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Guest Patrick_Waugh

Altitudes, their definitions and XML VARS

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Hi,I intend to make a drawing with the various altitudes visualised and would like some help. I am aware of the fact that in aviation we are dealing with severeal altitudes. It can become a little bit confusing once in a while.I am looking for a definition and above all the A:Var Paramater for the following altitudes of which some will defintely be the same:1. Ground altitude2. Ground elevation3. Ground level4. Above ground level5. Above sealevel6. Radio altitude7. Aircraft altitude8. Runway elevationI left pressure and transition altitude out as they would not serve a purpose in this list. Decision altitude and hight may also not belong there.There might be additional that I have not listed. Please feel free to add them.Thanks for your time,Roelof

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Hi,Some Google, Wikipedia, will give you the definitions of altitude and elevation.Mostly in reference with a certain point or pressure.Altitude is the elevation of an object from a known level or datum. Common datums are mean sea level and the surface of the WGS-84 geoid, used by GPS. The elevation of a geographic location is its height above a fixed reference point, often the mean sea level. Elevation, or geometric height, is mainly used when referring to points on the Earth's surface, while altitude or geopotential height is used for points above the surface, such as an aircraft in flight or a spacecraft in orbit.The Var's are:(A:DECISION HEIGHT,feet)(A:INDICATED ALTITUDE,feet)(A:KOHLSMAN SETTING HG,inHg) (A:KOHLSMAN SETTING MB,mbar)(A:PLANE ALT ABOVE GROUND,feet) (A:PLANE ALTITUDE,feet)(A:PRESSURE ALTITUDE,feet) (A:RADIO HEIGHT,feet)(A:SIM ON GROUND,bool)(A:GROUND ALTITUDE,feet) (A:ARTIFICIAL GROUND ELEVATION,feet)(A:AUTOPILOT ALTITUDE LOCK VAR,feet) (A:AUTOPILOT ALTITUDE LOCK,bool) (A:AUTOPILOT ALTITUDE MANUALLY TUNABLE,bool) (A:GPS POSITION ALT,feet) (A:GPS TARGET ALTITUDE,feet)(A:GPS WP NEXT ALT,feet) (A:GPS WP PREV ALT,feet) FlightPlanWaypointApproachAltitudeFlightPlanWaypointMinAltitudeFlightPlanWaypointAltitudeFlightPlanCruisingAltitude FlightPlanAlternateAltitude FlightPlanDestinationAltitudeFlightPlanDepartureAltitudeNearestAirspaceCurrentMaxAltitudeNearestAirspaceCurrentMinAltitudeNearestAirspaceNearAltitudeNearestAirspaceCurrentAltitudeWaypointVorElevationWaypointAirportRunwayElevationWaypointAirportElevationProbably some more and you can calculate altitude from pressure and temperature.Jan"Beatus ille qui procul negotiis..."

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Hi Jan,As always the first to help people out. This is a lot more than I expected but not exactly what I am looking for. To tell you the truth it adds a little to my confusion :-) I have seen most of them in the Avar-Parameters.doc I'll attach a pic.What I would like to know is the corresponding A:Vars for the various altitudes you see on it if they are available of course. Hope this clears it up a bit.Roelof

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Well, as far as i can see:Above field:(A:PLANE ALT ABOVE GROUND,feet) or (A:RADIO HEIGHT,feet)QFE is pressure at elevation.Otherwise:(A:INDICATED ALTITUDE,feet)QNH is indicated altitude at local barometer setting above MSL.QNE is indicated altitude at standard barometer setting (1013,25 mbar or 29,92 inhg) above MSL.Jan"Beatus ille qui procul negotiis..."

