The Navigation Stack, and What the Compass Actually Tells You

Introduction

This volume contains the finding that justifies the dive. The eTrex is on the dash to supply a heading the truck does not have, which is a sound reason — but the thing it supplies while driving is not what most people assume, and the assumption matters because it determines when the reading can be trusted.

3.1 The eTrex Touch 25, precisely

Table 1 — The eTrex Touch 25, precisely

Property	Value
Display	2.6 in touchscreen
Positioning	GPS and GLONASS
Compass	3-axis tilt-compensated electronic compass
Altimeter	Barometric
Storage	8 GB internal, microSD expandable
Power	AA cells — the only independent power source in the cab

The 3-axis tilt-compensated compass is the relevant feature. Tilt compensation means the device does not have to be held level to read correctly, which is why it works on a dash that is not horizontal. And crucially, a magnetic compass indicates heading while stationary, which a GPS cannot do at all.

3.2 The part that is usually assumed wrong

A handheld GPS displays heading from two entirely different sources depending on what it is doing:

Standing still, it reads the magnetic field — a true compass, giving the direction the device is pointed.
Moving above a speed threshold, it switches to GPS course-over-ground — the direction of travel, computed from successive position fixes, which has nothing to do with magnetism.

These are not the same quantity, and neither is strictly better. Course-over-ground is immune to magnetic interference and is what a driver actually wants while moving. The magnetic compass is what a person wants when stopped and orienting a paper map or pointing at a distant ridge.

Garmin devices switch between them automatically, which is sensible and is also why the distinction goes unnoticed.

Why it matters here: the magnetic reading, in this truck, is the less trustworthy of the two. A magnetic compass mounted on a steel dashboard is surrounded by ferrous mass, speaker magnets, wiring carrying current, and — in this specific cab — a RAM mount full of steel and three radio heads containing permanent magnets in their speakers. The resulting error is called deviation, and it is a property of the installation rather than of the instrument.

The practical consequences:

Calibrate the compass where it is mounted, not in the driveway with it in hand. The calibration routine builds a correction for the local field, and a calibration performed somewhere other than the operating position corrects for the wrong environment.
Re-calibrate after anything ferrous or magnetic moves nearby — a new radio head, a relocated mount, a different phone on the dash.
Sanity-check it against a known bearing. Park on a road whose direction is known from the map and compare. A consistent offset is deviation and can be lived with once it is known; an erratic reading means the mounting position is unusable and the device needs to move.
Trust the moving reading more than the stopped one. Course-over-ground is not affected by any of this.
The honest workaround, if deviation turns out to be severe: the reading that matters most while driving is course-over-ground, which is unaffected — so a bad magnetic environment degrades the eTrex’s usefulness when parked, not when underway. A baseplate compass in the door pocket, used away from the truck, is a five-dollar answer to the stopped case and does not care about the dashboard at all.

None of this makes the eTrex the wrong choice. It is a rugged, AA-powered device with its own GNSS and its own maps that works when everything else in the cab is dead, and the barometric altimeter is genuinely useful. It just should not be assumed to be a good magnetic compass simply because it contains one.

3.3 Gaia on the tablet: the workflow is the feature

The Galaxy Tab S8 running Gaia GPS is the primary off-pavement surface, and the thing that makes it work is a habit rather than a setting.

Download before departure, every time, without exception. This is the single most important operational rule in the whole connectivity story, and it is worth stating in the strongest terms: the Cellular Gateway dive concluded that roughly a third of Keweenaw County has no service from any carrier. A navigation plan that requires downloading a map in the field is a plan that fails in exactly the terrain it was made for. Any trip where maps are being fetched over cellular is a trip where the preparation failed.

Practical points for this rig:

Download generously. Storage is cheap and the boundary of a trip is never known precisely in advance. Downloading a whole county at a useful zoom is a few gigabytes and removes an entire class of problem.
Download the layers, not just the base map. Satellite imagery, topographic base, and the public-land and motor-vehicle-use overlays are separate downloads. The overlay that says whether a two-track is legally open is the one most likely to be forgotten and the one with the most consequential absence.
Record tracks. A recorded track is the cheapest insurance available: it is the route back out, it is the record of where a campsite actually was, and it is the raw material for the Trips dives this project intends to write.
Watch the thermal and power budget. A tablet navigating with the screen on and GNSS running is a meaningful load that will not survive a long day on its own battery. It needs charging, and the mount needs to keep it out of direct sun — see volume two.
The tablet’s GNSS is its weak point. Tablet GNSS receivers are generally less sensitive than a dedicated handheld’s, and a tablet mounted low behind glass has a compromised sky view. Under canopy this is where the eTrex’s separate receiver earns its place.

3.4 Android Auto, and knowing where its boundary is

Android Auto on the factory screen is the right tool for pavement and should be left alone. Two things are worth being explicit about, because both are discovered at inconvenient moments.

It requires the phone, and much of it requires connectivity. Waze is a connected application by nature — its entire value is other users’ live reports. Offline, it degrades to a basic map. Google Maps offline areas must be downloaded deliberately, exactly like Gaia’s.

It does not know about two-tracks, seasonal roads, or public-land boundaries. Its road network is a road network. Following it off pavement in the Upper Peninsula is a well-known way to end up on a seasonal road that is legally a road and physically a swamp. The handoff from Android Auto to Gaia should happen at the pavement, deliberately, rather than when the road runs out.

3.5 The redundancy, stated as a table

Table 2 — The redundancy, stated as a table

Failure	What still works
No cellular service	Gaia offline, eTrex, paper map. Android Auto degrades
Tablet dead, overheated, or unmounted	eTrex with its own maps and AA cells; phone with Gaia installed
Truck’s 12 V system dead	eTrex on AA cells; phone and tablet on internal batteries
eTrex fails	Tablet and phone; GPS course-over-ground on either
Everything electronic fails	Paper map and a baseplate compass — which is why both should be in the truck

That last row is not a joke. A rig with four GNSS receivers and no paper map has a single common-mode failure — a dead 12 V system on a cold morning takes more of the stack than is comfortable. A county map and a baseplate compass weigh nothing and are the only genuinely independent layer.

The radios themselves are covered in the Radios and Scanners projects. The connectivity question — whether a cellular gateway is worth buying, and what it would and would not fix — is this project’s Cellular Gateway dive. Powering all of this properly is the Vehicle Power dive.

References

Garmin — eTrex Touch 25 product specifications and user manual: 3-axis tilt-compensated electronic compass, barometric altimeter, GPS and GLONASS, 2.6 in touchscreen, 8 GB plus microSD.
Garmin — eTrex Touch 25/35 compass calibration and altimeter settings documentation.