NAV-06 · Lesson 06

Integrating GPS With Map and Compass

The core of the course. What to do with a fix before you act on it, and what to do when the device and the ground disagree.

Reading: about 12 minutes · Field drill: about 1 hour

Bottom Line Up Front

Everything so far has been preparation for one habit: a fix is a claim, and a claim gets checked before you act on it. The check costs about thirty seconds, it uses the ground you are already standing on, and it is the only thing standing between you and every silent failure this course has described.

This lesson assumes you can already read a grid and plot one, which is NAV-01, and that you can already relate a map to the ground in front of you, which is NAV-02. What it adds is the join: how a position moves between device and paper without corrupting, when to verify, and the decision rule for the moment the two sources disagree. That rule is the single most important sentence in the course.

Recall Check · L01

From Lesson 01, because this lesson rests on it: what is the accuracy figure your device displays?

A. The measured difference between the reported position and the true position, updated as the fix improves.
B. A manufacturer specification for ideal open-sky conditions, reprinted on screen without regard to conditions.
C. The spread between the individual satellite ranges, which widens whenever any one of them disagrees.
D. The receiver’s own estimate of its work, blind to the errors it cannot detect.

1. Three sources, and which one is the authority

Standing in the field you have three things that claim to know where you are, and they are not equal.

The device computes a position and reports it with a confidence figure it generated itself. It is precise, fast, and can be confidently wrong in the ways Lessons 01 through 03 described — reflected signals, a stale setting, a mistyped coordinate — without any outward sign.

The map is a model of ground that was surveyed at some point and printed at some point. It is right about relationships almost always and can be out of date about anything built, cut, dammed, burned or washed out.

The ground is not a claim at all. It is the thing itself. The re-entrant either runs down to your left or it does not.

The ground is the authority. The map is the reference. The device is a proposal. When they disagree, that order tells you which one to move first — and it is the reverse of the order most people trust them in.

Figure 6.1 Look up before you read the screen. Once you know what the fix claims, you will find the terrain it predicts rather than the terrain that is in front of you — the same reason you read a coordinate back off the device instead of off your own notes.

Knowledge Check

Why is the ground treated as the authority rather than the device, when the device is more precise than anything you can measure by eye?

A. Because handheld receivers are only accurate to several meters, which is coarser than the terrain features you would be checking against.
B. Because the ground is not a claim about the world — it is the world.
C. Because maps are surveyed to a higher standard than consumer receivers achieve, and the ground is what the survey was taken from.
D. Because devices fail more often than most users expect, so the safer assumption is that any electronic position is unreliable.

2. Moving a position between device and paper

The mechanics of plotting a grid belong to NAV-01 and are not repeated here. What matters at this join is what can corrupt in transit, and both directions have a confirmation step that people skip.

Device to paper

Read the position off the device, plot it, and then do the thing that makes it worth having done: look at where it landed and ask whether that is terrain you could be standing on. If the device puts you on the valley floor and you are demonstrably on a slope, you have caught a failure for free, before it cost you anything.

The failures that reach this point are the settings failures from Lesson 03 — a datum mismatch that shifts you eighty meters onto the wrong side of a creek, or a format read wrongly. Both produce a well-formed coordinate that plots onto real ground. The plot itself will not object. You have to.

Paper to device

Going the other way, you plot or read a grid from the map and enter it. The corruption risk here is transcription, which Lesson 04 covered, and the confirmation step is the same one: after entering it, look at the bearing and distance the device now reports, and ask whether that matches where you believe the point is. A point that should be a kilometer northeast and reads as six kilometers southwest is telling you something before you take a single step.

Both directions share one weakness

Every check described so far is a check against your own expectation. That is genuinely useful and it has a ceiling: if you are wrong about where you are in the first place, your expectation is wrong too, and the plot will look entirely reasonable. Only the ground breaks that loop, which is why the next section is a procedure and not a habit of mind.

Knowledge Check

A member plots a fix and it lands on plausible-looking ground. What has that told them?

A. Very little on its own, because a shifted or misread coordinate also lands on real ground that looks entirely reasonable.
B. That the fix is good to within the accuracy figure the device is displaying, which is what plausible ground confirms.
C. That the datum and format are correct, since an error in either would have placed the point somewhere obviously impossible.
D. That the map and the device share a common reference, since a mismatch between them would prevent the point plotting at all.

3. What verification actually is

“Check it against the terrain” is easy to nod at and vague enough to skip. Here is the procedure, and it takes about thirty seconds.

Step 1Look up firstBefore reading the screen, name three features you can actually see. A spur, a saddle, a bend in the stream, a road cut.
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Step 2Find them on the mapLocate those same three on the sheet. Not near the fix — anywhere on the sheet, on their own merits.
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Step 3Check the directionsFrom the plotted fix, is each feature in the direction and roughly the distance you observe it to be?
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Step 4DecideThree agreements: act on the fix. One disagreement: stop, and go to the decision rule below.

Two details make this work rather than merely feel diligent. Look up before you look at the screen — if you read the fix first, you will find the features it predicts rather than the ones that are there, and the check confirms itself the same way checking your typing against the paper you copied from does. And use features you can identify independently, not “that hill must be the one at 41”.

Do this at planned points rather than continuously. Every turning point from Lesson 05 is a natural checkpoint, plus any moment you are about to commit — before a long leg, before dark, before passing a position to somebody else, and before doing anything you cannot undo.

Knowledge Check

Why does the verification procedure have you name visible features before reading the position off the device?

A. Because reading the device first drains battery that the verification itself may need if the check takes several attempts.
B. Because the device needs time to settle after being woken, and the delay improves the fix you eventually compare against.
C. Because knowing what the fix claims makes you look for the features it predicts, so the check confirms itself.
D. Because features named from memory are more reliable than features identified while holding a device in your hand.