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>I intend to make a drawing with the various altitudes>visualised and would like some help. You will find such drawings in any basic flight training book.>I am looking for a definition and above all the A:Var>Paramater for the following altitudes of which some will>defintely be the same:>>1. Ground altitude>2. Ground elevation>3. Ground level>4. Above ground level>5. Above sealevel>6. Radio altitude>7. Aircraft altitude>8. Runway elevation>>I left pressure and transition altitude out as they would not>serve a purpose in this list. Decision altitude and hight may>also not belong there.>>There might be additional that I have not listed. Please feel>free to add them.Height measurements in aviation are expressed by the following altitudes:TRUE ALTITUDE - The exact distance above mean sea level (MSL). The heights of all fixed objects (such as mountains and obstructions such as radio towers) are given in true altitude, including field elevations. These altitudes DO NOT change with varying atmospheric conditions.ABSOLUTE ALTITUDE - The exact altitude of an aircraft above the surface over which it is flying. This altitude is commonly refered to as AGL or "above ground level", and is the altitude returned by a radar altimeter. Note that if you are over the ocean, your AGL altitude is not necessarily the same as true altitude, as the altitude of local sea level may be above or below MSL depending on atmospheric conditions and tides.INDICATED ALTITUDE - The aircraft's altitude displayed on your altimeter. It is only "true" (correct and equal to true altitude) when your altimeter is correctly adjusted to the local barometric setting. In Europe it is common when on final to adjust the Kohlsman to QFE (vs. the normal local QNH), which then sets your altimeter to display the altitude above the field reference point (usually the threshold of the runway). For a detail explaination of QFE and QNH, see my previous post in this forum on the subject.Note the local barometric setting is NOT the local atmospheric pressure, but rather the local pressure reduced (using the ISA standard atmosphere) to mean sea level (specificall QNH or pressure natical height). On the airfield, if you set the altimeter to read field elevation (a true altitude from your chart) your altimeter will read the local pressure setting.The only time the indicated altitude will be the same as the true altitude is when sea level pressure and temperature are the same as in the standard atmosphere, and he rate of decrease in temperature with altitude is the same as that of the standard atmoshere. Remember, altimeter indications are based on this assumed relationship of pressure to height.PRESSURE ALTITUDE - An altitude above a standard pressure level, such as 29.92 hPascals or 1013.25 mbars. This is the altitude you fly when above 18,000 ft in the US to assit in maintaining aircraft vertical separation without having to make continuous altimeter adjustments due to atmospheric variations over a route. You can think of it as the altitude equivialent of a given pressure. It is also used in computer solutions for things like true airspeed, density altitude (see below) and true altitude.DENISITY ALTITUDE - Pressure altitude corrected for non-standard temperature variations. Remember, the ISA standard atmosphere is not only a standard pressure and temperature, but also a standard temperature and pressure gradient, which is the map but not the territory of the actual atmospheric conditions. Aircraft performance charts are based on this altitude. Think of this as the effective altitude equivalent given actual pressure and temperature conditions. In other words, what altitude will it feel like to your engine.So, as an example, assume you are flying and your altimeter is correctly set to the local QNH of 29.72, your altimeter indicates 3,000 feet, and you note the outside air temperature is 22

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FYI, that diagram is not going to do you any good.See my earlier post on QFF, QFE, and QNH for detailed info. If it is not here on AVSIM, you may find it on Hovercontrol.com in the general section.

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To expand slightly, because atmopheric pressure can vary with location, many countries are divided into Altimeter Setting Regions each of which has its own QNH. In the UK this Regional QNH is updated hourly. For safety reasons the QNH is given as the lowest forecast QNH within the next hour. This means that aircraft fly a little higher above the ground than indicated.

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>atmopheric pressure can vary with locationMore precisely, atmospheric pressure DOES vary but by altitude, time of the day (atmospheric tides), temperature, and even gravity.>many countries are divided into Altimeter Setting RegionsYou would always use the QNH (or altimeter setting as we refer to it here in the US) of the closest reporting station. Perhaps they call that an "altimeter setting region" where you are from, I cannot say, but here in the US there is no such term.>aircraft fly a little higher above the ground than indicatedYou are in error, the true altitude could be above or below the indicated altitude (remember the lowest QNH might not be that low compared to the actual or high) and hence the absolute altitude higher or lower. See the above discussion to understand why.

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Hello Patrick >aircraft fly a little higher above the ground than indicated No error What he meant was within the defined regional altimeter setting area you are given the worst setting to assure terrain clearence anywhere in the defined regional area. I think that is a UK only quirk

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Thanks Gentlemen,That gives me enough to keep me going for a while.Roelof

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>> Hello Patrick> > >aircraft fly a little higher above the ground than>indicated >> No error>> What he meant was within the defined regional altimeter>setting area you are given the worst setting to assure terrain>clearence anywhere in the defined regional area.Yes, I (and you) get that however, as a psychologist, I would point out two things to you: a) Because we understand the topic, we could "assume" "what he meant" (at great risk), however, :( What he said is not correct (nor what you may think he meant), and therefore would confuse someone who does not understand, given they would take him to the letter of his word and not be able to interprete it as we could.If you said that on a commercial flight verbal, you'd get it wrong.> I think that is a UK only quirk And an interesting one to be aware of for sure. Given the accuracy matters only in relation to the true altitude of obstructions, pilots are required (here) to have a margin of safety that exceeds any error in QNH. As long as everyone in the vicinity is using the same setting, separation is maintained. As long as you maintain MSA (minimum safe altitude) you are further guaranteed clearance from terrain and obstructions.

