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Calculation of "minimum safe altitude"


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GeKaiser
GeKaiser
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I wonder how SD calculates the "minimum safe altitude". I know that I change in settings "vertical terrain clearance" and "abeam terrain clearance" (+ same for obsructions).

If you like, check this example:
- assuming a flight from the northwest to the southwest of "Millstätter See" (http://en.wikipedia.org/wiki/Millst%C3%A4tter_See)
- surface elevation of the lake is 1929ft
- even if I select for all four above values "0", the calculated MSA is 4300 ft
- with more realistic seetings (1000ft/1nm) I end up with 5000ft MSA to fly over a lake with 1900ft elevation

Why?

I noticed this when planning different transalpine routes: SD's MSA was always minimum 2000-3000ft higher than the MSA on the Swiss ICAO map (e.g. Swiss "skyguide" recommend 7500ft for GAFOR Route No 83 from Bad Ragatz to Biasca - the highest point on this route is 6285ft)

Can anybody explain?

regards
Georg
Edited 7/1/2012 8:22:20 PM by GeKaiser
Tim Dawson
Tim Dawson
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The link to Wikipedia doesn't work. Can you give me the coordinates of the lake in question? I'll be able to tell you how the MSA calculation is arrived at.
GeKaiser
GeKaiser
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Link to the lake on google maps:Millstaetter See

Or type "46.795,13.579722" in google's search window
Tim Dawson
Tim Dawson
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Basically, there is so much high terrain around that narrow lake that the MSA calculation is always going to include some of it. You can't currently decrease the terrain threshold below 1nm either side of your track, so it's a 2nm wide area in which SkyDemon is looking for the highest terrain elevations in its data. To me this explains the MSA you are seeing, as within 1nm of the northern shore of the lake the terrain is up above 4000ft.
GO

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