MSL vs AGL?

[Mitchell N. Perilstein]

Could someone please define MSL (mean sea level) versus AGL (above
ground level)? Why do we need both terms?

Thanks,
---
Mitchell N. Perilstein [/]
COMPASS Design Automation, Columbia MD USA
m...@compass-da.com
Tel: 410-992-5700 x1225, Fax: 410-992-3536

[Allen Miller]

In article <MNP.95No...@krishna.tibu>,

Mitchell N. Perilstein <m...@compass-da.COM> wrote:
>Could someone please define MSL (mean sea level) versus AGL (above
>ground level)?

MSL = altitude above mean sea level
AGL = altitude above ground level

> Why do we need both terms?

Because if your MSL altitude is lower than the local terrain
elevation your life expectancy gets very short.

--
Allen Miller PP-ASEL KB0NDD mil...@lamar.colostate.edu

[Dan Arias]

In Article<MNP.95No...@krishna.tibu>, <m...@compass-da.COM> writes:
> Could someone please define MSL (mean sea level) versus AGL (above

> ground level)? Why do we need both terms?

MSL is height above sea mean sea level. AGL is height above ground
level. Both are measured in feet in the USA.

MSL is what your altimeter reads and what the sectional charts use
(primarily). Fixed objects, like runways and mountains are measured
in MSL. AGL is used in weather reports, e.g., the surface observation
(SA) report. Transitory objects, like cloud bases, are measured in AGL.

Roughly speaking MSL is used to make sure you don't run into anything
and AGL is used to compare weather conditions at different airports.

--Dan Arias
Cupertino, California, USA

[Thomas W. Eggers]

Geoff Brown (G.B.D...@bnr.co.uk) wrote:
: m...@compass-da.COM (Mitchell N. Perilstein) wrote:

: For height (AGL) set your altimeter to zero (Qfe) before you take off - it
: will only give your height above home base, not your height above
: local terrain if you fly away.

I agree that is always possible in the UK, where the highest point, Ben
Nevis, is only some 4000+ feet above sea level.

Here in Colorado Springs, where my home field is 7050 feet above sea
level, and 50Km away Pikes Peak is 14,210, that is neither possible nor
feasible: most altimeters simply cannot be adjusted that far, and for
the very few that can be so adjusted, nobody is willing to crank them
that long. The result is that the altimeter is *always* set to MSL so
that it reads height above sea level. Qfe is never used, thus preventing
one form of forgetfulness (doing the Qfe<==>Qnh change at some height,
or forgetting which setting you started with), and is replaced by another:
forgetting the field elevation when you need to calculate AGL in your head.
On the other hand, aren't we who fly gliders supposed to be able to land by
visual angles without referring to the altimeter at all?

twe

[Benny]

G.B.D...@bnr.co.uk (Geoff Brown) wrote:

>When a flight is expected to cover a large range, even MSL can be a
>bit restrictive due to the change in air pressure for a given height above
>MSL. Therefore airways are defined relative to the height of the 1013
>mbar isobar.
In the U.S., low altitude airways (below 18,000 ft.) use MSL altitude
at the current altimiter setting. 1013 millibars (29.92" Hg) are only
used above FL180 (altitudes above 18,000 ft. pressure altitude).
--
>Geoff Brown
> G.B.D...@bnr.co.uk tel +44 1279 402716 fax +44 1279 402485
> BNR Europe Ltd., London Road, Harlow, Essex,U.K.

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_| _| _| _|_| _| _| Benny Easton-KD6CHA _|
_| _| _| _| _| _| _|be...@uclink4.berkeley.edu_|
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