altimeter: Display sea-level pressure, too, and allow calibration

Add display of sea-level pressure and temperature, and allow
calibration by setting the altitude. That way, you can get precise
altitude if you already know sea-level pressure (from example from
forecast), or you can calibrate using known altitude and have precise
readings for a while.

apps/altimeter/ChangeLog

@@ -1,2 +1,3 @@
 0.01: New App!
 0.02: Actually upload correct code
+0.03: Display sea-level pressure, too, and allow calibration

apps/altimeter/app.js

@@ -1,4 +1,4 @@
-Bangle.setBarometerPower(true, "app");
+Bangle.setBarometerPower(true, "altimeter");
 
 g.clear(1);
 Bangle.loadWidgets();
@@ -10,21 +10,62 @@ var MEDIANLENGTH = 20;
 var avr = [], median;
 var value = 0;
 
+function getStandardPressure(altitude) {
+  const P0 = 1013.25; // standard pressure at sea level in hPa
+  const T0 = 288.15; // standard temperature at sea level in K
+  const g0 = 9.80665; // standard gravitational acceleration in m/s^2
+  const R = 8.31432; // gas constant in J/(mol*K)
+  const M = 0.0289644; // molar mass of air in kg/mol
+  const L = -0.0065; // temperature lapse rate in K/m
+
+  const temperature = T0 + L * altitude; // temperature at the given altitude
+  const pressure = P0 * Math.pow((temperature / T0), (-g0 * M) / (R * L)); // pressure at the given altitude
+
+  return pressure;
+}
+
+function convertToSeaLevelPressure(pressure, altitude) {
+  return 1013.25 * (pressure / getStandardPressure(altitude));
+}
+
 Bangle.on('pressure', function(e) {
   while (avr.length>MEDIANLENGTH) avr.pop();
   avr.unshift(e.altitude);
   median = avr.slice().sort();
-  g.reset().clearRect(0,y-30,g.getWidth()-10,y+30);
+  g.reset().clearRect(0,y-30,g.getWidth()-10,R.h);
   if (median.length>10) {
     var mid = median.length>>1;
     value = E.sum(median.slice(mid-4,mid+5)) / 9;
-    g.setFont("Vector",50).setFontAlign(0,0).drawString((value-zero).toFixed(1), g.getWidth()/2, y);
+    t = value-zero;
+    if ((t > -100) && (t < 1000))
+      t = t.toFixed(1);
+    else
+      t = t.toFixed(0);
+    g.setFont("Vector",50).setFontAlign(0,0).drawString(t, g.getWidth()/2, y);
+    sea = convertToSeaLevelPressure(e.pressure, value-zero);
+    t = sea.toFixed(1) + " " + e.temperature.toFixed(1);
+    if (0) {
+      print("alt raw:", value.toFixed(1));
+      print("temperature:", e.temperature);
+      print("pressure:", e.pressure);
+      print("sea pressure:", sea);
+      print("std pressure:", getStandardPressure(value-zero));
+    }
+    g.setFont("Vector",25).setFontAlign(-1,0).drawString(t,
+                                                        10, R.y+R.h - 35);
   }
 });
 
+print(g.getFonts());
 g.reset();
-g.setFont("6x8").setFontAlign(0,0).drawString(/*LANG*/"ALTITUDE (m)", g.getWidth()/2, y-40);
+g.setFont("Vector:15");
+g.setFontAlign(0,0);
+g.drawString(/*LANG*/"ALTITUDE (m)", g.getWidth()/2, y-40);
+g.drawString(/*LANG*/"SEA L (hPa) TEMP (C)", g.getWidth()/2, y+62);
+g.flip();
 g.setFont("6x8").setFontAlign(0,0,3).drawString(/*LANG*/"ZERO", g.getWidth()-5, g.getHeight()/2);
-setWatch(function() {
-  zero = value;
-}, (process.env.HWVERSION==2) ? BTN1 : BTN2, {repeat:true});
+Bangle.setUI("updown", btn=> {
+  if (!btn) zero=value;
+  if (btn<0) zero-=5;
+  if (btn>0) zero+=5;
+});

apps/altimeter/metadata.json

@@ -1,6 +1,6 @@
 { "id": "altimeter",
   "name": "Altimeter",
-  "version":"0.02",
+  "version":"0.03",
   "description": "Simple altimeter that can display height changed using Bangle.js 2's built in pressure sensor.",
   "icon": "app.png",
   "tags": "tool,outdoors",