## Disclaimer:

This is a simple application that I wrote on a lark in 1997, just to learn a little java and the math involved in doing the conversion. I am not a good source of answers to more general questions about map projections, nor is this applet intended to be a serious tool, even though it has become the most popular page on the Cibola Search and Rescue web site.

## Looking for NAD27 to NAD83 conversion?

Please go to the National Geodetic Survey's NADCON site. This is the definitive software that does the conversion. There is both downloadable and interactive web-based software on that site.

## Looking for software to convert GIS data from one coordinate system to another?

Please go look at the Geospatial Data Abstraction Layer (GDAL) free software.

## Looking for general purpose utilities to convert geographic coordinates to projected coordinate systems?

Please go to Cartographic Projections Library for some excellent software that does everything you want.

## Looking for serious documentation of map projections?

Please go check out the USGS Publications Warehouse. You can find a huge number of papers on map projections, especially if you use their advanced query engine to search for articles by John Parr Snyder (I have found that doing a full search on the name doesn't work well, just use the last name and look among the 148 articles for the ones by J.P. or John Parr Snyder). I especially recommend Map Projections: A Working Manual, Map Projections used for large-scale quadrangles by the U.S. Geological Survey, and An Album of Map Projections

There are many other places to go for detailed discussions of map projections, GPS and geodetic datum. One is The Geographer's Craft. Another is Hunter College's Map Projection Home Page. If you are interested in general purpose conversion between arbitrary coordinate systems and datums, please look into the Cartographic Projections Library.

## The source code for this applet is not available for download

Please don't write asking for copies of the source code to this applet. It is the only Java I have ever written and I'm too ashamed of it to share it with anyone. It's a toy, let's leave it at that.

## About the UTM-Lat/Lon Converter

This converter applet will convert a single pair of UTM coordinates to latitude/longitude using formulas and constants for the North American Datum of 1927 (NAD27, Clarke Ellipsoid of 1866). If that's all you're looking for, you can skip all the text from here forward and just go straight to the applet. If you don't know what the North American Datum of 1927 is or whether it applies to you, you should probably read on.## About the Universal Transverse Mercator coordinate system

The Universal Transverse Mercator (UTM) system is widely used for locating positions on the USGS maps. What is this system, and how does it differ from the Latitude/Longitude system?The Lat/Lon system is a spherical coordinate system involving meridians of longitude -- great circles intersecting at the poles -- and parallels of latitude. It is not especially convenient for users of large-scale maps for several reasons:

- Differences in the coordinates of two points cannot readily be converted to a distance. This distance depends crucially on the distance from the equator, because lines of longitude converge at the poles.
- Latitude and longitude numbers are generally expressed in degrees, minutes (1/60th of a degree) and seconds (1/60th of a minute) or in degrees and decimal minutes. Use of decimal degrees is extremely uncommon except in software designed by people who don't actually use maps very much.
- Over short distances such as those encounters while hiking, the coordinates do not change very much --- an entire USGS topo quad is only 7.5 minutes across.

The UTM projection is a transverse cylindrical projection that maps the nearly-ellipsoidal Earth onto the surface of a cylinder that touches the surface along a meridian of longitude, and a rectangular coordinate system (what we call "the UTM coordinates" in most of our trainings) is assigned to points on the flattened cylinder after projection.

*[Note: In the following
paragraph, where I refer to "distance from the Equator" and "distance from the
central meridian" I do not mean "distance along the surface of the Earth," but
rather "distance on the flattened cylindrical projection surface."
The further you get from the central meridian, the less "one meter" of
difference between UTM coordinates corresponds to "one meter" of distance on
the ground. However, since zones are only used within three degrees of the
central meridian on either side, the approximation is good enough for our
purposes and we'll leave it at that.]*

The globe is divided up into "zones" of 6 degrees longitude with the first zone running from 180 degrees west longitude to 174 degrees west longitude. The central meridian (the meridian along which the projection cylinder contacts the surface) in each zone is assigned the arbitrary "Easting" coordinate of 500 kilometers, and all points within the zone are assigned coordinates based on their distance from the equator ("Northing") and from the hypothetical 0 point of Easting coordinate; so at the equator and at the central meridian the coordinate is (500.0km,0km). Since zones are less than 1000 kilometers wide there is no point which is actually given the coordinate 0,0, and all UTM coordinates are positive. Zones are also divided into 8-degree strips designated by a letter, starting at C at 80 degrees South Latitude and skipping I and O. Here in Albuquerque that is what the "S" stands for in our UTM coordinates (here where we live it is usually "13S" followed by eastings and northings), but this letter is mostly is redundant information: it merely denotes the range in which the northing coordinate falls. Some brands of GPS units do not use these NATO zone labels and display "N" or "S" for to indicate which side of the equator you're on (i.e. "North" or "South"). Do not confuse the "13S" displayed by Garmin GPS units for "Zone 13 South", as Garmin does display the correct NATO zone label.

