coordinate conversion when required. Listing 1 presents the
relevant public methods, along with private inner classes that are included in the CoordinateConversion
class:
Listing 1. CoordinateConversion
public class CoordinateConversion { public CoordinateConversion() { } public double[] utm2LatLon(String UTM) { UTM2LatLon c = new UTM2LatLon(); return c.convertUTMToLatLong(UTM); } public String latLon2UTM(double latitude, double longitude) { LatLon2UTM c = new LatLon2UTM(); return c.convertLatLonToUTM(latitude, longitude); } //..implementation omitted private class LatLon2UTM { public String convertLatLonToUTM(double latitude, double longitude) { //..implementation omitted } //..implementation omitted } private class LatLon2MGRUTM extends LatLon2UTM { public String convertLatLonToMGRUTM(double latitude, double longitude) { //..implementation omitted } //..implementation omitted } private class MGRUTM2LatLon extends UTM2LatLon { public double[] convertMGRUTMToLatLong(String mgrutm) { //..implementation omitted } //..implementation omitted } private class UTM2LatLon { public double[] convertUTMToLatLong(String UTM) { //..implementation omitted } //..implementation omitted } private class Digraphs { //used to get digraphs when doing conversion between //lat/long and MGRS //..implementation omitted } private class LatZones { //include methods to determine latitude zones //..implementation omitted }
The next section looks at the latitude/longitude and UTM conversions in more detail.
Conversion from latitude/longitude to UTM
Coordinates are converted from latitude/longitude to UTM with the String latLon2UTM(double latitude, double longitude)
method. The method implementation creates a new instance of the inner class LatLon2UTM c = new LatLon2UTM();
and returns
UTM coordinates as a 15character string (that is, to 1meter precision). The
implementation of the LatLon2UTM
methods is shown in Listing 2:
Listing 2. public String convertLatLonToUTM(double latitude, double longitude)
public String convertLatLonToUTM(double latitude, double longitude) { validate(latitude, longitude); String UTM = ""; setVariables(latitude, longitude); String longZone = getLongZone(longitude); LatZones latZones = new LatZones(); String latZone = latZones.getLatZone(latitude); double _easting = getEasting(); double _northing = getNorthing(latitude); UTM = longZone + " " + latZone + " " + ((int) _easting) + " "+ ((int) _northing); return UTM; }
This method performs its conversion by calling various methods to get the latitude and longitude
zone and calculating the easting and northing and so on. Input is validated using the validate()
method; if the clause (latitude < 90.0  latitude > 90.0  longitude < 180.0  longitude >= 180.0)
is true,
it throws an IllegalArgumentException
.
The
setVariables()
method in Listing 3 sets various variables
required for calculating conversions (take a look at "The Universal Grids" for more information; see Resources for a link):
Listing 3. protected void setVariables(double latitude, double longitude)
protected void setVariables(double latitude, double longitude) { latitude = degreeToRadian(latitude); rho = equatorialRadius * (1  e * e) / POW(1  POW(e * SIN(latitude), 2), 3 / 2.0); nu = equatorialRadius / POW(1  POW(e * SIN(latitude), 2), (1 / 2.0)); double var1; if (longitude < 0.0) { var1 = ((int) ((180 + longitude) / 6.0)) + 1; } else { var1 = ((int) (longitude / 6)) + 31; } double var2 = (6 * var1)  183; double var3 = longitude  var2; p = var3 * 3600 / 10000; S = A0 * latitude  B0 * SIN(2 * latitude) + C0 * SIN(4 * latitude)  D0 * SIN(6 * latitude) + E0 * SIN(8 * latitude); K1 = S * k0; K2 = nu * SIN(latitude) * COS(latitude) * POW(sin1, 2) * k0 * (100000000) / 2; K3 = ((POW(sin1, 4) * nu * SIN(latitude) * Math.pow(COS(latitude), 3)) / 24) * (5  POW(TAN(latitude), 2) + 9 * e1sq * POW(COS(latitude), 2) + 4 * POW(e1sq, 2) * POW(COS(latitude), 4)) * k0 * (10000000000000000L); K4 = nu * COS(latitude) * sin1 * k0 * 10000; K5 = POW(sin1 * COS(latitude), 3) * (nu / 6) * (1  POW(TAN(latitude), 2) + e1sq * POW(COS(latitude), 2)) * k0 * 1000000000000L; A6 = (POW(p * sin1, 6) * nu * SIN(latitude) * POW(COS(latitude), 5) / 720) * (61  58 * POW(TAN(latitude), 2) + POW(TAN(latitude), 4) + 270 * e1sq * POW(COS(latitude), 2)  330 * e1sq * POW(SIN(latitude), 2)) * k0 * (1E+24); }
The
getLongZone()
method in Listing 4 and
the LatZones
class (available in the source code)
are used to get longitude and latitude zones. The longitude zone is calculated from the longitude parameter, and
latitude zones are basically hard coded using an array in the LatZones
class.
