Choosing a Coordinate System

Which coordinate system should you use?

Most GPS receivers come out of the box set to lat/lon, and many people never change it — they don't realize there's a choice, or that another system might suit them better. There is a choice, and for most land navigation I'd steer you toward a UTM-based system. UTM, USNG, and MGRS are all built on the same square-grid foundation; they're easy to plot, easy to measure, and the numbers translate directly to distance on the ground. Lat/lon still wins in a few situations — very small-scale maps, and any work where your peers are already using it. This page is about making that choice deliberately instead of by default.

The four systems worth knowing about, all of which work with MapTools plotting tools:

Lat/lon — the global angular system (degrees of latitude and longitude). Universal, but awkward to measure with and easy to fumble the minutes-and-seconds arithmetic.
UTM — Universal Transverse Mercator. A metric, square-grid system. The workhorse for land navigation.
USNG — United States National Grid. UTM's grid with a letter-based labeling scheme, designed for civilian and emergency use in the U.S.
MGRS — Military Grid Reference System. Essentially the same grid and labeling as USNG, used by the military and by search-and-rescue.

A useful framing: USNG and MGRS are ways of writing down a position on the UTM grid. If you learn UTM, you've already learned most of what you need for the other two. All MapTools tools that work with UTM also work with USNG and MGRS.

Three questions that settle the choice

Use the same system your peers use

If you're passing coordinates back and forth with other people — a search team, a hunting party, a class — use whatever they use. A coordinate is only useful if the person receiving it can plot it without translating it first. Most aviation and maritime users are already on lat/lon, so if you're coordinating with pilots or boaters, lat/lon is the practical answer regardless of what you'd prefer on land. Search-and-rescue and military work is usually MGRS or USNG. Match the room.

Use a system that works with your maps

Map scale drives this one:

On large-scale maps (a USGS 1:24,000 topo, most hiking maps), a UTM-based system is usually the better choice. Many of these maps already carry a UTM grid, and where they don't you can add one. USGS prints both lat/lon and UTM references on its large-scale maps.
On small-scale maps — anything covering more than about 6° of longitude, or 1:1,000,000 and smaller — prefer lat/lon. UTM grids on small-scale maps stop being square (the projection has to keep lines of longitude looking parallel), which defeats the main advantage of UTM.

A caution about lat/lon references on maps: some mapmakers print "decorative" lat/lon marks that are approximate at best. Before GPS, almost nobody checked them. Many maps still haven't caught up. Verify before you trust.

Use a system that's easy to use

When the first two questions don't decide it for you — you've got a large-scale map, you have UTM information, and nobody's forced your hand — UTM is usually the easiest system to actually work with in the field. Its advantages:

Square grids.
East-west units are the same as north-south units.
Decimal-based: no fussing with minutes and seconds.
Coordinates translate directly to distances on the ground.
Precision is intuitive — you don't have to wonder how far a tenth of a second of longitude is.
Easy to abbreviate when you're working in a small area.

When lat/lon is still the right call

Don't read "UTM for land navigation" as "lat/lon is obsolete." Reach for lat/lon when:

Your map is small-scale (continental, marine charts, world maps).
You're working with aviation or maritime people.
A coordinate arrives in lat/lon and you just need to plot it once — convert it or plot it as-is rather than reworking your whole setup.

The Map Datums still matters in every system. Whatever notation you pick, the datum is the Earth-model the numbers are measured against, and it has to match between your map and your GPS.