Merge pull request #37 from oyve/copilot/add-altitude-calculation-feature

Add altitude calculation from pressure time series

Author: oyve

src/common.ts

@@ -4,4 +4,15 @@ export interface Reading {
     altitude: number, //meter
     relativeHumidity: number, //relative humidity in %
     timestamp: number
+}
+
+/**
+ * Sort readings by timestamp (oldest to newest).
+ * Returns a new sorted array without modifying the original.
+ * 
+ * @param {Reading[]} readings - Array of readings to sort.
+ * @returns {Reading[]} New array of readings sorted by timestamp (oldest to newest).
+ */
+export function sortOldestToNewest(readings: Reading[]): Reading[] {
+    return [...readings].sort((a, b) => a.timestamp - b.timestamp);
 }

src/formulas/altitude.ts

@@ -1,3 +1,6 @@
+import { Reading } from '../common';
+import * as c from '../constants';
+
 /**
  * Estimate the altitude (in meters) where the temperature drops below freezing (0°C).
  * Assumes a linear lapse rate (default: 0.0065 K/m).
@@ -16,3 +19,42 @@ export function freezingLevelAltitude(
     const altitudeDiff = (surfaceTemp - 273.15) / lapseRate;
     return surfaceAltitude + altitudeDiff;
 }
+
+/**
+ * Calculate the altitude difference based on the difference in pressure between two points.
+ * Uses the hypsometric formula (barometric formula) to calculate altitude change.
+ * 
+ * The formula used is: Δh = (R * T_avg / g) * ln(P1 / P2)
+ * where:
+ * - R is the specific gas constant for dry air (287.05 J/(kg·K))
+ * - T_avg is the average temperature between the two points
+ * - g is gravitational acceleration (9.80665 m/s²)
+ * - P1 is the pressure at the reference (lower) point
+ * - P2 is the pressure at the higher point
+ * 
+ * @param {number} referencePressure - Pressure at the reference altitude in Pascals (Pa).
+ * @param {number} currentPressure - Pressure at the current point in Pascals (Pa).
+ * @param {number} temperature - Average temperature between the two points in Kelvin (K). Defaults to 288.15 K (15°C).
+ * @returns {number} Altitude difference in meters (m). Positive values indicate higher altitude.
+ * 
+ * @example
+ * // Calculate altitude change from sea level pressure to current pressure
+ * const altitudeDiff = altitudeFromPressureDifference(101325, 95000, 288.15);
+ * console.log(altitudeDiff); // ~540 meters higher
+ * 
+ * @see https://en.wikipedia.org/wiki/Hypsometric_equation
+ */
+export function altitudeFromPressureDifference(
+    referencePressure: number,
+    currentPressure: number,
+    temperature: number = c.STANDARD_MEAN_TEMPERATURE_KELVIN
+): number {
+    const R = c.DRY_AIR_CONSTANTS.gasConstant; // 287.05 J/(kg·K) for dry air
+    const g = c.DRY_AIR_CONSTANTS.gravity; // 9.80665 m/s²
+    
+    // Hypsometric formula: Δh = (R * T / g) * ln(P1 / P2)
+    const scaleHeight = (R * temperature) / g;
+    const altitudeDifference = scaleHeight * Math.log(referencePressure / currentPressure);
+    
+    return Math.round(altitudeDifference * 100) / 100;
+}

tests/formulas/altitude.test.ts

@@ -1,4 +1,7 @@
-import { freezingLevelAltitude } from '../../src/formulas/altitude';
+import { 
+    freezingLevelAltitude, 
+    altitudeFromPressureDifference
+} from '../../src/formulas/altitude';
 import { cloudBaseHeight } from '../../src/phenomena/cloud';
 
 describe('freezingLevelHeight', () => {
@@ -88,3 +91,56 @@ describe('cloudBaseHeight', () => {
         expect(result).toBeCloseTo(1249.4, 2);
     });
 });
+
+describe('altitudeFromPressureDifference', () => {
+    it('should return 0 when pressures are equal', () => {
+        const result = altitudeFromPressureDifference(101325, 101325);
+        expect(result).toBe(0);
+    });
+
+    it('should calculate positive altitude difference when current pressure is lower', () => {
+        // Sea level pressure to ~1000m altitude pressure
+        // Using standard temperature 288.15K (15°C)
+        const result = altitudeFromPressureDifference(101325, 89874.46, 288.15);
+        // Expected: ~1000m (approximately, based on barometric formula)
+        // The hypsometric formula may give slightly different results than barometric
+        expect(result).toBeCloseTo(1011, 0); // Within 1 meter
+    });
+
+    it('should calculate negative altitude difference when current pressure is higher', () => {
+        // From higher altitude to sea level (going down)
+        const result = altitudeFromPressureDifference(89874.46, 101325, 288.15);
+        // Expected: ~-1000m (going down)
+        expect(result).toBeCloseTo(-1011, 0); // Within 1 meter
+    });
+
+    it('should calculate altitude difference at standard sea level to 500m', () => {
+        // Sea level standard: 101325 Pa
+        // At 500m altitude: ~95461 Pa (approximately)
+        // Using hypsometric formula: Δh = (R * T / g) * ln(P1/P2)
+        // With R=287.05, T=288.15K, g=9.80665
+        // Scale height = 287.05 * 288.15 / 9.80665 = 8434.5 m
+        // For ~500m: P2 = P1 * exp(-500/8434.5) = 101325 * 0.9424 = 95461 Pa
+        const result = altitudeFromPressureDifference(101325, 95461, 288.15);
+        expect(result).toBeCloseTo(500, -1); // Within 10 meters
+    });
+
+    it('should account for temperature in calculations', () => {
+        // Same pressure difference at different temperatures should yield different altitudes
+        const coldTemp = 273.15; // 0°C
+        const warmTemp = 303.15; // 30°C
+        
+        const resultCold = altitudeFromPressureDifference(101325, 95000, coldTemp);
+        const resultWarm = altitudeFromPressureDifference(101325, 95000, warmTemp);
+        
+        // Warmer air is less dense, so same pressure drop represents larger altitude change
+        expect(resultWarm).toBeGreaterThan(resultCold);
+    });
+
+    it('should use default temperature when not provided', () => {
+        const resultWithDefault = altitudeFromPressureDifference(101325, 95000);
+        const resultWithExplicit = altitudeFromPressureDifference(101325, 95000, 288.15);
+        
+        expect(resultWithDefault).toBe(resultWithExplicit);
+    });
+});