GPS Setup for Map Coordinates

What GPS setup is for

A modern handheld GPS, out of the box, will tell you where you are — but in a format and against a reference that may not match the map you're carrying. GPS setup is the small handful of configuration choices that make the receiver and the map agree, so the position the GPS reports can be plotted directly on the paper map without conversion or correction.

The two settings that matter most are:

• Position format — the format the GPS uses to display your coordinates (UTM, MGRS, USNG, or one of several latitude/longitude variants).
• Map datum — the geodetic reference the GPS uses to compute those coordinates. Must match the datum of the map.

A handful of other settings — north reference, distance and elevation units, data field choices, battery type, satellite system — affect how usable the GPS is in the field but rarely make the difference between a usable position fix and a wrong one. They're covered here in roughly that order of importance.

These settings exist on almost every land-recreation GPS receiver, even though the menu paths and exact wording differ. Most of the concrete examples use Garmin terminology (the eTrex line in particular), because Garmin dominates the recreational handheld market and because the MapTools instructor materials use Garmin units. The concepts travel to other manufacturers; only the button paths change.

Position format

This is the format your GPS uses to display your current coordinates. The common choices, in roughly the order most useful for land navigation with topographic maps:

• UTM/UPS — Universal Transverse Mercator, plus the polar variant. The default recommendation for most land users.
• MGRS — Military Grid Reference System. A specialized form of UTM.
• US National Grid (USNG) — civilian sibling of MGRS. See USNG Coordinates.
• hddd° mm′ ss.s″ — latitude/longitude, degrees-minutes-seconds.
• hddd° mm.mmm′ — lat/lon, degrees and decimal minutes.
• hddd.ddddd° — lat/lon, decimal degrees.

For ordinary land navigation against a topographic map with a UTM grid, set the format to UTM/UPS and the rest of your workflow gets simpler — coordinates plot directly with a UTM tool on the printed grid, with no conversion. The metric basis of UTM also pairs naturally with metric distance units.

Pick lat/lon when the people you're working with use lat/lon (sea, air, marine charts, search-and-rescue traffic in some regions) or when the map you're using doesn't have a UTM grid and you have a tool for plotting lat/lon coordinates.

Most receivers ship configured for lat/lon decimal degrees out of the box. The first setup step is almost always to change the position format to whatever you actually plan to use.

Map datum

The map datum is the geodetic reference frame the GPS uses when it converts the satellite-derived position into ground coordinates. The datum setting on the GPS must match the datum of the map you're plotting against. A datum mismatch produces a consistent offset of typically a few hundred meters — large enough to put you on the wrong side of a stream or ridge, but small enough that you may not notice the error until something else doesn't add up.

The common datum choices on a recreational GPS:

• NAD 27 CONUS — North American Datum of 1927, Continental United States. The datum used on most USGS paper topographic maps printed before the late 1990s and on many that are still in circulation. If your map's collar says "NAD 27" or doesn't say anything at all and it's a USGS topo, assume NAD 27.
• NAD 83 — North American Datum of 1983. The current USGS datum. Essentially identical to WGS 84 for civilian use.
• WGS 84 — World Geodetic System 1984. The default datum for most GPS receivers as shipped, the datum used on most modern marine charts, and the datum the GPS satellites themselves broadcast against.

The single highest-yield setup mistake to avoid: leaving the GPS at its default WGS 84 while using a USGS NAD 27 paper map. This is the most common datum mismatch in the field, and it consistently puts your reported position about 100–200 meters away from where you actually are, depending on where in the continent you are.

For more on the geometry — what a datum is, why the offset exists, and how to recognize and correct one in old map data — see Map Datums.

A practical habit: any time you pick up a new map, check the collar for the datum and set the GPS to match before you leave the trailhead. Treat it as part of preparing the map.

North reference

The north reference setting determines what direction the GPS calls 0° / North in any heading or bearing it displays. The common choices:

• True — bearings and headings are referenced to True (geographic) North. The left and right edges of most topographic maps are lines of longitude, which are also True North references — making True-referenced bearings straightforward to plot.
• Magnetic — bearings are referenced to Magnetic North, matching readings taken on a compass that has not been adjusted for declination. If you want the GPS bearing and the compass bearing to agree directly, use Magnetic. Most receivers will compute the local declination automatically; some allow manual entry — pick the automatic option unless you have a specific reason to override it.
• Grid — bearings are referenced to Grid North, the direction the UTM grid lines on your map run. Convenient when plotting bearings on a UTM-gridded map, since the grid lines are everywhere and the bearing plots without any rotation correction.
• User — manual declination entry.

There's no single right answer here; the right choice depends on which other tools you're using.

