# Selecting a Geographic Coordinate System

Most GPS receivers come out of the box set to use latitude/longitude coordinates. And many GPS users never consider that there may be other coordinate system that would better meet their needs.

I would urge you to consider coordinate formats based on the Universal Transverse Mercator map projection and coordinate system. Many land based users will find them to be easy to use and well suited to their needs. But, as much as I like using UTM, there are some circumstances where lat/lon is a better choice. This page provides information to help you make the choice between using UTM based coordinates or lat/lon coordinates.

When it comes to selecting tools for plotting and measuring coordinates, all of our tools that work with the UTM system also work with the other UTM based systems; USNG, MGRS, and kUTM.

## Use the same system that your peers are using

If you are working with other folks and passing map coordinates around, then you will want to use the same coordinate system and notations that they use. Unless of course you're convinced they should all change to something better and you're willing to be the force behind the change.

You will find that most aviation and maritime users are already using lat/lon coordinates.

## Use a system that works well with your maps

If you are working with maps that cover more than 6 degrees of longitude or are 1:1,000,000 scale or less, you will probably want to use lat/lon coordinates. Small scale maps are often projected using a map projection that will result in UTM grids that are not square. Small scale UTM gridded maps need to use a projection where lines of longitude appear to be parallel, such as a Mercator projection.

On larger scale maps the choice of coordinate system is often determined by the coordinate references that have been supplied by the map maker. USGS puts both lat/lon and UTM coordinates on all of their large scale maps. Many other map makers only provide lat/lon references. It is possible to add a UTM grid to a map that is only marked with lat/lon references. But it is a tedious process, so you might be better off just using lat/lon coordinates.

Many maps have no coordinate references at all. You can add coordinate references by either comparing known features with another map that has coordinate information or by locating several know point in the field and "surveying" them with your GPS receiver. This is a difficult process, but at least you can choose what coordinate system to use.

If you are using or considering buying computer based maps, check what coordinate systems are available in the software.

Beware of "decorative" lat/lon references. Some map makers have provided lat/lon information that is approximate at best. Before GPS receivers were common place, it was rare that a casual map user would even notice coordinate references, much less care if they are accurate. The times have changed, but many maps haven't caught up yet.

## Use a system that easy to use

If your using a large scale map, you have UTM coordinates information, and your peers haven't already made the decision for you, then UTM is likely to be your best bet.

Some of the advantages of UTM coordinates include...

• Square grids
• East-West units of measure are the same as North-South units
• Decimal based, no fussing with minutes and seconds.
• Coordinates translate directly to distances on the ground.
• Coordinate precision is easily understood. No need to wonder what distance a tenth of a second of longitude represents.
• It's easy to abbreviate coordinates when working in a small area.