Geoimage: Incorrect scale

[bcd]

I'm seeing persistent scaling errors of 1.5 on Geoimages of Ireland. Am I holding it wrong?

[bcd]

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

[zoomer]

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.

[Tom W.]

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

[bcd]

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.

[Tom W.]

27700 seems to include Dublin as well:

 

[Tom W.]

Or:

[zoomer]

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 🙂

[bcd]

Thank you

 

[bcd]

Test

[Benson Shaw]

@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

[bcd]

This is very interesting - @Benson Shaw thanks.

[bcd]

I'm having difficulty with my replies on this thread for some reason

[bcd]

Thank you @Tom W. & @zoomer

 

[bcd]

The correct answer was  ESPG: 2157 -IRENET95 / Irish Transverse Mercator

[bcd]

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.

[bcd]

I originally searched for 'Ireland' and didn't get any results.

[bcd]

This would be a helpful VW improvement

[bcd]

 to tag the country along with the name of its

[bcd]

coordinate

[bcd]

sorry everyone - I found the gremlin

don't type "system" followed by an "s" in parenthesis or you get a Forbidden Error 403 response

[bcd]

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.

 

[bcd]

 

 

[Tom W.]

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.

[bcd]

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.