To remain safe and legal we fly according to Visual Flight Rules, and we have to avoid controlled airspace. Normally for a given area there is a transition layer, above which airspace will be defined at a certain flight level. The altimeter will indicate altitude AMSL. Restricted airspace below the transition layer will be expressed as altitude AMSL.
When flying above the transition layer, altimeters should be set to the standard pressure of The altimeter will indicate Pressure Altitude PA.
To avoid confusion between Pressure Altitude and Altitude, PA generally has the last two zeroes removed and is then known as flight level: PA is more commonly known as FL Restricted airspace above the transition layer will be expressed as a Flight Level. It is essential to know the airspace restrictions in the area you are flying. It is possible to have a combination of Flight Level and Altitude restrictions during a flight. For any given geometric height, the indicated altitude of the altimeter the one that defines airspace will vary depending on the conditions of the day.
This assumption is not very accurate for two major reasons. The second is the atmosphere usually has hot and cold layering making the rate of change of pressure non-uniform. The GPS will measure a height of 10, ft above the geoid and everyone is happy. At a pressure of hPa, the yellow altimeter will read ft higher than the green altimeter at any altitude. This geometric height is quite close to the 10, ft indicated by the altimeter set to QNH.
For illustration purposes the geometric height of 10, ft, indicated by the yellow altimeter, equates to an altitude AMSL of 10, ft at a pressure of hPa. The green and yellow altimeters will indicate the same as Day 2 because they are temperature compensated. It has moved up. For illustration purposes the geometric height of 10, ft equates to 9, ft AMSL as indicated by the yellow altimeter at a pressure of hPa.
Diagram 3 click on it once and then again to see a bigger version illustrates how Flight Levels change their height depending on the day. FL is used as an example and is always at hPa. From this we see that a change in QNH pressure alone does not cause a large difference between GPS height and indicated altimeter altitude.
The error introduced by the temperature change is 0. The reason for this is that the ICAO formula assumes that the density of the air at a particular altitude is standard but in reality this density changes with temperature. Does this theory work in reality? Diagram 4a, b and c below shows three paraglider flights made by Meredyth Malocsay in Australia in using a Flytec As it explains below, these flights show conclusively that there is a difference between GPS height and barometric altitude and that the difference can vary depending on the day — and it can be worse than the theoretical prediction.
She used a Flytec to record them. The first was four hours, the second three hours and the third three hours. The day temperature was about the same in each. If m is 1 then the GPS height increases at the same rate as the Pressure Altitude; we would like this in an ideal world but we are expecting m to increase as the temperature increases indicating the percentage error.
In Flight 1, c needs to be forced to a value of —40 m to account for the pressure of hPa whereas for Flights 2 and 3 c is zero because the day pressure happened to be hPa. We are more interested in the m value. In Flight 1, m is 1. In Flight 2 the GPS altitude is 7. The theory According to theory, the GPS altitude should be 5. So in these examples reality is actually worse than the prediction. One explanation of this is that the atmosphere layering could have an influence.
Almost certainly the atmosphere was not standard. Notice that the graphs have a distribution around the trend line. This could be due to temperature and QNH changes throughout the duration of the flight.
Also, due to the delay in the GPS height calculation, strong climbs will plot points slightly to the right and strong sink slightly to the left of the correct value, giving the line a marked thickness. The conclusion These flights show conclusively that there is a percentage difference between GPS height and barometric altitude and that the difference can vary depending on the day, and can be worse than the theoretical prediction.
If you want to check this out, all the calculations, information and graphs for this are here. Detailed here and in Diagram 5 below are some of the instruments we use, what they display and what they record — this is not always the same thing. Their E an N are still only accurate to 3 or 4m and elevation to 5 or 6 m due to atmospheric conditions unless told where the base is, either by correction, eg set up, survey, manipulate on drawings like CAD, apply adjustments or as above with a smartnest licence.
Or with a previous survey E. My Leice kit, similar plus the licence, paid annually. This topic has 45 replies, 32 voices, and was last updated 8 years ago by CountZero. Viewing 40 posts - 1 through 40 of 46 total.
MidlandTrailquestsGraham Free Member. Posted 8 years ago. RealMan Free Member. Drac Full Member. But they do. GrahamS Full Member. I think the has a barometric altimeter. But I may be wrong. Trimix Free Member. Trig via several good signals. Hope that helps… Posted 8 years ago.
Flaperon Free Member. Dibbs Free Member. They do just not very well Read this Posted 8 years ago. CountZero Full Member. But before sending his field crew out with the technology, Michael wanted to make sure the solution worked.
So, Michael took his iPad and Arrow Gold into the field and tested them by recording the elevation of a survey monument. A survey monument is a point with published high accuracy horizontal and vertical coordinates. Michael recorded his elevation measurement from his devices, compared it with the published coordinates, and was surprised.
His GPS receiver elevation data was off by tens of meters. The earth is shaped like this, with the north and south poles acting as the top and bottom points of an approximate egg. So, when a receiver collects elevation data, it is referenced to the ellipsoid. However, there is a problem with ellipsoidal elevations. Although they are very accurate, they are not practical for every day operations, such as field work. The issue is that the earth is not a perfect ellipsoid.
It has mountains, craters, and other features above or below the mathematically perfect ellipsoidal reference. MSL is a local tidal datum that can be used as a reference for elevation when close to the shoreline.
However, once you get more than a few kilometers inland, MSL becomes impractical. MSL can be calculated two ways. A less accurate model that uses less data called EGM96 is also available. Google both of these to understand these better. It's not quite that simple -- the Android API has either changed or has bugs. I have two Android devices -- a 'generic' phone Android 2. On the Nexus 7 , the altitude returned is uncorrected. The documentation for the API does not specify which is returned -- so in some sense both are 'correct'.
So it looks as though one has to parse the messages oneself to get the correct altitude, and the getAltitude method is untrustworthy. The altitude value returned by any GPS receiver is always the least accurate value. One fundamental reason is that it is impossible to get an even spread of satellites in the altitude direction; they are always going to be above you, and if there is any obscuration of the sky at low elevations, they will be located in a cone above you.
In the X and Y directions there will always, assuming a good sky view, be satellites spread left and right; in front and behind the receiver. This spread enhances the accuracy of the position solution.
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