4. The decision rule

You have done the check and something does not fit. The map says the spur runs east and you are looking at it running north. This is the moment the course exists for, and it needs a rule decided in advance, because deciding it in the moment goes badly.

The rule

If the device and the ground disagree, you are lost until proven otherwise. Stop moving. Do not split the difference.

Not “probably a glitch.” Not “close enough, keep going and it will sort itself out.” Disagreement means one of your inputs is wrong and you do not yet know which, and every step taken in that condition makes the recovery longer.

Then work through the cheap explanations before the expensive ones, because most disagreements are a setting rather than a catastrophe:

FirstAre you looking at the right point?Navigating to the waypoint you think you are? A neighboring name on the list is the single most common cause.
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SecondFormat and datumDoes the device match the sheet? An eighty-meter-class disagreement is this until proven otherwise.
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ThirdIs the fix healthy?Accuracy figure, satellite view, position stable or jumping. Terrain and canopy from Lesson 01.
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ThenRe-establish by terrainIf none of the above explains it, put the device away and fix your position from the ground. NAV-02 procedure.

Note what the last step does not say. It does not say average the two, or take the midpoint, or trust whichever feels closer. A disagreement is not noise to be smoothed; it is information that one source is wrong, and the resolution is to re-establish position by a method that does not depend on the source you now doubt.

There is one useful asymmetry to hold. The device is usually right about where a stored point is; the ground is authoritative about where you are. A coordinate somebody sent you does not become wrong because you cannot find the feature — but your own position most certainly can be, and that is the one to re-establish first.

Knowledge Check

The device puts a member about 150 meters from where the terrain says they are. The accuracy figure reads four meters and the position is stable. What is the right first move?

A. Continue on the current bearing and re-check at the next checkpoint, since a stable fix with a good accuracy figure is unlikely to be badly wrong.
B. Stop, and check the cheap explanations first — the point selected, then the datum and format against the sheet.
C. Split the difference and proceed from a position midway between the two, which bounds the error while keeping the team moving.
D. Switch the device off and re-establish position entirely by terrain, since a disagreement of that size means it cannot be trusted.

5. Recognizing a bad fix before it costs you

Some disagreements announce themselves earlier if you know the tells. None of these is proof; each is a reason to look harder.

What you notice	What it suggests
Position jumps while you stand still, accuracy figure unchanged	Reflected signals. Move to more open ground and take the fix again.
Accuracy figure climbing steadily as terrain closes in	Restricted sky view. Expected, honest, and a reason to widen your tolerance rather than distrust the device.
Elevation disagrees with the map by more than the contour interval	Elevation reference or an uncalibrated altimeter — and a hint the horizontal may be off too.
Bearing to a known feature is off by a consistent amount	North reference or a stale manual declination, not a position error at all.
Everything looks perfect and the ground does not match	Datum or format. This is the one with no other symptoms.

That last row is the reason this lesson comes after Lesson 03 rather than before it. The most dangerous failure mode produces no tells whatsoever — and the only reason you would ever catch it is that you looked up.

Knowledge Check

Bearings taken with the device read consistently about twelve degrees off from bearings to features you can identify. What does that pattern point at?

A. A position error, since a fix displaced to one side produces bearings that are wrong by a roughly constant amount.
B. Reflected signals, which shift the computed position and therefore rotate every bearing taken from it in the same direction.
C. A coordinate format mismatch, which changes how the position is expressed and shifts derived bearings by a fixed offset.
D. The north reference or a stale declination, since both rotate every bearing equally without moving the position.

6. Stepping down without stopping

When the resolution is that the device is not usable, the transition to map and compass should be a step rather than an event. That means the conditions for it were set up long before: the paper map is on you rather than in the vehicle, the route is marked on it, the grids are written down, and you know roughly where you have been since the last confirmed point.

If all of that is true, the transition costs you a pause at a known point and nothing else. If none of it is true, the transition is a crisis, and the difference was decided at the map table days earlier.

Lesson 09 covers the degraded and denied cases in detail — recognizing them, and moving through them without halting. The point here is narrower: the step down is not a failure of the plan, it is the plan. A member who moves to map and compass on a disagreement and turns out to have been wrong about the disagreement has lost twenty minutes. A member who presses on has lost their position and does not know it yet.

What to carry out of this lesson

The ground is the authority, the map is the reference, the device is a proposal — the reverse of the order most people trust them in.
Name three features you can see before you read the screen. Knowing what the fix claims makes you find the terrain it predicts.
Plausible ground proves almost nothing. An eighty-meter error lands on terrain that looks entirely reasonable.
If the device and the ground disagree, you are lost until proven otherwise. Stop moving, check the cheap explanations, and re-establish by terrain. Never split the difference.

Field Drill

On ground you know, with the map you would actually carry.

Walk to three points and at each one run the four-step verification, out loud, in order — features first, screen second. Time it. Note whether you cheated and looked at the screen first.
Deliberately induce a disagreement: change your device’s datum to a different one, then take a fix and plot it. Measure how far off it lands, and notice how ordinary the wrong point looks. Set the datum back and confirm you did.
Have somebody put a waypoint in your device without telling you which point it is. Navigate to it, verifying at least once on the way. If the fix and the ground ever disagree, run the decision ladder rather than improvising.
Write your own version of the decision rule in one sentence, in words you would actually use under pressure, and put it on the settings card from Lesson 03.

This drill is the rehearsal for the capstone in Lesson 10.

Answer all five knowledge checks to unlock Lesson 07

← Lesson 05: Routes, Tracks, and Retracing Your Path