Listing 4. protected String getLongZone(double longitude)
protected String getLongZone(double longitude) { double longZone = 0; if (longitude < 0.0) { longZone = ((180.0 + longitude) / 6) + 1; } else { longZone = (longitude / 6) + 31; } String val = String.valueOf((int) longZone); if (val.length() == 1) { val = "0" + val; } return val; }
The
getNorthing()
(in Listing 5) and getEasting()
(in Listing 6) methods
calculate the northing and easting values. Both methods use variables that were set in the
setVariables()
method in Listing 3.
Listing 5. protected double getNorthing(double latitude)
protected double getNorthing(double latitude) { double northing = K1 + K2 * p * p + K3 * POW(p, 4); if (latitude < 0.0) { northing = 10000000 + northing; } return northing; }
Listing 6. protected double getEasting()
protected double getEasting() { return 500000 + (K4 * p + K5 * POW(p, 3)); }
Listing 7 includes some sample output, including some latitude/longitude coordinates and the corresponding UTM coordinates:
Listing 7. Latitude/longitudetoUTM test values
( 0.0000 0.0000 ) "31 N 166021 0" ( 0.1300 0.2324 ) "30 N 808084 14385" (45.6456 23.3545 ) "34 G 683473 4942631" (12.7650 33.8765 ) "25 L 404859 8588690" (80.5434 170.6540) "02 C 506346 1057742" ( 90.0000 177.0000) "60 Z 500000 9997964" (90.0000 177.0000) "01 A 500000 2035" ( 90.0000 3.0000 ) "31 Z 500000 9997964" ( 23.4578 135.4545) "08 Q 453580 2594272" ( 77.3450 156.9876) "57 X 450793 8586116" (89.3454 48.9306 ) "22 A 502639 75072"
Conversion from UTM to latitude/longitude
Converting from UTM coordinates to latitude and longitude is a little easier than the reverse process. Again, "The Universal Grids" (see Resources) includes the formulas for the conversions. Listing 8 shows the code for the convertUTMToLatLong()
method. This
returns a double array, where the first element (array index [0]
) is latitude and second element (array index [1]
) is longitude.
Because the UTM string parameter has an accuracy of 1 meter, the latitude/longitude coordinates have the same accuracy.
Listing 8. public double[] convertUTMToLatLong(String UTM)
public double[] convertUTMToLatLong(String UTM) { double[] latlon = { 0.0, 0.0 }; String[] utm = UTM.split(" "); zone = Integer.parseInt(utm[0]); String latZone = utm[1]; easting = Double.parseDouble(utm[2]); northing = Double.parseDouble(utm[3]); String hemisphere = getHemisphere(latZone); double latitude = 0.0; double longitude = 0.0; if (hemisphere.equals("S")) { northing = 10000000  northing; } setVariables(); latitude = 180 * (phi1  fact1 * (fact2 + fact3 + fact4)) / Math.PI; if (zone > 0) { zoneCM = 6 * zone  183.0; } else { zoneCM = 3.0; } longitude = zoneCM  _a3; if (hemisphere.equals("S")) { latitude = latitude; } latlon[0] = latitude; latlon[1] = longitude; return latlon; }
The
convertUTMToLatLong()
method splits the incoming UTM string (which is in the format
34 G 683473 4942631) and uses the getHemisphere()
method to determine the hemisphere that the location denoted by the string is in. Determining the hemisphere is easy: latitude zones A
, C
, D
, E
, F
, G
, H
, J
, K
, L
, and M
are
in the Southern Hemisphere, and the rest are in the Northern Hemisphere.
The
setVariables()
method, shown in Listing 9, sets the variables required for calculation and then immediately calculates latitude. Longitude is calculated using the longitude zone.
