Field Reference
Working notes distilled from TC 3-25.26 — Map Reading and Land Navigation (Nov 2013, HQ Dept. of the Army). Quick reference, not a substitute for the manual or training.
Grid coordinates (MGRS)
Read right, then up. Give the easting (left→right) before the northing (bottom→top). “Right and up.”
Precision. A grid like 12S AB 1234 5678 is 8 digits = 10 m. 6 digits = 100 m, 10 digits = 1 m. Always give the same number of digits for easting and northing.
Grid zone + 100k square. Full MGRS = grid-zone designator (e.g. 12S) + two-letter 100,000 m square (e.g. AB) + the numeric easting/northing.
St. George. Southern Utah sits in grid zone 12S. The Coordinates tab reads your GPS straight into MGRS.
The three norths & the G-M angle
True north (TN). Toward the geographic North Pole — the star on the declination diagram.
Grid north (GN). The direction the map’s vertical grid lines point.
Magnetic north (MN). Where the compass needle points; it drifts over time and varies by location.
G-M angle. The angle between grid north and magnetic north, printed in the declination diagram. Use it to convert between grid and magnetic azimuths.
Two values appear in the diagram. If GN and MN are on opposite sides of TN, add the absolute values; if on the same side, subtract them — that total is the G-M angle.
LARS — converting azimuths
Left Add, Right Subtract — applied when converting a grid azimuth → magnetic (compass) heading:
MN left of GN. ADD the G-M angle to the grid azimuth.
MN right of GN. SUBTRACT the G-M angle from the grid azimuth.
Going the other way. Magnetic → grid reverses it (right add, left subtract).
In Southern Utah magnetic north is east (right) of grid north, so you subtract ~11° going grid→magnetic. The Compass tab does this for you.
Azimuths & distance
Azimuth. A horizontal angle measured clockwise from a north reference, 0–360°. Grid azimuths are measured from grid north with a protractor.
Back azimuth. Reverse direction: add 180° if the azimuth is under 180°, subtract 180° if it’s 180° or more.
Mils. 6400 mils = 360°. 1° ≈ 17.78 mils. Artillery and many compasses use mils.
Measuring on the map. Plot start and end, draw the line, then read the angle against grid north. The Map tab’s protractor does this once the map is calibrated.
Terrain features
Five major. Hill, ridge, valley, saddle, depression.
Three minor. Draw, spur, cliff.
Two supplementary. Cut and fill (man-made).
Contour lines. Connect points of equal elevation. Close together = steep; far apart = gentle. The contour interval is in the marginal data.
Terrain association. Navigate by matching the ground to the map — handrails, catching features, and checkpoints — rather than azimuth alone.
Finding your location
Resection. Determine your position by shooting back-azimuths to two or three known, identifiable features and plotting them; you’re where the lines cross.
Modified resection. Same idea when you’re on a linear feature (road, stream) — one azimuth to a known point fixes you along it.
Intersection. Locate an unknown distant point by shooting azimuths to it from two known positions.
Dead reckoning. Hold a known azimuth and track distance by pace count to arrive at a point you can’t see.
Moving smart
Pace count. Count every time the same foot strikes. Calibrate over 100 m; most people run ~110–125 paces per 100 m on flat ground (more uphill/rough).
Steering marks. Pick a distant feature on your azimuth, walk to it, then pick the next — keeps you straight without staring at the compass.
Deliberate offset. Aim intentionally left or right of a point on a linear feature (~3° per 100 m of offset wanted) so you know which way to turn when you hit it.
Attack points & handrails. Navigate to an obvious nearby feature (attack point), then fine-navigate to the objective. Follow linear features (handrails) when they parallel your route.
Don't get killed
GPS supplements the map. It doesn’t replace map-and-compass skills. Canyons, cliffs, and tree cover degrade GPS; carry analog tools and know them cold.
Magnetic interference. Vehicles, weapons, power lines, and even a phone deflect a compass — and a phone’s magnetometer is worse. Step away from metal and verify.
Battery & water. Electronics fail. In Southern Utah, heat and dehydration kill faster than a wrong azimuth. Plan water, tell someone your route, set a turnaround time.
Check the math yourself
Don't take the app's word for it — these are worked examples you can reproduce by hand:
Grid → magnetic (FM example). Grid azimuth 199.5° with a 9.5° EAST G-M angle: 199.5 − 9.5 = 190.0° magnetic. (East G-M angle subtracts going grid→magnetic; the Compass tab gives the same answer.)
Back azimuth. From 65°: 65 + 180 = 245°. From 245°: 245 − 180 = 65°.
Mils. 90° × 6400/360 = 1600 mils.
Your position. On the Coordinates tab, your GPS MGRS should match a paper plot of the same lat/long to within your fix accuracy — if it doesn't, suspect datum (WGS84 vs NAD27) or a bad fix.