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scott967

LWM2 vs LWM3 for Airport Flattens

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Curious if there are any drawbacks to using LWM2 for airport flattens? It doesn't seem like there should be, except that MS is using LWM3 for some reason.scott s..

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As far as I am aware of the only difference between LWM2 and LWM3 is that you can give an altitude to each point of your polygon with LWM3. So that means you can also make sloped flattens. For an airport you want it to be flat in general, so then there is no difference I guess.

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Thanks. Now I have been looking at bgls made with Ground2k4, sbuilder, and the defaults and see this (LWMViewer 1.3 used to obtain source code):Ground2k4 builds the flatten as:LWMDataAreaDrawPolygons 2,3,1,26,28 <--- 3 means transparent LWMPoly2 4, 0, _Land_, 255, 15Sbuilder builds the flatten as:LWMDataAreaDrawPolygons 1,2,1,26,28 <--- 2 means flatten? LWMPoly2 21, 0, _Water_, 255, 15Default FL file builds flatten as :LWMDataAreaDrawPolygons 1,_Flatten_,1,26,28 LWMPoly3 4, 0, _Water_, 255, 15, 255, 15Looking at the raw source (scasm 2.96 for sbuilder and bglc for ground2k4) confirm the LWMViewer results. I guess the three methods used result in the same thing.scott s..

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Hello Scott,The meaning of the parameters is:_Water_ EQU 0_Land_ EQU 1_Flatten_ EQU 2_Transparent_ EQU 3According to Microsoft you can not use _Flatten_ in:LWMDataAreaDrawPolygons 1,_Flatten_,1,26,28Microsoft refers that the second parameter should allways be _Transparent_. But as per your example they also use _Flatten_!Following is my understanding on how it works. You fix the attributes in LWMDataAreaDrawPolygons and in LWMPoly2 (or LWMPoly3). Take this case:LWMDataAreaDrawPolygons 1,_XXXX_,1,26,28 LWMPoly2 21,0,_xxxx_, 255, 15If _XXXX_ is transparent (=3) than the _xxxx_ of the following polygons will define the type of the polygons.If _XXXX_ is not transparent then its value value is forced into (is applied to) the following polygons (the _xxxx_ are ignored)You should knoe that _XXXX_ is coded in 2 bits and so it can take the values of 0 1 2 and 3. On the other hand _xxxx_ is coded in a single bit and so it can only take the values of 0 and 1.Therefore it is not possible to make a flatten with:_XXXX_ = 3_xxxx_ = 2 <<<< 2 does not exists here!But you can with these 2 possibilites:_XXXX_ = 2_xxxx_ = 0 < water but ignored_XXXX_ = 2_xxxx_ = 1 < land but ignoredIf you use:_XXXX_ = 3_xxxx_ = 1 it works as a flatten if applied over a land region but turns water to land if applied over water. Because flattens are not applied over water areas you can also use this last construction (G2K case).Regards, Luisps: sorry if I am wrong!

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Thanks Luis. No doubt you are right about this.There's this one airport in Hawaii (PHHI) that MS made some mistakes on. It's not one I normally fly to, but every time I fly around it the looks bug me, so I decided to finally fix it, at least the basic geometry of it.scott s..

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