From bbbdef0770bccfa1b0dd62fd32bc4fc3c196b2cb Mon Sep 17 00:00:00 2001
From: justin4046 <71111866+justin4046@users.noreply.github.com>
Date: Thu, 2 May 2024 22:45:12 -0500
Subject: [PATCH] add altimeter pressure correction

---
 html/config.js   |  1 +
 html/defaults.js |  1 +
 html/index.html  |  9 +++++++--
 html/script.js   | 48 ++++++++++++++++++++++++++++++++++++++++++++++--
 4 files changed, 55 insertions(+), 4 deletions(-)

diff --git a/html/config.js b/html/config.js
index f31b2af..8f316c3 100644
--- a/html/config.js
+++ b/html/config.js
@@ -331,6 +331,7 @@ HideCols = [
 
 // labelsGeom = false; // labels: uses geometric altitude (WGS84 ellipsoid unless geomUseEGM is enabled
 // geomUseEGM = false; // use EGM96 for displaying geometric altitudes (extra load time!)
+// baroUseQNH = false;
 
 // windLabelsSlim = false;
 // showLabelUnits = true;
diff --git a/html/defaults.js b/html/defaults.js
index 74f0b6e..f877499 100644
--- a/html/defaults.js
+++ b/html/defaults.js
@@ -344,6 +344,7 @@ let showSil = false;
 
 let labelsGeom = false; // labels: uses geometric altitude (WGS84 ellipsoid unless geomUseEGM is enabled
 let geomUseEGM = false; // use EGM96 for displaying geometric altitudes (extra load time!)
+let baroUseQNH = false;
 
 let windLabelsSlim = false;
 let showLabelUnits = true;
diff --git a/html/index.html b/html/index.html
index e0f7259..93c7c49 100644
--- a/html/index.html
+++ b/html/index.html
@@ -297,7 +297,7 @@
               </tr>
               <tr>
                 <td>
-                  <span class="infoHeading" title="The uncorrected pressure-derived height of the aircraft above mean sea level (based on barometric pressure)">Baro. Altitude</span>:
+                  <span class="infoHeading" id="selected_altitude1_title" title="The uncorrected pressure-derived height of the aircraft above mean sea level (based on barometric pressure)">Baro. Altitude</span>:
                   <span class="infoData" id="selected_altitude1"></span>
                 </td>
               </tr>
@@ -519,7 +519,7 @@
           <div class="infoBlockSection">
             <div>
               <div class="infoHeading">
-                <span title="The uncorrected pressure-derived height of the aircraft above mean sea level (based on barometric pressure)">Barometric</span>:
+                <span id="selected_altitude2_title" title="The uncorrected pressure-derived height of the aircraft above mean sea level (based on barometric pressure)">Barometric</span>:
               </div>
               <div class="infoData">
                 <span id="selected_altitude2"></span>
@@ -901,6 +901,11 @@
                       <td id="dump1090_total_history_td"><span class="infoBlockTitleText">History:</span> <span id="dump1090_total_history">n/a</span> positions</td>
                     </tr>
                   </table>
+                  <table style="width: 100%" id="infoblock_altimeter" class="hidden">
+                    <tr>
+                      <td><span class="infoBlockTitleText">Altimeter:<span> <span><input id="settings_altimeter" type="number" step="0.01" value="29.92">
+                    </tr>
+                  </table>
                 </td>
               </tr>
             </table>
diff --git a/html/script.js b/html/script.js
index 9e74d48..9b010b7 100644
--- a/html/script.js
+++ b/html/script.js
@@ -1247,6 +1247,28 @@ jQuery('#selected_altitude_geom1')
         }
     });
 
+    new Toggle({
+        key: "baroUseQNH",
+        display: "Baro. alt.: Standard pressure -> Altimeter corrected",
+        container: "#settingsLeft",
+        init: baroUseQNH,
+        setState: function(state) {
+            baroUseQNH = state;
+            if (baroUseQNH) {
+                jQuery('#selected_altitude1_title').updateText('Baro. altitude');
+                jQuery('#selected_altitude2_title').updateText('Adjusted Barometric');
+                jQuery('#infoblock_altimeter').removeClass('hidden');
+            } else {
+                jQuery('#selected_altitude1_title').updateText('Baro. altitude');
+                jQuery('#selected_altitude2_title').updateText('Adjusted Barometric');
+                jQuery('#infoblock_altimeter').addClass('hidden');
+            }
+            if (loadFinished) {
+                remakeTrails();
+                refreshSelected();
+            }
+        }
+    });
 
     if (usp.has('labelsGeom')) {
         toggles['labelsGeom'].toggle(true, 'init');
@@ -3127,8 +3149,8 @@ function refreshSelected() {
     }
 
 
-    jQuery("#selected_altitude1").updateText(format_altitude_long(selected.altitude, selected.vert_rate, DisplayUnits));
-    jQuery("#selected_altitude2").updateText(format_altitude_long(selected.altitude, selected.vert_rate, DisplayUnits));
+    jQuery("#selected_altitude1").updateText(format_altitude_long(adjust_baro_alt(selected.altitude), selected.vert_rate, DisplayUnits));
+    jQuery("#selected_altitude2").updateText(format_altitude_long(adjust_baro_alt(selected.altitude), selected.vert_rate, DisplayUnits));
 
     jQuery('#selected_onground').updateText(format_onground(selected.altitude));
 
@@ -8434,6 +8456,28 @@ function getn(n) {
     limitUpdates=n; RefreshInterval=0; fetchCalls=0;
 }
 
+function adjust_baro_alt(alt) {
+    if (baroUseQNH && alt != null) {
+        let altimeter = document.getElementById("settings_altimeter").value
+        if (altimeter < 100) {
+            return corrected_alt(alt, inhg_to_hpa(altimeter));
+        } else {
+            return corrected_alt(alt, altimeter);
+        }
+    } else {
+        return alt;
+    }
+}
+
+function corrected_alt(alt, qnh) {
+    let station_pressure = 29.92 * Math.pow( ( (288-0.0065*(0.3048 * alt)) / 288), 5.2561);
+    return ( 1 - Math.pow( (inhg_to_hpa(station_pressure) / qnh ), 0.190284) ) * 145366.45;
+}
+
+function inhg_to_hpa(inhg) {
+    return 33.8639 * inhg
+}
+
 function globeRateUpdate() {
     if (adsbexchange) {
         dynGlobeRate = true;