bcd Posted October 9, 2023 Share Posted October 9, 2023 I'm seeing persistent scaling errors of 1.5 on Geoimages of Ireland. Am I holding it wrong? Quote Link to comment
bcd Posted October 9, 2023 Author Share Posted October 9, 2023 I'm not sure exactly how runway length is measured. In the cases of Dublin Airport's new north runway the length should measure 3110m. What gives? Geoimage Scaling problem.vwx Quote Link to comment
zoomer Posted October 9, 2023 Share Posted October 9, 2023 Uhm, it makes me always nervous when I look at Airport closeups and see crossing runways or even taxiways crossing runways. Like Zurich Airport in Switzerland. Yours is a quite mild example, but still ... they could meet if things happen. Unfortunately no clue about your scaling problem as I am a georeference novice and do not know how to officially dimension a runway. I personally am not used to which georef system I have to choose from. So I realize that often the coordinates I read out of web maps will lead to an offset of the site in VW which I think is also a scaling/georef distortion standard thing. Once I got the correct position of my site, the Geoimage looked reasonable in scale .... But not sure if that has anything to do with your case. 1 Quote Link to comment
Tom W. Posted October 9, 2023 Share Posted October 9, 2023 It looks scaled-up to me. Is the problem the georeferencing i.e. you're using the wrong EPSG code? I don't think 3857 is correct for Ireland so you could be suffering all sorts of distortion as a result. In GB I use EPSG 27700 1 Quote Link to comment
bcd Posted October 9, 2023 Author Share Posted October 9, 2023 1 minute ago, Tom W. said: It looks scaled-up to me. Is the problem the georeferencing i.e. you're using the wrong EPSG code? I don't think 3857 is correct for Ireland so you could be suffering all sorts of distortion as a result. In GB I use EPSG 27700 Thanks Tom, I was hoping it was something like this. I'll look in that direction and revert. Quote Link to comment
Tom W. Posted October 9, 2023 Share Posted October 9, 2023 27700 seems to include Dublin as well: Quote Link to comment
zoomer Posted October 9, 2023 Share Posted October 9, 2023 41 minutes ago, Tom W. said: It looks scaled-up to me. Is the problem the georeferencing i.e. you're using the wrong EPSG code? I don't think 3857 is correct for Ireland so you could be suffering all sorts of distortion as a result. In GB I use EPSG 27700 Basically what I tried to say 🙂 2 Quote Link to comment
Benson Shaw Posted October 31, 2023 Share Posted October 31, 2023 @bcd did switching CRS to 27700 work? I think one needs to accept the transformation option when switching. If nothing else works, images, incl geotiffs, can be conformed (scale, rotate, stretch) in QGIS by locating a few known ground control points. This is in the Georeferencer tool. I think one of the polynomial transformation algorithms does the rubber sheet stretch. -B Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 This is very interesting - @Benson Shaw thanks. Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 I'm having difficulty with my replies on this thread for some reason Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 Thank you @Tom W. & @zoomer Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 The correct answer was ESPG: 2157 -IRENET95 / Irish Transverse Mercator Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 Luckily, in this case, the difference between the default and the site was significant - pointing up the error and the importance of setting the correct coordinate system. Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 I originally searched for 'Ireland' and didn't get any results. Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 This would be a helpful VW improvement Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 to tag the country along with the name of its Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 sorry everyone - I found the gremlin don't type "system" followed by an "s" in parenthesis or you get a Forbidden Error 403 response Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 Last time: Thank you @Tom W. & @zoomer. This is the correct answer: ESPG: 2157 -IRENET95 / Irish Transverse Mercator Luckily, in this case, the difference between the default and the site was significant - pointing up the error and the importance of setting the correct coordinate system. I originally searched for 'Ireland' and didn't get any results. This would be a helpful VW improvement to tag the country along with the name of its coordinate system/s so that searches are more reliable. 2 Quote Link to comment
Tom W. Posted October 31, 2023 Share Posted October 31, 2023 1 minute ago, bcd said: Yep EPSG 29902 is there too as suggested earlier. I'd be interested to know whether there's any discernible difference (distortion) between these different projections. Plus 27700 which is what I use for the UK + which includes most of Ireland. Quote Link to comment
bcd Posted October 31, 2023 Author Share Posted October 31, 2023 I understand EPSG 29902 TM65 was replaced by EPSG 29903 TM 75 by EPSG 2157 IRENET95 The question I guess is which one agrees with the transformation that the Arcgis is using for the satellite imagery. I'll run a comparison of the various coordinate systems you mention. I have a feeling that the differences between them, while real, will not be discernible with the satellite imagery that we're getting. 2 Quote Link to comment
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