diff --git a/samples/blackmiscquantities/samplesphysicalquantities.cpp b/samples/blackmiscquantities/samplesphysicalquantities.cpp
index ad9056bc1..2bc22c39c 100644
--- a/samples/blackmiscquantities/samplesphysicalquantities.cpp
+++ b/samples/blackmiscquantities/samplesphysicalquantities.cpp
@@ -11,6 +11,7 @@
 //! \ingroup sampleblackmiscquantities
 
 #include "samplesphysicalquantities.h"
+#include "blackmisc/aviation/altitude.h"
 #include "blackmisc/pq/acceleration.h"
 #include "blackmisc/pq/angle.h"
 #include "blackmisc/pq/frequency.h"
@@ -28,6 +29,7 @@
 #include <QTextStream>
 
 using namespace BlackMisc;
+using namespace BlackMisc::Aviation;
 using namespace BlackMisc::PhysicalQuantities;
 
 namespace BlackSample
@@ -96,7 +98,7 @@ namespace BlackSample
         w2.switchUnit(CMassUnit::lb());
         out << w1 << " " << w1.valueRoundedWithUnit(CMassUnit::kg()) << " " << w2 << endl;
 
-        CPressure p1(1013.25, CPressureUnit::hPa());
+        CPressure p1(CAltitude::standardISASeaLevelPressure());
         out << p1 << " " << p1.valueRoundedWithUnit(CPressureUnit::psi()) << " " << p1.valueRoundedWithUnit(CPressureUnit::inHg()) << endl;
 
         CTemperature t1;
@@ -130,5 +132,4 @@ namespace BlackSample
         out << "-----------------------------------------------" << endl;
         return 0;
     }
-
 } // namespace
diff --git a/src/blackgui/editors/situationform.cpp b/src/blackgui/editors/situationform.cpp
index 03dd42415..2fbd1be85 100644
--- a/src/blackgui/editors/situationform.cpp
+++ b/src/blackgui/editors/situationform.cpp
@@ -17,6 +17,7 @@
 
 using namespace BlackMisc;
 using namespace BlackMisc::Aviation;
+using namespace BlackMisc::Geo;
 using namespace BlackMisc::PhysicalQuantities;
 
 namespace BlackGui
@@ -57,12 +58,12 @@ namespace BlackGui
 
         CAircraftSituation CSituationForm::getSituation() const
         {
-            const BlackMisc::Geo::CCoordinateGeodetic position = ui->comp_Coordinate->getCoordinate();
+            const CCoordinateGeodetic position = ui->comp_Coordinate->getCoordinate();
+            const CAltitude pressureAltitude(position.geodeticHeight().toPressureAltitude(this->getBarometricPressureMsl()));
+
             CAircraftSituation s(position);
             s.setBank(this->getBankAngle());
             s.setPitch(this->getPitchAngle());
-
-            CAltitude pressureAltitude(position.geodeticHeight().toPressureAltitude(this->getBarometricPressureMsl()));
             s.setPressureAltitude(pressureAltitude);
             return s;
         }
@@ -100,13 +101,13 @@ namespace BlackGui
             const QString v(ui->le_Pressure->text().replace(',', '.'));
             bool ok;
             double vd = v.toDouble(&ok);
-            if (!ok) { vd = 1013.25; }
+            if (!ok) { vd = CAltitude::standardISASeaLevelPressure().value(CPressureUnit::mbar()); }
             return vd;
         }
 
         CPressure CSituationForm::getBarometricPressureMsl() const
         {
-            return CPressure (getBarometricPressureMslMillibar(), CPressureUnit::mbar());
+            return CPressure(getBarometricPressureMslMillibar(), CPressureUnit::mbar());
         }
 
         void CSituationForm::setReadOnly(bool readonly)
@@ -207,8 +208,10 @@ namespace BlackGui
 
         void CSituationForm::resetPressure()
         {
-            ui->le_Pressure->setText("1013.00");
-            ui->hs_Pressure->setValue(1013);
+            static const int v = CAltitude::standardISASeaLevelPressure().valueInteger(CPressureUnit::mbar());
+            static const QString vs(dotToLocaleDecimalPoint(QString::number(CAltitude::standardISASeaLevelPressure().valueRounded(CPressureUnit::mbar(), 2))));
+            ui->le_Pressure->setText(vs);
+            ui->hs_Pressure->setValue(v);
         }
     } // ns
 } // ns
diff --git a/src/blackgui/editors/situationform.h b/src/blackgui/editors/situationform.h
index 56030ac81..4dce47123 100644
--- a/src/blackgui/editors/situationform.h
+++ b/src/blackgui/editors/situationform.h
@@ -74,7 +74,7 @@ namespace BlackGui
             //! Get pitch angle
             double getPitchAngleDegrees() const;
 
