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scott967

Determining below sea level altitude flattens???

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I was looking at EHAM in nl, and found that the various scenery utils seem to have different answers for the flatten altitudes (default files only) 52.312022/4.757501:LWMViewer 1.3 -3 mLWMViewer 1.3 source code -3 134LWMViewer 2 -1.34mFlight Sim 9 -3.3528mTDFCalc2004 -4 -44 (44/128 = .34375 so looks promising)Sbuilder 2.05 append lwm FL949130.bgl -3.34375mAFCAD 2.11 (airport alt) -3.4mJABBGL (airport) 4,294,963.944m (apparently not set up for signed int)So, what (if any) is the right answer?scott s..

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Hello Scott,Jacky's JABBgl does not return the Flatten value, but rather the airport reference altitude that is in the AP files. This should be as close to the FL value as possible, given that the airport reference altitude is in fractional meters, whereas the FL value is in 128ths of a meter. So, there will always be some discrepancies between these two.In the case of this AP file, there seems to be something preventing JABBgl from reading the correct value, and perhaps it is as you say that it cannot handle negative values.AFCAD also shows the ARP, but not a precise value.I would go with Dick's TDFCalc, since he is measuring the precise value of the ground, and also with Luis' SBuilder, since you have appended the original FL file. All the others are rounding the value.Jim's LWMViewer 2 is still not finished, so that value may be incorrect.Best regards.Luis

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Hi Scott,-11 ft according to the airport charts, so that makes -3.3528 meters.

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Thanks Luis, I understand the AP9 alt versus the FL9 alt. It seems like both LWMViewer 1.3 and LWMViewer 2 are decoding this wrong. The TDFCalc2004 says:LWM Height: -4 Height fraction: -44I don't see how to compute the actual flatten from this. But, like I said, 44/128 = .34375 which is the same frztional part that sbuilder is reporting, so I assume they are both obtaining the same fractional part. But how should it be applied? If we just take the arithmatic sum-4 + (-44)/128 = -4 +(-.34375) = -4.34375so I don't see how we get to -3.34375based on my experience with AP9 files, the airport elevation seems to be derived from the altitude in feet, converted to nearest .001 meter. At -11 feet that would be -3.353, which would be bellow the flatten if -3.34375 was correct.So I did some testing.First thing, I compiled the sbuilder appended file back to bgl and then appended it again. I got -3.34375, so at least whatever sbuilder and scasm are doing, they are consistent (and more likely correct). I then loaded the sbuilder created flatten into the game, and it looks exactly like the default file. (LWMViewer 2 still reports it as -1.34, so that looks consistent too -- and wrong). But I noticed some runway texture flickering, both with my sbuilder copmpiled flatten and the default. That I would expect, if the airport scenery is -11.0 feet = -3.3528m. So as a further test, I edited my sbuilder project with a flatten elevation of -3 47/128 meters (-3.36719). I compiled and installed this, and my texture flicker was gone.Comclusions:Sbuilder is probably correctly reading default flatten (LWM) altitudes that are below sea level.LWMViewer does not properly decode the altitude in FL9 files that are below sea level.FS9 has some bugs in the default flattens, when below sea level the flatten altitude is not always below the airport scenery, causing flickering (I haven't seen this in any above sea level airports, but can't rule it out).I hope someone has experience with below sea level designs and can comment.scott s..

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Very sorry, Scott, my mistake. I assumed that when you indicated TDFCalc's result as -4 that this was just a typographic error and that it should have read -3. But, if this is so, then TDFCalc is wrong there.Best regards.Luis

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Hi Scott.If you use TCalc2004 ( not the version2 which reports incorrect altitude in minus values! )...Slew slowly over the runways, and you'll see the actual runway surface is:-3.3515625 meterswhile the flatten is:-3.34375 metersThe terrain under the apron and taxiways is:-3.3515625 meters... to align with the runway.The actual flattens are -3.34375. The terrain under the aprons appears to be raised to the level of the airfield automatically. Decompilation using newBGLAnayze or bglxml of the AP949130 uses -3.35M as the airfield/runway height. ( That could be a rounding error.. or the sim may default to the nearest LWM height fraction! )What is odd, is the surrounding terrain is 1/128th meter HIGHER than the runway and aprons. Usually the terrain is 1/128th lower than the runway. That is probably a mistake in the program MS uses to determine the flatten. I think the AFCAD elevation is a rounding error.Dick

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Thanks Dick --I am using version 3. I have to admit, I didn't even look at the elevation number, I was thinking that was to the A/C reference point like in the FS9 supplied values in slew / shift-Z. That's why I was always using the LWM height as being "more accurate".I see that the numbers in the Elevation box do look correct, and had I been smart enough to look at that at first, I would have seen that it agrees with sbuilder. It looks like the fractional height number is correct, just the whole number of meters is off -- maybe consistently by 1 m ?But LWMViewer I think has a decode bug here.Here is the reading with my revised file from sbuilder which I set at -3.36719 which I compute by manually rounding 47/128 meters:http://forums.avsim.net/user_files/152812.jpgscott s..

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