precise_compass

An honest, high-accuracy Flutter compass. Every reading carries a continuous heading-uncertainty estimate in degrees, a 0..1 confidence score, debounced calibration detection, sensor fusion, and graceful degradation — so your app never has to lie to its users about how good the heading is.

The one-sentence difference: other compass packages report a coarse, sticky 3-level "accuracy" derived from Android's onAccuracyChanged status. precise_compass reports the OS-provided continuous heading accuracy (TYPE_ROTATION_VECTOR.values[4] on Android, CLHeading.headingAccuracy on iOS), cross-checks it against the magnetic-field magnitude to catch interference the status misses, and folds everything into one trustworthy confidence value and a non-flickering shouldCalibrate flag.

Why it exists

Compass headings are only useful if you know when not to trust them — qibla finders, AR overlays, navigation and surveying all break silently when the magnetometer is uncalibrated or sitting next to a speaker magnet. The popular packages can't tell you this reliably:

	precise_compass	flutter_compass / _v2	compassx	flutter_device_compass
Continuous accuracy (degrees)	✅ values[4] / headingAccuracy	❌ status→15/30/45 buckets	⚠️ undocumented	❌ none
Confidence score (0..1)	✅	❌	❌	❌
Magnetic-interference detection	✅ field magnitude	❌	❌	❌
Debounced shouldCalibrate	✅ hysteresis FSM	❌	⚠️ undocumented	❌
Sensor-fusion modes	✅ RV / geomagnetic / manual	⚠️ RV + fallback	RV	RV
Capability introspection	✅	❌	❌	❌
Full-orientation heading	✅	✅ (v2)	❌ portrait-only	⚠️
Pure-Dart, unit-tested core	✅ ≥90%	❌	❌	❌

Quick start

import 'package:precise_compass/precise_compass.dart';

final subscription = PreciseCompass.heading.listen((reading) {
  if (!reading.hasHeading) return; // no compass on this device

  print('${reading.heading.toStringAsFixed(0)}° '
      '± ${reading.accuracyDegrees.toStringAsFixed(0)}° '
      '· ${(reading.confidence * 100).toStringAsFixed(0)}% confident');

  if (reading.shouldCalibrate) {
    // Prompt a figure-8 motion, or let iOS show its native HUD.
  }
});

// Sensors register lazily on first listen and release on cancel:
await subscription.cancel();

What a reading tells you

class CompassReading {
  final double? headingMagnetic;        // degrees [0,360) or null
  final double? headingTrue;            // degrees [0,360) or null (needs location)
  final double  heading;                // true if available, else magnetic
  final double  accuracyDegrees;        // continuous ± estimate; -1 = unknown
  final CompassAccuracy accuracy;       // high / medium / low / unreliable / unknown
  final double  confidence;             // 0.0 … 1.0
  final bool    shouldCalibrate;        // debounced recommendation
  final CalibrationStatus calibrationStatus;
  final HeadingSource source;           // rotationVector / geomagnetic / fusion / …
  final double? magneticFieldMagnitude; // µT (Earth ≈ 25–65 µT)
  final double? pitch, roll;            // degrees, for AR
  final DateTime timestamp;
}

It never throws for a missing sensor: you get source: unavailable, accuracy: unknown, confidence: 0 instead.

Configuration

PreciseCompass.headingStream(
  config: const CompassConfig(
    reference: HeadingReference.auto,     // trueNorth / magnetic / auto
    fusionMode: FusionMode.auto,          // rotationVector / geomagnetic / fusion / auto
    rate: SensorRate.ui,                  // fastest / ui / normal / batterySaving
    smoothingFactor: null,                // null = light default; 0 = off; →1 = smoother
    detectCalibration: true,
    suppressIosCalibrationHud: false,     // true = hide native HUD, show your own
    magneticFieldBand: ClampRange(25, 65),
  ),
).listen(...);

Probe the hardware before offering features:

final caps = await PreciseCompass.capabilities();
if (caps.supportsTrueHeading) enableTrueNorthToggle();
print('Recommended mode: ${caps.recommendedMode}');

Platform setup

Android

No permission is needed for magnetic heading. For true heading the plugin reads the device's last known location to compute declination, so the host app must declare and request a location permission:

<!-- android/app/src/main/AndroidManifest.xml -->
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />

Minimum SDK: 21.

iOS

Add a usage string so the system can provide true heading (location):

<!-- ios/Runner/Info.plist -->
<key>NSLocationWhenInUseUsageDescription</key>
<string>Used to show your true (geographic) heading.</string>

Minimum iOS: 12. The compass is unavailable on the iOS Simulator.

How accuracy & calibration work

• Continuous accuracy. Android exposes the OS estimate as TYPE_ROTATION_VECTOR.values[4] (radians); iOS as CLHeading.headingAccuracy (degrees). We convert to a single accuracyDegrees (-1 when the OS provides no estimate) and bucket it into CompassAccuracy (≤10° high, ≤20° medium, ≤45° low, else unreliable).
• Interference detection. Earth's field is ≈25–65 µT. A magnitude well outside the configured band — even when the OS still claims "high" — drives the field score and shouldCalibrate down. This catches the most common real-world failure that status-based compasses miss.
• Confidence. A documented, monotonic blend of accuracy and short-term heading stability, gated by source quality and field health, in [0,1]. The exact formula is in ARCHITECTURE.md.
• Debounced calibration. A hysteresis state machine asserts shouldCalibrate only after ~1.5 s of sustained poor quality and clears it only after ~1 s of sustained good quality — no flicker, and no cold-start false positive.

See ARCHITECTURE.md for the data-flow diagram, the confidence formula, the fusion math and the calibration FSM.

Migrating

Coming from flutter_compass, flutter_compass_v2 or compassx? See MIGRATION.md for API mapping tables.

Example

The example/ app is a live test harness: a rotating dial, numeric heading, accuracy/confidence meters, a figure-8 calibration banner, a capability readout, and toggles for reference/rate/fusion mode.

Attribution & license

precise_compass is MIT-licensed. It originated as a fork of flutter_compass_v2 (itself a fork of flutter_compass by Hemanth Raj V), both MIT. The native Android orientation/​math code was rewritten from scratch to keep the entire package MIT-clean. See LICENSE.