-            //! Get barometric pressure at mean sea level
+            //! Get barometric pressure at MSL (mean sea level)
             BlackMisc::PhysicalQuantities::CPressure getBarometricPressureMsl() const;
 
             //! Get pressure at mean sea level
diff --git a/src/blackmisc/aviation/altitude.cpp b/src/blackmisc/aviation/altitude.cpp
index 8fcaf4e91..3441b08bd 100644
--- a/src/blackmisc/aviation/altitude.cpp
+++ b/src/blackmisc/aviation/altitude.cpp
@@ -11,6 +11,7 @@
 #include "blackmisc/iconlist.h"
 #include "blackmisc/icons.h"
 #include "blackmisc/pq/measurementunit.h"
+#include "blackmisc/pq/constants.h"
 #include "blackmisc/pq/pqstring.h"
 
 #include <Qt>
@@ -22,7 +23,7 @@ namespace BlackMisc
 {
     namespace Aviation
     {
-        CAltitude::CAltitude(const QString &altitudeAsString, PhysicalQuantities::CPqString::SeparatorMode mode) : CLength(0, BlackMisc::PhysicalQuantities::CLengthUnit::m()), m_datum(MeanSeaLevel)
+        CAltitude::CAltitude(const QString &altitudeAsString, CPqString::SeparatorMode mode) : CLength(0, CLengthUnit::m()), m_datum(MeanSeaLevel)
         {
             this->parseFromString(altitudeAsString, mode);
         }
@@ -31,7 +32,7 @@ namespace BlackMisc
         {
             if (this->m_datum == FlightLevel)
             {
-                int fl = qRound(this->CLength::value(CLengthUnit::ft()) / 100.0);
+                const int fl = qRound(this->CLength::value(CLengthUnit::ft()) / 100.0);
                 return QString("FL%1").arg(fl);
             }
             else
@@ -45,7 +46,7 @@ namespace BlackMisc
                 {
                     s = this->CLength::valueRoundedWithUnit(CLengthUnit::ft(), 0, i18n);
                 }
-                return s.append(this->isMeanSeaLevel() ? " MSL" : " AGL");
+                return s.append(this->isMeanSeaLevel() ? QStringLiteral(" MSL") : QStringLiteral(" AGL"));
             }
         }
 
@@ -81,10 +82,10 @@ namespace BlackMisc
 
         void CAltitude::parseFromString(const QString &value)
         {
-            this->parseFromString(value, PhysicalQuantities::CPqString::SeparatorsCLocale);
+            this->parseFromString(value, CPqString::SeparatorsCLocale);
         }
 
-        void CAltitude::parseFromString(const QString &value, PhysicalQuantities::CPqString::SeparatorMode mode)
+        void CAltitude::parseFromString(const QString &value, CPqString::SeparatorMode mode)
         {
             QString v = value.trimmed();
 
@@ -94,8 +95,8 @@ namespace BlackMisc
                 v = v.replace("FL", "", Qt::CaseInsensitive).trimmed();
                 bool ok = false;
                 double dv = v.toDouble(&ok) * 100.0;
-                CAltitude a(ok ? dv : 0.0, FlightLevel,
-                            ok ? CLengthUnit::ft() : nullptr);
+                const CAltitude a(ok ? dv : 0.0, FlightLevel,
+                                  ok ? CLengthUnit::ft() : nullptr);
                 *this = a;
                 return;
             }
@@ -113,7 +114,7 @@ namespace BlackMisc
                 rd = AboveGround;
             }
 
-            const CLength l = BlackMisc::PhysicalQuantities::CPqString::parse<CLength>(v, mode);
+            const CLength l = CPqString::parse<CLength>(v, mode);
             *this = CAltitude(l, rd);
         }
 