• If you're plotting bearings on a UTM-gridded map and rarely sighting them with a compass, Grid is the most convenient choice.
• If you're sighting bearings with a compass and walking them, Magnetic keeps the GPS bearing and the compass reading numerically identical.
• If neither of those is dominant, True is the safe general-purpose default.

See North References (True, Magnetic, Grid) and Declination for the geometry of how the three norths relate.

Distance and elevation units

Mostly a preference setting, with one alignment worth honoring: if you've set the position format to UTM, set distance to metric. UTM is a metric grid, your map is gridded in 1-km squares, and your plotting tool measures within those squares in meters. Mixing metric coordinates with statute distances forces unit conversion at every step.

If the GPS lets you set elevation units separately, match the units to whatever the map's contour lines and elevation markings use — feet on a USGS topo, meters on most other countries' topographic maps.

Data field configuration

Most handheld GPS receivers let you configure which data fields appear on which screens. This is one of the differences between a GPS that's useful in the field and one that's mostly screens of irrelevant information. The fields that earn their place on the screens you actually look at:

On the Trip Computer / position screen, large data fields:

• Location (your current coordinates, in your chosen position format)
• Time of day
• Battery level
• GPS accuracy (the receiver's own estimate of position error — see Accuracy vs. Precision)
• Heading
• Speed and moving average speed
• Elevation and vertical speed

On the Trip Computer, small data fields (a denser layout for when you're working with mixed audiences) — useful to add a second Location field set to lat/lon while the first stays on UTM, so you can read out either format without changing the position format setting.

On the Compass screen:

• Bearing (direction to the next waypoint)
• Distance to next (distance to the next waypoint)
• Heading (your current direction of travel)
• Destination (the name of the waypoint you're navigating to)

On the Map screen: no data fields. The map screen on a handheld is small to begin with; data fields shrink the actual map further, and the same data is already on the Trip Computer and Compass screens. Set the map screen's data field count to 0.

The Garmin Profiles feature is worth knowing about: it saves an entire configuration — settings, data field choices, screen layout — under a name, so a group can standardize on a shared profile and individual users can switch between configurations (e.g., a "Class" profile and a personal profile) without losing either. Most other vendors have an analogous feature with a different name.

Battery type, satellite system, and other ancillary settings

Worth setting once and forgetting:

• Battery type. Set this to match the chemistry you're actually using — alkaline, lithium, or rechargeable NiMH. The battery-level indicator only reads correctly when this matches; mismatched, the receiver will either report a healthy battery shortly before shutdown or report a near-dead battery with most of its capacity remaining.
• Satellite system. GPS + GLONASS (or GPS + GLONASS + Galileo, on newer receivers) gives faster fixes and better performance under tree cover or canyon walls than GPS alone, at the cost of slightly higher battery draw. Use the multi-system option unless battery life is the dominant constraint.
• WAAS / EGNOS. Enables the regional satellite-based augmentation system. Turn it on in North America (WAAS) and Europe (EGNOS); it improves position accuracy noticeably.
• USB mode. Set to Mass Storage so the GPS appears as a drive when plugged into a computer — easier transfer of waypoints, tracks, and offline maps than the vendor-proprietary mode.

How the GPS fits into the workflow

Setup is the foundation, not the point. The point is the way the GPS gets used in the field. The pattern that works well for most land users:

• Carry the GPS turned off, most of the time. Power it on when you specifically want a position fix, wait about a minute for the computed position to stabilize, read off your coordinates, then turn it off again. With this pattern a set of batteries lasts months, not days.
• Use the GPS for the "where am I?" question. That's what it does best: read off your UTM (or lat/lon) coordinates, plot them on the paper map with a UTM tool, and you have your position to about the size of two parking spaces — anywhere on Earth, day or night, in any visibility. For this specific question, the GPS beats a sighted-and-plotted compass resection handily.
• For navigation along a planned route, mark waypoints at decision points — places where you change direction, choose between options, or want a confidence check on a long leg. The GPS shows you when you're drifting off-line; the compass alone tells you only what direction you're pointed.
• For bearing-and-distance work, project waypoints rather than walking compass bearings. Most GPS receivers can create a new waypoint a stated distance and bearing from a known one. The GPS then recalculates a fresh course to the projected waypoint continuously, which means you can detour around obstacles or follow easier terrain without losing the line — the compass-and-pacing approach can't do that.

The receiver's built-in Map screen is mostly for confirmation, not for primary navigation. The screen is small, the detail-vs-context trade-off forces constant zooming, and on most units the installed maps are coarser than the paper you're carrying. Use the paper map for the map work; use the GPS for the position fix.