Nipper2
4th Sep 2008, 08:05
I note that the 'GUND' value for elevation now appears on an increasing number of AIS plates including many of the small VFR only airfields. A typical example here http://www.nats-uk.ead-it.com/aip/current/ad/EGHR/EG_AD_2_EGHR_2-1_en.pdf
(See below for a brief explanation of GUND if you are not familiar).
Two questions:
1. Why? The only reasons I can think of is that it is a prelude to the more widespread use of GPS vertical navigation (e.g. approaches) at these airfields or, more likely, it has simply become an ICAO requirement.
2. Why does it have to be given individually for each runway threshold, thus adding unnecessary clutter and complexity to the chart? The value of GUND is highly unlikely to vary by more than one foot over the area of a typical GA airfield and in any event it appears to be promulgated as a single value for larger fields with instrument procedures.
[What is 'GUND'? It is the Geoid UNDulation. This is the difference in height between the simple ellipsoid used to approximate to the surface of the earth and the more complex geoid that better represents a 'level' surface as measured by surveying instruments.
In simple terms, it is a measure, for any point in the world, of the difference in height between the simplified shape of the earth used by GPS (the WGS84 datum) and the locally defined 'Mean Sea Level'.
MSL is between about 150 feet above the GPS defined shape of the earth in southern England, rising to about 190 feet in Scotland. Not enough to be significant for VFR navigation (even if you were stupid enough to use GPS for altitude) but very significant for the design of GPS approaches. There is a more thorough explanation here http://kartoweb.itc.nl/geometrics/reference%20surfaces/body.htm] (http://kartoweb.itc.nl/geometrics/reference%20surfaces/body.htm%5D)
(See below for a brief explanation of GUND if you are not familiar).
Two questions:
1. Why? The only reasons I can think of is that it is a prelude to the more widespread use of GPS vertical navigation (e.g. approaches) at these airfields or, more likely, it has simply become an ICAO requirement.
2. Why does it have to be given individually for each runway threshold, thus adding unnecessary clutter and complexity to the chart? The value of GUND is highly unlikely to vary by more than one foot over the area of a typical GA airfield and in any event it appears to be promulgated as a single value for larger fields with instrument procedures.
[What is 'GUND'? It is the Geoid UNDulation. This is the difference in height between the simple ellipsoid used to approximate to the surface of the earth and the more complex geoid that better represents a 'level' surface as measured by surveying instruments.
In simple terms, it is a measure, for any point in the world, of the difference in height between the simplified shape of the earth used by GPS (the WGS84 datum) and the locally defined 'Mean Sea Level'.
MSL is between about 150 feet above the GPS defined shape of the earth in southern England, rising to about 190 feet in Scotland. Not enough to be significant for VFR navigation (even if you were stupid enough to use GPS for altitude) but very significant for the design of GPS approaches. There is a more thorough explanation here http://kartoweb.itc.nl/geometrics/reference%20surfaces/body.htm] (http://kartoweb.itc.nl/geometrics/reference%20surfaces/body.htm%5D)