Listing 9. protected void setVariables()
protected void setVariables() { arc = northing / k0; mu = arc / (a * (1  POW(e, 2) / 4.0  3 * POW(e, 4) / 64.0  5 * POW(e, 6) / 256.0)); ei = (1  POW((1  e * e), (1 / 2.0))) / (1 + POW((1  e * e), (1 / 2.0))); ca = 3 * ei / 2  27 * POW(ei, 3) / 32.0; cb = 21 * POW(ei, 2) / 16  55 * POW(ei, 4) / 32; cc = 151 * POW(ei, 3) / 96; cd = 1097 * POW(ei, 4) / 512; phi1 = mu + ca * SIN(2 * mu) + cb * SIN(4 * mu) + cc * SIN(6 * mu) + cd * SIN(8 * mu); n0 = a / POW((1  POW((e * SIN(phi1)), 2)), (1 / 2.0)); r0 = a * (1  e * e) / POW((1  POW((e * SIN(phi1)), 2)), (3 / 2.0)); fact1 = n0 * TAN(phi1) / r0; _a1 = 500000  easting; dd0 = _a1 / (n0 * k0); fact2 = dd0 * dd0 / 2; t0 = POW(TAN(phi1), 2); Q0 = e1sq * POW(COS(phi1), 2); fact3 = (5 + 3 * t0 + 10 * Q0  4 * Q0 * Q0  9 * e1sq) * POW(dd0, 4) / 24; fact4 = (61 + 90 * t0 + 298 * Q0 + 45 * t0 * t0  252 * e1sq  3 * Q0 * Q0) * POW(dd0, 6) / 720; lof1 = _a1 / (n0 * k0); lof2 = (1 + 2 * t0 + Q0) * POW(dd0, 3) / 6.0; lof3 = (5  2 * Q0 + 28 * t0  3 * POW(Q0, 2) + 8 * e1sq + 24 * POW(t0, 2)) * POW(dd0, 5) / 120; _a2 = (lof1  lof2 + lof3) / COS(phi1); _a3 = _a2 * 180 / Math.PI; }
setVariables()
uses the easting and northing values to set required variables. These are both class variables and are set in the convertUTMToLatLong(String UTM)
method (see Listing 8).
Other methods
The source code also includes other public and private methods and classes. For example, methods and classes to convert coordinates between latitude/longitude and some helper methods to perform degreetoradian conversions (and vice versa) and for various mathematical operations (such as POW, SIN, COS, and TAN).
Summary
This article introduced a little bit of the theory of world coordinate systems, along with a Java class to perform coordinate transformation. Although not all of the coordinate transformation formulas were presented in detail here, they are available in the Resources section. Typically, this theory is not needed in everyday development work  except in rare cases where there is no good way to do otherwise, as I recently discovered when I encountered a coordinate transformation task.
I needed to perform conversion between latitude and longitude, UTM, and MGRS, and so I did basic research and implemented the transformations
in a Java class. For me, the development work took several hours; I hope that you will save those hours for
other tasks and that you find CoordinateConversion
useful in your own work.
Download
Description  Name  Size 

Coordinate conversion source code^{1}  jcoordconvert.zip  4KB 
Note
 The download also includes MGRS methods, but conversion from MGRS is incorrect.
Resources
Learn
 "Gartner's 2006 emerging technologies hype cycle highlights key technology themes": This press release from Gartner contains some interesting information on the prevalence of locationbased services.
 "The Universal Grids: Universal Transverse Mercator (UTM) and Universal Polar Stereographic (UPS)": This National Imagery and Mapping Agency (NIMA) publication includes UTM formulas as well as other details about UTM. (This document is a PDF.)
 "Locationbased services" (Valerie Bennett and Andrew Capella, developerWorks, March 2002): Get an introduction to the concepts behind locationbased services.
 "Put yourself on the map with Google Maps API, DB2/Informix, and PHP on Linux" (Marty Lurie and Aron Y. Lurie, developerWorks, March 2006): This article includes code that can convert a ZIP code into latitude and longitude coordinates.
 "Creating an automatically maintained spatial table from latitudelongitude column data" (Robert Uleman, developerWorks, April 2006): Learn how DB2 can help you turn raw, coordinate data into useful information.
 Wikipedia is a great place to start learning about several important concepts explored here, including:
 Browse the technology bookstore for books on these and other technical topics.
 developerWorks Java technology zone: Hundreds of articles about every aspect of Java programming.
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