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I was tired, and posted a minor mistake myself.Where I said:> Since your altimeter is lower than the standard pressure of 29.92,> the 3,000 foot pressure altitude (where you would be in the sky if> you changed to 29.92 and the altimeter read 3,000 ft) is below> your current altitude. In fact, if you change the Kohlsman to> 29.92, your indicated altitude would be 2,800 feet.It should read:Since your altimeter is lower than the standard pressure of 29.92, we find your current pressure altitude of 2,800 feet by setting the altimeter to 29.92. This altitude is "lower" than your indicated altitude of 3,000 feet.This can get quite confusing at 4am. =)

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I very carefully said in the UK to counter those who assume that US practice is universal."...the UK has been divided into an number of ASRs for each of which the National Meteorological Office calculates the lowest forecast QNH (Regional Pressure Setting) for each hour. These values are available for the period H+1 to H+2 and may be obtained from all aerodromes having an Air Traffic Service, form the London, Manchester and Scottish ACCs or by telephone."Source: UK AIP ENR 1.7 Altimeter Setting Procedures.To be on the conservative side, the Regional QNH is the lowest forecast QNH value for that hour, and so will be at sea-level or slightly higher. This ensures that the aircraft will be at or slightly higher than the altitude indicated, and not lower."Source: The Air Pilot's Manual Vol 3 Air Navigation (UK) Using regional settings has advantages when the pressure is changing geographically - which it does and is of far more practical importance than changes with gravity, for example. Suppose an aircraft uses a QNH of 995mb based in its nearest source and another uses 1008mb from its nearest source. If the first flies at 3000ft indicated and the other at 4000ft indicated when they meet halfway between the vertical separation would be 610 ft not 1000ft. That's a quite significant loss of separation.

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>I very carefully said in the UK to counter those who>assume that US practice is universal.Yeap, we saw that, and no one ever declared differently.>"...the UK has been divided into an number of ASRs for each>of which the National Meteorological Office calculates the>lowest forecast QNH (Regional Pressure Setting) for each hour.>These values are available for the period H+1 to H+2 and may>be obtained from all aerodromes having an Air Traffic Service,>form the London, Manchester and Scottish ACCs or by>telephone.">>Source: UK AIP ENR 1.7 Altimeter Setting Procedures.Yeap, we know. No one disputed this either.>To be on the conservative side, the Regional QNH is the>lowest forecast QNH value for that hour, and so will be at>sea-level or slightly higher. This ensures that the aircraft>will be at or slightly higher than the altitude indicated, and>not lower.">>Source: The Air Pilot's Manual Vol 3 Air Navigation (UK) Yes. Note: "will be at sea-level or slightly higher" is refering to QNH (a pressure actually), whereas we in your original post you asserted:> "This means that aircraft fly a little higher above > the ground than indicated."asserting that the aircraft's altitude AGL (absolute altitude) would be higher than "indicated" (indicated altitude).That statement is incorrect.All the above is saying is that the indicated altitude with the altimeter set to the proper regional setting, is guarenteed to be at or above the indicated altitude which would be indicated if you set the altimeter to the actual QNH (vs. forcast).So, the above does nothing to change what has been stated earlier. Challenge yourself to determine what might happen under different conditions to absolute altitude given a forcast QNH that is only 0.01 hPascals below actual, as an easy example.Absolute altitude may be either above or below your indicated altitude (at least with small differences between actual and forcast QNH).>Using regional settings has advantages when the pressure is>changing geographically - which it does and is of far more>practical importance than changes with gravity, for example.>Suppose an aircraft uses a QNH of 995mb based in its nearest>source and another uses 1008mb from its nearest source. If the>first flies at 3000ft indicated and the other at 4000ft>indicated when they meet halfway between the vertical>separation would be 610 ft not 1000ft. That's a quite>significant loss of separation.> "Using regional settings has advantages "Compared to what?Do not be fooled, this is how "reporting stations" in the US function... as a kind of "region". There is really no difference in the two systems, except in size and name of "regions" (and maybe we use actual pressure vs. your forcast pressure).That statement (poorly written though it may be) is comparing these systems to one in which you continuously change your pressure to the theoretical QNH of the point on the earth below you as you move (given infinite reporting stations). This is why we ALL use regions, although of different sizes.FYI, (for anyone interested) my discussion of QFF, QNE, and QNH is on hovercontorl.com and pprune.com, not here as I originally mentioned.

