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Guest JohnC

Terraserver DOQ s from USGS..right or wrong?

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I downloaded some DOQ's advertised as 1 m/pixel, and used them in a background image in Ground2k4. After minutely tweaking the NW and SE extents so that my x and y resolutions are exactly 1 m/pixel in GR2k4 (+/- 0.1 percent) I find that the runway in the DOQ's translates to 129.5 degrees from true north clockwise. The FAA AirNav site says it is "131"...runway number is 33..magvar is approx 15 west of north. Here's the problem...I can distort the backgound image (on the y) axis to make the runway angle match up to the FAA (and the FS9) angle, but this means distorting who knows what else? If I submitted this to AVSIM with the runway angle at 129.5 and changed all the taxiways,etc. AFCAD taxiing would need to be changed. Has anyone else considered this or ran into something like it?. I understand published data isn't always accurate, but I thought DOQ's were the final word accuracy wise."When you're building a house, make sure the foundation is right."Any thoughts?

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Hi John,the Terraserver data are in UTM projection and need to be re-projected to lat/long WGS84 to best fit with FS. Do a search for "reprojection" here to find the relevant threads.On the other hand, small sections should be very close even without re-projection. What I have found on my recently uploaded Columbia River Gorge project is that the AirNav data, which the FS team used to build the default AFCAD files, are simply wrong for many small airfield. The discrepancies are in runway location, runway direction, and elevation, and can be quite large. In fact, your 1.5 degree discrepancy is rather small to what I've seen in some places (10-15 degrees, with a horizontal displacement of several hundred feet).In other words, if you're confident in your DOQQ image you should correct the default AFCAD to match your data. That's what I did with 20+ airfields and now they fit very nicely with the high-res mesh and road data.Cheers, Holger

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Hi, Holger...glad I heard from you..just as a point of information for others,when you are in Terraserver, and you click on info at the top right of your image you get an overlay of gridlines with hi resolution lat/lon values. Those gridlines are exactly vertical and horizontal and leave the distinct impression that they are precise and correct. I "was" using topozone to nail the exact values at the NW and SE corners of my background image. If something is in UTM projection vs the ...uh...other, would you expect a 1 or 2 degree difference in a runway when the actual image size is only 1000 by 800 meters? Would a small area undergo that much rotation? I assume I will find a way to convert if I decide to, but I have a very large area to do.I actually laid down a VTP2 poly using my background image runway in GR2k4, and so I have an ice runway laid down beside my FS9 default runway. Not a good match...no sireee...Cold up there in Calgary? Hate to say it but my red ruffle azaleas are looking pretty good out the window. I am an ex-Montrealer so I know WAY too much about the frozen North.

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Hi John.A while back I discovered you can get TerraServer images in geographic projection:http://terraserver.microsoft.com/ogcmap.as...ceptions=se_xmlOr go to:http://terraserver.microsoft.com/ogcwms.aspxUsing longitudes and latitudes forces geographic projection. DOQ is standard photo, DRG is topographic, or UrbanArea for finer detail where availble. Use EPSG:43262000 x 2000 pixels is a maximum ( I think ). But you could merge them in a paint program...Dick

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I've found that in SceneGenX, if I rotate the Terraserver DOQ (built by USAPhotoMaps/BigJPEG) by the UTM Grid North declination, that the runways line up perfectly. I guess if you reproject for Gound2k4, that will take care of it too, but if doing the projection is too much hassle you might try just rotating it (I use Gimp) and see how that looks.I'm thinking of buying Global Mapper just to make life easier.For larger US airports, NGS has runway and ILS survey data available as ASCII column delimited text files that can be read into Excel for easier use.http://www.ngs.noaa.gov/AERO/aero.htmlThere is also data available (at least for now) from the NGA DAFIF, but I have found that data flat wrong for a military airport I'm working on.With that caveat, though, DAFIF has runway polygon shapefiles for the whole world (at least ICAO airports).scott s..

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Thanks, rf...the runways line up now and I can get 4MB at one gulp instead of 0.8 MB....additionally every second retrieval butts up against the next one so no overlaying two layers and lining up is necessary.

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