*[N.B.: There are some notable exceptions to the zone definitions above.
Band "X" is 12 degrees of latitude high. Bands A and B, and Y and Z are near
the South and North poles, respectively, and in those bands a different
projection is used, called the UPS (Universal Polar Stereographic) projection.
South of the Equator a "false northing" of 10000 kilometers is added to the
northing coordinate. And zone widths and central meridians of UTM Zones 31V
and 32V, and zones 31-37 in band X have been redefined so certain Scandinavian
countries fall mostly in a single zone. See this Norwegian site for
specifics. That page links back here for background information, but be
warned: in addition to the converter here not using the correct datum for
sites outside the continental United States as they mention, I also assume
that all zones are 6 degrees wide, making it doubly incorrect to use the
converter on this page for anything in Norway.]*

A map of UTM zones appears below, courtesy of the Geographer's Craft Project web site. The image is copyrighted by Prof. Peter Dana, and is used here with his permission.

The advantages of using UTM coordinates instead of Lat/Lon are several: the coordinates are a base-10 metric coordinate system rather the cumbersome base-60 coordinate system of the Lat/Lon system; because the system is a rectangular coordinate system measured in kilometers, one can use the coordinates to calculate approximate distances directly. A point which is in UTM zone 13 and has coordinates (315.1,3925.1) is exactly 1 kilometer away from the point in zone 13 (315.1,3924.1).

A little bit of information about using UTM coordinates and USGS topo maps is contained in one of the handouts for our land navigation training. UTM coordinates are printed on those maps if you know what to look for. It does take a little preparation of your maps to use these coordinates (such as drawing in grid lines) but it is worth the trouble and is more convenient than using the Lat/Lon system.

Unfortunately conversion from Lat/Lon coordinates to UTM coordinates
involves some fairly complicated applications of spherical
trigonometry. The appropriate formulas are called "Redfearn's
Formulas" and may be read about on the transverse
mercator page
from the geotiff
project's projections list. *[Note: the web page just listed is not the one I actually
used when I wrote this applet, but that one is long gone. If you want to get
the various constants that are used in the calculation for a given datum,
please, please go download PROJ.4.]*

A crude Java applet which implements these formulas appears below.

Enter either a latitude and longitude in the upper data fields or the UTM zone, easting and northing in the lower, then hit the enter key. The coordinates in the other system will be displayed in the appropriate boxes. Enter West longitudes with a negative sign before the degrees, and enter South latitudes with a negative sign before the degrees. (The applet starts with the latitude 35 degrees North, 106 degrees West in the upper boxes, and the corresponding UTM coordinates in the lower boxes.)

On 11 July 2004 I modified this applet a little bit, to bring it more in line with what we use nowadays. The applet now displays UTM coordinates in meters, the way they appear on GPS units (when the original applet was written GPS was too expensive to be common, and most of our UTM usage was based on reading from maps --- where it was most convenient to work in decimal kilometers). Also, since the above text talks a lot about the NATO designators of latitude bands, the code now displays them. But you MUST type in the zone character if you are trying to convert from UTM to Lat/Lon -- the code expects the last character of the zone to be the band and won't give correct numbers if you leave it off. I also fixed a bug in the handling of the false northing for southern hemisphere lat/lon's --- for which this applet is inappropriate anyway (vide infra).

The applet was written by Tom Russo, Cibola SAR, who hopes you find it useful, but disclaims all liability related to this applet; the applet was written for fun and his own edification, not for life-critical navigation purposes. The user assumes all responsibility for the accuracy of his or her navigational calculations.

# EXTREMELY IMPORTANT: THIS APPLET HAS VERY SEVERE LIMITATIONS!

The converter uses constants derived from USGS Bulletin 1532 (which has been superceded by USGS Professional Paper 1395 (Map Projections: A Working Manual)) using the Clark 1866 ellipsoid. This is appropriate for USGS maps marked "North American Datum of 1927." : The newest USGS maps use the North American Datum of 1983, and so this converter will NOT give the correct Lat-Lon/UTM correspondence for those maps.**The NAD 27 datum used by this applet is almost**One of these days I will include an option which lets you toggle between NAD-27 and NAD-83 data, but for now you can only use this converter for the older maps. (To calibrate "one of these days" --- I wrote those words in mid-1997 and I write these words in late 1999, reinforced them in early 2001, re-emphasized them again in mid-2001, and here it is 2006 and it's still not done. That is to say, don't hold your breath. Cibola Search and Rescue disclaims all liability for readers of its web pages who render themselves unconcious --- or worse, who achieve core body temperatures near room temperature --- by holding their breath waiting for features promised "real soon now.")

*certainly*incorrect for any use outside of North America! Apologies are extended to the many visitors who are from another continent, but I am unlikely at this point to be including geodetic data appropriate for your use.# Oh, for cryin' out loud, enough of this yammering, just show me the applet already.

Having trouble running this applet? Check your browser's configuration
--some systems need you to have a special file installed or have "enable java"
checked in an options box. If you think you have Java running properly but
this still doesn't work, check your Java setup by visiting the Java web site and trying some of the
applets there. If those work and you *still* can't get this one
running it is unlikely that we can help you. This software is no longer
supported by Cibola's webmaster and will remain here only so long as it seems
to work for most people.

Users who live behind firewalls should note: some places (like where the author and several of his team mates work) don't allow Java applets to pass through their firewalls, so you may be outta luck. Your only option would be to get a commercial account and try it from home.