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Guest smithf

A question for everyone...

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Hello!I am working on some funky Flash teaching aids - it seems there is a lot of confusion about basic geographic principles and how they translate to FS World, and I thought that a well-illustrated teaching aid will greatly clear up confusion.The question is: what is the actual geometric shape of Earth in FS (as in, 3D model of it, not the conceptual)? Is it FLAT, and when you fly to the edges, you are transported ("wrapped around") to the other edge, or is it a true SPHERE?I would have to conclude that it is indeed flat, with a true lat-long grid superimposed to it so that it correlates to proper navigation. This would explain why we can't reach north or south poles (things get weird beyond +/- 87 deg latitude) because of the enormous distortions. Also, Flat system fits to the rectangular LOD model. But I am confused in what the SDK documentation says:"In Flight Simulator, the earth is defined as an

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From what I can tell, FS uses an oblate spheroid. The earth itself is an oblate spheroid. Due to the rotation of the earth it bulges slightly at the equator and the poles are flattened. I believe the difference is about 25 miles in circumference or diameter or something like that.If you look at the conversion from lat lon to UTM you will notice a conversion factor to accomodate the ellipsoidal shape of the earth.In order to make the computer number crunching more efficient MS projects the square textures on a cylindrical projection so they can efficiently locate them, then lets the video card remap them onto squashed triangles.I hope this is not too obtuse.Frank

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Thanks for some very insightful info. However, I haven't had my question answered. While it is apparent that FS World follows the physical characteristics of the real world, this can all be a fairly simple mathematical operation, i.e. application of polar coordinate grid (as opposed to Cartesian). However, this very approach exhibits most of the shortcomings that arise when you try to map the spherical surface to the FLAT plane, namely, the meridian convergence and distortions near the poles.So, let me ask the same question in a different form:If you COULD slew to, say, 300,000 feet altitude, would you see curvature of the Earth?(you can see this in Map View, but this doesn't apply - map view is faked beyond certain altitude. You can tell this by a "lovely" earth-wide photorealistic texture applied to globe beyond certain zoom level)

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Misho,I just tried an experiment. I slewed out over the ocean and up to 100000 feet. I adjusted me "seat" so that the horizon went across the middle of the screen, and paused the simulator. I laid a piece of paper across the horizon line, carefully lining up my eye to avoid a parallax error. It looked to me like like the middle of the line was about 3 pixels higher at the center, so it appears that FS accounts for the curvature of the earth.The way the FS graphics account for curvature is that images are stretched horizontally at higher latitudes when run through resample to make them equiangular. The graphics engine and video card then scrunch them back down when they apply the texture to the 3D surface. There is a lot more distortion of the image due to mapping the texture on the altitude mesh than there is due to fitting it on the curvature of the earth.The LOD13 grid is based on a texture that is 256 pixels square being laid on a geograpic grid. This grid is square at about 36.8

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I have the same thread running in the FS Scenery Design forum. Check there for some interesting PRO/CON arguments.Thanks,

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Thanks for pointing out the other posts to me.I have known for a long time that the FS texture mapping depends on equiangular textures. As pointed out in one of the posts on the Scenery Design forum, the textures are mapped to a cylindrical projection. This is why I use LOD grid based images that are resized to a multiple of 256. I like to use an LOD 8 size image (that is about as big as my machine will handle at 4096x4096). Whatever size image I use, I locate the corners on the LOD grid and square up and resize the image as the final step before running it through resample. That way the image resizing is done in Photoshop, and not resample. I get very good image line-up between adjacent sceneries done at different times. I don't worry at all about 4.8 m/pixel resolution as it becomes irrelevant. I start with a base image of 2 to 4m/pixel. When you square up the image and resize it to a multiple of 256 pixels you have taken care of the distortion needed, and FS will then "undistort" the image and correctly map it onto the FS world.Resample can stretch these images out for you, but I like to have the control of doing it myself.If someone could figure out how to create the bgl file without going through resample, I wouldn't use resample at all.Frank

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