src/Atmosphere/Pressure.hpp

   1 /*
   2 Copyright_License {
   3
   4   XCSoar Glide Computer - http://www.xcsoar.org/
   5   Copyright (C) 2000-2013 The XCSoar Project
   6   A detailed list of copyright holders can be found in the file "AUTHORS".
   7
   8   This program is free software; you can redistribute it and/or
   9   modify it under the terms of the GNU General Public License
  10   as published by the Free Software Foundation; either version 2
  11   of the License, or (at your option) any later version.
  12
  13   This program is distributed in the hope that it will be useful,
  14   but WITHOUT ANY WARRANTY; without even the implied warranty of
  15   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  16   GNU General Public License for more details.
  17
  18   You should have received a copy of the GNU General Public License
  19   along with this program; if not, write to the Free Software
  20   Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
  21 }
  22 */
  23
  24 #ifndef XCSOAR_ATMOSPHERE_PRESSURE_H
  25 #define XCSOAR_ATMOSPHERE_PRESSURE_H
  26
  27 #include "Math/fixed.hpp"
  28 #include "Compiler.h"
  29
  30 /**
  31  * ICAO Standard Atmosphere calculations (valid in Troposphere, alt<11000m)
  32  *
  33  */
  34 class AtmosphericPressure
  35 {
  36   /** Pressure in hPa */
  37   fixed value;
  38
  39   /**
  40    * @param value the pressure in hPa
  41    */
  42   explicit constexpr
  43   AtmosphericPressure(fixed _value):value(_value) {}
  44
  45 public:
  46   /**
  47    * Non-initialising constructor.
  48    */
  49   AtmosphericPressure() = default;
  50
  51   /**
  52    * Returns an object representing zero pressure.  This value doesn't
  53    * make a lot of practical sense (unless you're an astronaut), but
  54    * it may be used internally to mark an instance of this class
  55    * "invalid" (IsPlausible() returns false).
  56    */
  57   static constexpr
  58   AtmosphericPressure Zero() {
  59     return AtmosphericPressure(fixed(0));
  60   }
  61
  62   /**
  63    * Returns an object representing the standard pressure (1013.25
  64    * hPa).
  65    */
  66   static constexpr
  67   AtmosphericPressure Standard() {
  68     return AtmosphericPressure(fixed(1013.25));
  69   }
  70
  71   static constexpr
  72   AtmosphericPressure Pascal(fixed value) {
  73     return AtmosphericPressure(value / 100);
  74   }
  75
  76   static constexpr
  77   AtmosphericPressure HectoPascal(fixed value) {
  78     return AtmosphericPressure(value);
  79   }
  80
  81   /**
  82    * Is this a plausible value?
  83    */
  84   constexpr
  85   bool IsPlausible() const {
  86     return value > fixed(100) && value < fixed(1200);
  87   }
  88
  89   fixed GetPascal() const {
  90     return GetHectoPascal() * 100;
  91   }
  92
  93   /**
  94    * Access QNH value
  95    *
  96    * @return QNH value (hPa)
  97    */
  98   fixed GetHectoPascal() const {
  99     return value;
 100   }
 101
 102   /**
 103    * Calculates the current QNH by comparing a pressure value to a
 104    * known altitude of a certain location
 105    *
 106    * @param pressure Current pressure
 107    * @param alt_known Altitude of a known location (m)
 108    */
 109   gcc_const
 110   static AtmosphericPressure FindQNHFromPressure(const AtmosphericPressure pressure,
 111                                                  const fixed alt_known);
 112
 113   /**
 114    * Converts altitude with QNH=1013.25 reference to QNH adjusted altitude
 115    * @param alt 1013.25-based altitude (m)
 116    * @return QNH-based altitude (m)
 117    */
 118   gcc_pure
 119   fixed PressureAltitudeToQNHAltitude(const fixed alt) const;
 120
 121   /**
 122    * Converts QNH adjusted altitude to pressure altitude (with QNH=1013.25 as reference)
 123    * @param alt QNH-based altitude(m)
 124    * @return pressure altitude (m)
 125    */
 126   gcc_pure
 127   fixed QNHAltitudeToPressureAltitude(const fixed alt) const;
 128
 129   /**
 130    * Converts a pressure value to the corresponding QNH-based altitude
 131    *
 132    * See http://wahiduddin.net/calc/density_altitude.htm
 133    *
 134    * Example:
 135    * QNH=1014, ps=100203 => alt = 100
 136    * @see QNHAltitudeToStaticPressure
 137    * @param ps Air pressure
 138    * @return Altitude over QNH-based zero (m)
 139    */
 140   gcc_pure
 141   fixed StaticPressureToQNHAltitude(const AtmosphericPressure ps) const;
 142
 143   /**
 144    * Converts a QNH-based altitude to the corresponding pressure
 145    *
 146    * See http://wahiduddin.net/calc/density_altitude.htm
 147    *
 148    * Example:
 149    * alt= 100, QNH=1014 => ps = 100203 Pa
 150    * @see StaticPressureToAltitude
 151    * @param alt Altitude over QNH-based zero (m)
 152    * @return Air pressure at given altitude
 153    */
 154   gcc_pure
 155   AtmosphericPressure QNHAltitudeToStaticPressure(const fixed alt) const;
 156
 157   /**
 158    * Converts a pressure value to pressure altitude (with QNH=1013.25 as reference)
 159    * @param ps Air pressure
 160    * @return pressure altitude (m)
 161    */
 162   gcc_const
 163   static fixed StaticPressureToPressureAltitude(const AtmosphericPressure ps);
 164
 165   /**
 166    * Converts a 1013.25 hPa based altitude to the corresponding pressure
 167    *
 168    * @see StaticPressureToAltitude
 169    * @param alt Altitude over 1013.25 hPa based zero(m)
 170    * @return Air pressure at given altitude
 171    */
 172   gcc_const
 173   static AtmosphericPressure PressureAltitudeToStaticPressure(const fixed alt);
 174 };
 175
 176 #endif