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I should have quoted the next paragraph from the UK Air Pilot:"For example, with Regional QNH set and the altimeter indicating 2000ft the aircraft should be at 2000ft amsl or slightly higher. No aerodrome QNH in that region will be lower than the value of the Regional QNH."You stated that "You would always use the QNH (or altimeter setting as we refer to it here in the US) of the closest reporting station.". If, as in the example I gave, two aircraft use their closest reporting stations then could be a loss of separation when they meet half way between them if the stations report different QNH.You say "...this is how "reporting stations" in the US function... as a kind of "region". There is really no difference in the two systems...". Where are the regional boundaries in the US and how are they defined?

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"For example, with Regional QNH set and the altimeter indicating 2000ft the aircraft should be at 2000ft amsl or slightly higher. No aerodrome QNH in that region will be lower than the value of the Regional QNH."An alttitude "above mean sea level" (AMSL) is true altitude. Notice it is NOT an absolute ("above ground level" (AGL)) altitude, as the actual sea level may be above or below mean sea level (tides etc.).Now, imagine that actual sea level is higher than mean (high tide, storm, etc.), and you can see that your AGL altitude (one example) would be lower (read less than) true altitude, and your indicated altitude. There are other cases as well, but I'm too busy to detail an example right now. "If, as in the example I gave, two aircraft use their closest reporting stations then could be a loss of separation when they meet half way between them if the stations report different QNH."You are absolutely correct, if two aircraft are at the borders of the "reporting stations" or "regions" and the regions have different QNH (in the US we call that altimeter setting) then the altitude separation will not be the difference between their indicated altitudes. This is one reason why the required minimum vertical separation is 1,000 feet (in the US at least). "Where are the regional boundaries in the US and how are they defined?"They are not clearly defined (with lines on a chart) as they are in the UK, VFR pilot's are just required to use the "nearest" reporting station and if memory serves me correctly within 100 nm of the aircraft. If no station is that close (you are flying over Area 51 and don't have the freq's of the military station that does not exist) you would use 29.92, if I recall correctly. (See the AIM on the FAA web site for details).So, they are an imaginary system. When IFR, the controller gives you the altimeter setting to use as you enter and fly through their airspace, and maintains your separation for you.

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"An alttitude "above mean sea level" (AMSL) is true altitude. Notice it is NOT an absolute ("above ground level" (AGL)) altitude, as the actual sea level may be above or below mean sea level (tides etc.)."It is an absolute level because actual ground levels are referenced to mean sea level. My original post, that you said was wrong, referred to ground level. End of discussion.

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>"An alttitude "above mean sea level" (AMSL) is true>altitude. Notice it is NOT an absolute ("above ground level">(AGL)) altitude, as the actual sea level may be above or below>mean sea level (tides etc.).">>It is an absolute level because actual ground levels are>referenced to mean sea level. My original post, that you said>was wrong, referred to ground level. End of discussion.I love how you do not quote the part of your original post, or the definitions to support your statements. Of course, you can not do so when the facts do not support what you are claiming. Again I refer you to the post of definitions above:TRUE ALTITUDE - The exact distance above mean sea level (MSL). ABSOLUTE ALTITUDE - The exact altitude of an aircraft above the surface over which it is flying.The altitude, as you have clearly also stated, is above mean sea level and hence true altitude, not the altitude above the surface (the water) and therefore not absolute altitude. In fact, the only time that true altitude would be the same as your absolute altitude over the water would be a brief period during which the actual sea level happened to match the mean sea level.So, again you are in error. But then these are confusing issues even to licensed pilots, and you probably have not trained and recieved your pilots license. Even if you had, don't feel bad, most pilots do not have this level of understanding of these issues, and fly along quite happily just following procedures.

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