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rhumbaflappy

Correcting airport elevations

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I've just finally gotten my first terrain mesh working. It's a remesh of the area surrounding KRDG using the SRTM 30 arc second data. It looks great overall, but now I'm noticing that several small airfields near KRDG are exhibiting the "plateau/crater" phenomenon due to having flatten altitudes that are incompatible with the new mesh. Is there an accepted method for changing the airfield elevations to make them blend in better with the new mesh?thanks,

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Thanks, Ken. The flight back was easy but long. I took some time to kick around Philly before heading home... went over to Independence Hall and such. It was a pretty good time. BTW, since I got the new SRTM data working, Mt. Penn looks fabulous and exactly as I remember it in real life. Time for a pagoda!thanks for showing me around on Sunday,

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Hi Bill.LWM flattens can usually correct an elevation... but I'm not sure if there is another elevational code in the FS2004 BGL... and we don't know the format of that code.There is a problem with correcting these runways. The map ( and GPS ), autoland, and the AI traffic all expect the runway at a specific elevaton. MS usually does a very good job locating runways, as they use official data ( whenever possible ) to obtain the locations, heading, lengths, and elevations. If our scenery displays the runways as valleys or plateaus... it probably is due to problems with our data.Dick

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Hi Bill.I made some meshes with data from the USGS seamless data website. They have NED, SRTM_30, and SRTM_90 data for the KRDG area.The NED data is the "official" data. It's from a variety of sources, and is adjusted and "cleaned" to attempt a real-world elevational set.The 2 SRTM data sets are raw. They have no corrections. In other words... they are wrong. I looked at the mesh sets in FS2004, and the SRTM_30 seems close... but the SRTM_90 data is awful for that area.Eventually, the mathematicians and scientists will adjust the raw SRTM data, and they will be added to the NED data to form a new, corrected set. Until that time, the meshes based on that technology are bogus. They look nice, and in some areas do reflect actual elevations, but they are not reliable.Most of the world will eventually be covered by the SRTM data at 90 meters. But as to whether that data is ever corrected is unknown. I would think it would be a very laborious job.For now, in the US, NED is the way to go. Eventually, corrected SRTM data will far surpass it in accuracy.About NED:http://gisdata.usgs.gov/NED/About.aspThe SRTM mission objectives:" Mission Objective:To use C-band and X-band interferometric synthetic aperture radars (IFSARs) to acquire topographic data over 80% of Earth's land mass (between 60degN and 56degS) during an 11-day Shuttle mission. Produce digital topographic map products which meet Interferometric Terrain Height Data (ITHD)-2 specifications (30 m x 30 m spatial sampling with <=16 m absolute vertical height accuracy, <= 10 m relative vertical height accuracy and <=20 m absolute horizontal circular accuracy). All accuracies are quoted at the 90% level, consistent with National Map Accuracy Standards. "According to these objectives, 10% of the mapped area could have missing or grossly misaligned data, and still be considered a success. In addition, heights could be off by 16 meters ( + or - ) for any one point. That's a span of 32 meters ( about 105 feet vertically ), and still be a success.The 20 meter circular horizontal accuracy of 20 meters means any elevational point sampled would be guaranteed to be within a 20 meter diameter ( about 65 feet ).The mission has been a success by these standards, and the processing team has been given some awards... and the mission is pretty much considered a done deal. The whold world at 90 meters is scheduled to be released in early-mid 2004.But, until the world's data can be calibrated and adjusted, and holes filled with some accuracy, it's just some nice looking, bumpy mesh.Dick

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Thanks for yet another informative post, Dick. I'm attempting to get the NED data right now (although the site seems veeeerrrrrry slow this morning). I had been fooled, newbie that I am, by the apparent correctness of the SRTM data. The particular mountains I'm interested in correcting seemed quite good using the 30m stuff. I'll d/l the NED data and give it another whack... maybe those airports will fall into line.thanks,

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Hi Bill.Don't be surprised if the NED data disappoints you. It is as correct as they can make that data set, but, it still doesn't have the superior sampling rate of the SRTM data.Bottom line: no great solution until the SRTM data is cleaned and filled.Dick

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Well, the SRTM 30m data for this area looks very realistic with the exception of the two airports that are mis-located. I might live with them in order to have the overall effect enhanced.thanks,

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Hi there:two additional caveats with the SRTM data:1. the radar beam traces the surface of the earth, which includes vegetation and man-made structures. In some places, particularly inner-city cores, this can lead to interesting problems; see my post here:http://forums.avsim.net/dcboard.php?az=sho...g_id=1095&page=2. the other, more general problem is that of missing data, particularly in mountainous areas - have a look at SRTM tiles of the Grand Canyon or the B.C. Coast. We've been discussing various ways of interpolation or merging (a search for "SRTM" in the mesh design forum should bring up some posts) but good solutions involve some tricky geostatistics x( In any case, Dick is perfectly correct in stating that the NED data will be your best choice for the US. In case you don't use them directly but through MicroDem, please read this article on Steve Greenwood's website for what to watch out for:http://www.fs-traveler.com/sdds-test.shtmlCheers,Holger

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Hi Holger.Steve's site gives some good information... but the use of Microdem probably is a waste with NED. We can use the SDK directly with the BIL format, as the NED seamless data is already in WGS84 datum... and the quickly repoduced MESH can be viewed with TMFViewer.The necessary info for the data is in the BLW file ( cell dimensions and NW corner ) and the header file ( number of rows and columns ).The SRTM 30 and 90 data needs work, but I think it's problems will go beyond Microdem's capabilities. The KRDG area Bill uses has serious problems with the SRTM_90 ( the 30 doesn't seem so bad ).Reworking the SRTM with any program leads us further from the real world, unless we have a good set of topograhic data to check elevations at specific points , or a GPS device... and travel to those points ourselves to verify the elevations.I know the USGS will eventually absorb the SRTM data into the NED data... but I don't know what will happen to the rest of the world. We'll have access to 90 meter data, but the world's governments will have access to 30 meter data, and we don't know when they will process that, or if they'll ever make it available.I don't think the USGS will ever correct the 90 meter data, for either the US or for anywhere else.Dick

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