@@ -313,5 +314,12 @@ namespace BlackMisc
             static const CAltitude null(0, CAltitude::MeanSeaLevel, CLengthUnit::nullUnit());
             return null;
         }
+
+        const CPressure &CAltitude::standardISASeaLevelPressure()
+        {
+            // Average sea-level pressure is 1013.25mbar or 1013.25hPa
+            static const CPressure standardPressure(CPhysicalQuantitiesConstants::ISASeaLevelPressure());
+            return standardPressure;
+        }
     } // namespace
 } // namespace
diff --git a/src/blackmisc/aviation/altitude.h b/src/blackmisc/aviation/altitude.h
index 61eeef597..dd68ab381 100644
--- a/src/blackmisc/aviation/altitude.h
+++ b/src/blackmisc/aviation/altitude.h
@@ -168,6 +168,9 @@ namespace BlackMisc
             //! Null altitude (MSL)
             static const CAltitude &null();
 
+            //! Standard pressure 1013.25mbar/hPa
+            static const PhysicalQuantities::CPressure &standardISASeaLevelPressure();
+
         private:
             ReferenceDatum m_datum; //!< MSL or AGL?
             AltitudeType m_altitudeType = TrueAltitude;
diff --git a/src/blackmisc/pq/constants.h b/src/blackmisc/pq/constants.h
index bcb9e1682..cac179ce3 100644
--- a/src/blackmisc/pq/constants.h
+++ b/src/blackmisc/pq/constants.h
@@ -19,7 +19,6 @@ namespace BlackMisc
 {
     namespace PhysicalQuantities
     {
-
         /*!
          * Physical quantities constants
          */
@@ -43,14 +42,14 @@ namespace BlackMisc
             //! International Standard Atmosphere pressure at mean sea level, 1013.25hPa
             static const CPressure &ISASeaLevelPressure()
             {
-                static CPressure p(1013.25, CPressureUnit::hPa());
+                static CPressure p(1013.25, CPressureUnit::mbar());
                 return p;
             }
 
             //! ICAO standard pressure datum for flight levels, 1013.2hPa
             static const CPressure &ICAOFlightLevelPressure()
             {
-                static CPressure p(1013.2, CPressureUnit::hPa());
+                static CPressure p(1013.2, CPressureUnit::mbar());
                 return p;
             }
 
diff --git a/src/blackmisc/pq/pressure.h b/src/blackmisc/pq/pressure.h
index 3829f1f4e..fdb666c9e 100644
--- a/src/blackmisc/pq/pressure.h
+++ b/src/blackmisc/pq/pressure.h
@@ -34,8 +34,8 @@ namespace BlackMisc
             //! \copydoc CPhysicalQuantity(const QString &unitString)
             CPressure(const QString &unitString) : CPhysicalQuantity(unitString) {}
         };
-    }
-}
+    } // ns
+} // ns
 
 Q_DECLARE_METATYPE(BlackMisc::PhysicalQuantities::CPressure)
 
diff --git a/src/blackmisc/stringutils.cpp b/src/blackmisc/stringutils.cpp
index 7a565895e..1f9a8379d 100644
--- a/src/blackmisc/stringutils.cpp
+++ b/src/blackmisc/stringutils.cpp
@@ -300,6 +300,12 @@ namespace BlackMisc
         return input.replace('.', QLocale::system().decimalPoint());
     }
 
+    QString dotToLocaleDecimalPoint(const QString &input)
+    {
+        QString copy(input);
+        return copy.replace('.', QLocale::system().decimalPoint());
+    }
+
     bool stringCompare(const QString &c1, const QString &c2, Qt::CaseSensitivity cs)
     {
         if (cs == Qt::CaseSensitive) { return c1 == c2; }
diff --git a/src/blackmisc/stringutils.h b/src/blackmisc/stringutils.h
index 7a39c63d1..b2f87da76 100644
--- a/src/blackmisc/stringutils.h
+++ b/src/blackmisc/stringutils.h
@@ -145,6 +145,9 @@ namespace BlackMisc
     //! Replace dot '.' by locale decimal point
     BLACKMISC_EXPORT QString dotToLocaleDecimalPoint(QString &input);
 
+    //! Replace dot '.' by locale decimal point
+    BLACKMISC_EXPORT QString dotToLocaleDecimalPoint(const QString &input);
+
     //! Int to hex value (per byte, 2 digits)
     BLACKMISC_EXPORT QString bytesToHexString(const QByteArray &bytes);
 
diff --git a/src/blackmisc/weather/gridpoint.h b/src/blackmisc/weather/gridpoint.h
index 844e583d8..4e58960ea 100644
--- a/src/blackmisc/weather/gridpoint.h
+++ b/src/blackmisc/weather/gridpoint.h
@@ -12,18 +12,19 @@
 #ifndef BLACKMISC_WEATHER_GRIDPOINT_H
 #define BLACKMISC_WEATHER_GRIDPOINT_H
 
