1.4 · Getting Started

Calibration

Tie your survey to the real world: entrance GPS and true north.

Why calibrate: What you lose by skipping it, and what exports gain.

Without calibration, a survey floats in its own space. Calibration gives it a GPS position and a true-north heading, so exports line up on a real map.

Three states: Pick the level your survey needs before going in.

State A: no calibration. Bearings start from wherever you began. Fine for a quick sketch.
State B: entrance GPS locked. Lines share an origin; bearings are still local.
State C: GPS plus a true-north lock. True bearings, declination in Survex, and your survey sits on the map in KML and GeoJSON. Use C for any survey you keep.

In State B, the Navigator, Go and Rescue work in this trip's own frame; lines from other trips join it by inheriting from one of their stations.

Before the trip: A pre-trip step that keeps every bearing honest.

Calibrate the compass outdoors, away from metal, under Calibration settings. Your phone's own magnets (MagSafe, battery, speaker) bias the compass; calibrating measures that so SpeleoMap can remove it. Redo it after changing your case. Turn the phone slowly through every direction until the globe is fully lit. SpeleoMap asks you to calibrate each time it opens fresh (Remind in Settings → Alerts → Survey). The North step at the entrance shows how old the calibration is, with Recalibrate right there (amber after 24 hours). A calibration from the last 24 hours is trusted even where the cave's own magnetism reads differently from the world model.

Do it every time you head out: surveys are more accurate when you calibrate every session.

Calibration order: Compass first, then the FLIR or the laser meter.

Do these in one session, in this order:

Compass, with nothing attached.
Thermal camera: clip the FLIR on, then Overlay (good light, a scene with clear edges 2 m or more away) and Thermal (tumble the phone with the FLIR on).
Laser meter: mount the meter, then Meter.

The FLIR and the meter each pull the compass a little. Once measured, SpeleoMap tells from the readings which one is on the phone and takes its pull out. A new compass calibration clears both; measure them again after it. Re-align the FLIR if the clip moves.

Laser meter setup: set the meter to measure from its front edge and mount it with that edge flush with the iPhone's top edge, so shots measure from the phone. All of these are in Calibration settings.

At the entrance: Your last chance for GPS before heading underground.

Start a caving workout and SpeleoMap walks you through it. Lock the entrance GPS (it averages, so waiting tightens it), hold the phone steady and north locks on its own, then point the phone down your first line and tap Lock heading, then lock the datum. Back at a cave you've locked before? Use the saved entrance. No GPS in a deep sink? Start a local survey; distances and shape are still good. GPS stays off underground, so lock it before you go in.

Drift: What happens when bearings wander, and how to recover.

SpeleoMap watches heading drift against the compass as you go. Drift tolerance is in Calibration settings. Hand-held lines are never corrected against the compass; Stow Mode has its own drift correction switch, off by default. Each workout keeps its own entrance fix, so a new entrance means a new workout and a new calibration. Lost north after a crash underground? Recover it from a station with Inherit calibration (Cave Line).

Drift tolerance is the drift allowed, as a percent of distance traveled, before the map suggests you recalibrate; lower keeps the survey tighter but asks more often. If a hand-held line drifts, snap to a station.

The heading lock row: Why an odd compass reading underground is nothing to worry about.

New Cave Line shows a heading lock row. It is a compass check only and never changes the line. Underground it often reads no steady compass, because of rock and metal. That is normal and does not affect the survey.

Mentioned in: 2.1 Hike Line · 3.1 A Cave Trip, Start to Finish · 3.2 Cave Line · 7.1 Caving Workouts · 8.1 Team Merge · 8.2 Export · 9.7 Calibration