-#include "blackmisc/blackmiscexport.h"
-#include "blackmisc/geo/coordinategeodetic.h"
-#include "blackmisc/metaclass.h"
-#include "blackmisc/pq/pressure.h"
-#include "blackmisc/pq/units.h"
-#include "blackmisc/propertyindex.h"
-#include "blackmisc/valueobject.h"
-#include "blackmisc/variant.h"
 #include "blackmisc/weather/cloudlayerlist.h"
 #include "blackmisc/weather/temperaturelayerlist.h"
 #include "blackmisc/weather/visibilitylayerlist.h"
 #include "blackmisc/weather/windlayerlist.h"
+#include "blackmisc/geo/coordinategeodetic.h"
+#include "blackmisc/pq/pressure.h"
+#include "blackmisc/pq/constants.h"
+#include "blackmisc/pq/units.h"
+#include "blackmisc/blackmiscexport.h"
+#include "blackmisc/propertyindex.h"
+#include "blackmisc/valueobject.h"
+#include "blackmisc/variant.h"
+#include "blackmisc/metaclass.h"
 
 #include <QMetaType>
 #include <QString>
@@ -41,7 +42,7 @@ namespace BlackMisc
             //! Properties by index
             enum ColumnIndex
             {
-                IndexIdentifier = BlackMisc::CPropertyIndex::GlobalIndexCGridPoint,
+                IndexIdentifier = CPropertyIndex::GlobalIndexCGridPoint,
                 IndexPosition,
                 IndexCloudLayers,
                 IndexTemperatureLayers,
@@ -69,37 +70,37 @@ namespace BlackMisc
             void setIdentifier(const QString &identifier) { m_identifier = identifier; }
 
             //! Get identifier
-            QString getIdentifier() const { return m_identifier; }
+            const QString &getIdentifier() const { return m_identifier; }
 
             //! Set position
-            void setPosition(const BlackMisc::Geo::CCoordinateGeodetic &position) { m_position = position; }
+            void setPosition(const Geo::CCoordinateGeodetic &position) { m_position = position; }
 
             //! Get position
-            const BlackMisc::Geo::CCoordinateGeodetic getPosition() const { return m_position; }
+            const Geo::CCoordinateGeodetic &getPosition() const { return m_position; }
 
             //! Set cloud layers
             void setCloudLayers(const CCloudLayerList &cloudLayers) { m_cloudLayers = cloudLayers; }
 
             //! Get cloud layers
-            CCloudLayerList getCloudLayers() const { return m_cloudLayers; }
+            const CCloudLayerList &getCloudLayers() const { return m_cloudLayers; }
 
             //! Set temperature layers
             void setTemperatureLayers(const CTemperatureLayerList &temperatureLayers) { m_temperatureLayers = temperatureLayers; }
 
             //! Get temperature layers
-            CTemperatureLayerList getTemperatureLayers() const { return m_temperatureLayers; }
+            const CTemperatureLayerList &getTemperatureLayers() const { return m_temperatureLayers; }
 
             //! Set visibility layers
             void setVisibilityLayers(const CVisibilityLayerList &visibilityLayers) { m_visibilityLayers = visibilityLayers; }
 
             //! Get visibility layers
-            CVisibilityLayerList getVisibilityLayers() const { return m_visibilityLayers; }
+            const CVisibilityLayerList &getVisibilityLayers() const { return m_visibilityLayers; }
 
             //! Set wind layers
             void setWindLayers(const CWindLayerList &windLayers) { m_windLayers = windLayers; }
 
             //! Get wind layers
-            CWindLayerList getWindLayers() const { return m_windLayers; }
+            const CWindLayerList &getWindLayers() const { return m_windLayers; }
 
             //! Copies all weather data from other without modifying identifier and position.
             void copyWeatherDataFrom(const CGridPoint &other);
@@ -108,7 +109,7 @@ namespace BlackMisc
             void setSurfacePressure(const PhysicalQuantities::CPressure &pressure) { m_surfacePressure = pressure; }
 
             //! Get surface pressure
-            PhysicalQuantities::CPressure getSurfacePressure() const { return m_surfacePressure; }
+            const PhysicalQuantities::CPressure &getSurfacePressure() const { return m_surfacePressure; }
 
             //! \copydoc BlackMisc::Mixin::Index::propertyByIndex
             CVariant propertyByIndex(const BlackMisc::CPropertyIndex &index) const;
@@ -120,14 +121,13 @@ namespace BlackMisc
             QString convertToQString(bool i18n = false) const;
 
         private:
-            // Identifier is intentionally string based. MSFS uses ICAO but others don't.
-            QString m_identifier;
-            BlackMisc::Geo::CCoordinateGeodetic m_position;
+            QString m_identifier; //!< Identifier is intentionally string based. MSFS uses ICAO, but others don't.
+            Geo::CCoordinateGeodetic m_position;
             CCloudLayerList m_cloudLayers;
             CTemperatureLayerList m_temperatureLayers;
             CVisibilityLayerList m_visibilityLayers;
             CWindLayerList m_windLayers;
-            PhysicalQuantities::CPressure m_surfacePressure = { 1013.25, PhysicalQuantities::CPressureUnit::hPa() };
+            PhysicalQuantities::CPressure m_surfacePressure = { PhysicalQuantities::CPhysicalQuantitiesConstants::ISASeaLevelPressure() };
 
             BLACK_METACLASS(
                 CGridPoint,
diff --git a/src/blackmisc/weather/weathergrid.cpp b/src/blackmisc/weather/weathergrid.cpp
index 31cf99ed4..425547ffb 100644
--- a/src/blackmisc/weather/weathergrid.cpp
+++ b/src/blackmisc/weather/weathergrid.cpp
@@ -110,7 +110,7 @@ namespace BlackMisc
                 CTemperatureLayerList { temperatureLayer },
                 CVisibilityLayerList { visibilityLayer } ,
                 CWindLayerList { windLayer },
-                { 1013.25, PhysicalQuantities::CPressureUnit::hPa() }
+                { CAltitude::standardISASeaLevelPressure() }
             };
 
             static const CWeatherGrid weatherGrid = { gridPointGLOB };
@@ -162,7 +162,7 @@ namespace BlackMisc
                 CTemperatureLayerList { temperatureLayer },
                 CVisibilityLayerList { visibilityLayer },
                 CWindLayerList { windLayer1, windLayer2 },
-                { 1013.25, PhysicalQuantities::CPressureUnit::hPa() }
+                { CAltitude::standardISASeaLevelPressure() }
             };
 
             static const CWeatherGrid weatherGrid({ gridPointGLOB });
diff --git a/src/plugins/weatherdata/gfs/weatherdatagfs.cpp b/src/plugins/weatherdata/gfs/weatherdatagfs.cpp
index 1996b7b32..89ac7501c 100644
--- a/src/plugins/weatherdata/gfs/weatherdatagfs.cpp
+++ b/src/plugins/weatherdata/gfs/weatherdatagfs.cpp
@@ -46,8 +46,9 @@ namespace BlackWxPlugin
 
         double millibarToLevel(double millibar)
         {
+            static const double hPaStandardPressure = CPhysicalQuantitiesConstants::ICAOFlightLevelPressure().value(CPressureUnit::mbar());
             millibar /= 100;
-            double level = (1 - std::pow(millibar / 1013.25, 0.190284)) * 145366.45;
+            double level = (1 - std::pow(millibar / hPaStandardPressure, 0.190284)) * 145366.45;
             return level;
         }
 
diff --git a/tests/blackmisc/testphysicalquantities.cpp b/tests/blackmisc/testphysicalquantities.cpp
index fbbb2fa6e..768b7fd03 100644
--- a/tests/blackmisc/testphysicalquantities.cpp
+++ b/tests/blackmisc/testphysicalquantities.cpp
@@ -162,7 +162,7 @@ namespace BlackMiscTest
      */
     void CTestPhysicalQuantities::pressureTests()
     {
-        const CPressure p1(1013.25, CPressureUnit::hPa());
+        const CPressure p1(CPhysicalQuantitiesConstants::ISASeaLevelPressure());
         const CPressure p2(29.92, CPressureUnit::inHg());
         CPressure p4(p1);
         p4.switchUnit(CPressureUnit::mbar());