Image to image georeferencing in QGIS 3.4
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- เผยแพร่เมื่อ 25 ส.ค. 2024
- This video shows how to perform an image to image georeferencing of a scanned map. The Ground Control Points are taken from a reference Google Satellite image from the QuickMapServices plugin in QGIS.
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In QGIS 3.28.1 it is not in the raster menu but in the layer menu. I found out the hard way (involving hair loss)
thank you so much
Hot tip. Thanks for saving my hair :)
thanks for saving my hair ... very little hair I have ....
Dude you have no idea how much you have been of help to me. Keep up the amazing work man!
Thank YOU!! from Down Under.
Exactly what I needed, thank you very much!
Thank you Hans, appreciate this!
Hi Hans. You have accurately described the georeferencing process. thank you. There are a number of details that you warned in the video as wrongdoing but you did it anyhow, probably to show the problem they cause. Let me stress some of them for my students that are watching the video. For good georeferencing Google Earth or Google Maps are not adequate sources of GCP's (ground control points), As a matter of fact, they are rather a poor as they are done from image fusion. There are some funny overlaps that beginners might not notice creating serious cracks in the georeferencing. You will never get a 3 pixel RMSE from a good source of GCP, but much less. As it is indicated in the video GCP's must be taken inside and outside of the area of interest in a proportionate manner (not necessarily equidistant) and indeed, it is not necessary to take them far away from the working site. Under any circumstance, really never, use the lake borders as GCP's. That will end in disaster. This is something we can do in some lakes in The Netherlands since they are controlled to the millimeter.
In some cases, it is important to zoom in to pixel level before entering the GCP. QGIS accepts subpixel accuracy!!. The table that is building below as GCP's are entered is the essential "control" feature to achieve a reasonable georeferencing. You can check and uncheck the input points to see the ones producing most of the error. The errors are seen in the columns dX and dY. When you uncheck an input point and the error of the others reduce, then this particular GCP is probably not well selected. Delete it (DO NOT MOVE it as you create a systematic error) and bring a better one. The move point feature of the Georeferencer is quite temptive BUT IT SHOULD NEVER be used, as it is normally misused.
The number of GCP's you need to build depends on the transformation model, but a number between 6 to 12 are good for a first and second order polynomial. Never less than 4 and absolutely never 3 since you can't evaluate the error in a linear transformation. The RMSE on the lower right, gives you the RMSE of the points you entered and it might not help you much to see the goodness of the georeferencing as a whole. In order to see the the accuracy of the georeferencing I prefer using a dedicated digitized shape file of roads (if available) that is dropped over the georeferenced image and some map swipe tool to evaluate the mismatch. Hope this addendum complements some side aspects of this excellent formative video.
Important remarks indeed. The video was created to provoke discussion with my students. Good practice is very important here.
Thank you for the input! How unprecise can my referencepoints be to still have accepatlbe outcomes? i have a rather terrible scan of a map of germany, where i can maybe locate reference points to roughly 20meters accuracy, but i am interested in the data it includes (data on pollution).
It would be fine, if also the result would be about 20 meters off, but i think this would not work at all?
Would there be potentially other ways within qgis to get those very broad images into mybeloved rasters? Best regards
Thank you! Well explained :)
Thank you very much, very helpful
Very useful, thanks!
after i hit 'start georeferencing' the layer does not appear where it should
Thanks For This Video........
Pls tell How we can remove the Mean Error in QGIS
Hello Hans. Your videos are amazing. I've been using the Bing Maps from QMS, for georeferencing some aerial images from the '60s. But I don't know the accuracy of the Bing Images or even the Google Earth Images for my study area. From a research point of view, I can see from the map swipe function that my georeferencing is pretty crisp, but I can't ascertain the georeferencing accuracy because I don't know the authenticity of the base map layers. Could you please suggest me a way to determine the accuracy, such that I can determine the RMSE successfully?
Hello. Thanks. The only thing you can do is compare features that are steady through time (60's until now). The high resolution reference images from Google Satellite should be good as a reference for those locations. The most accurate is to measure those stable objects with DGPS in the field and compare with your georeferenced layer and calculate the error stats.
unable to find Google Satellite image from the plugins
In the QuickMapServices plugin go to settings and install the additional services.
couldnt find georeferancer tool on my qgis 3.16.2 how can ı get this tool and whats the problem with my qgis ?
In 3.16 you don't need to install/activate the Georeferencer GDAL plugin. It should be available in the main menu under Raster | Georeferencer...
@@HansvanderKwast thankss!
I couldn't find this plugin
This is an old video. In QGIS 3.22 you can find it in the Raster menu. In newer versions it's in the Layers menu.
@@HansvanderKwast got it 🙏🙏🙏🙏🙏
bjr je cherche une personne pour m'apprendre le qgis , j'ai un peux de la connaissance sur Arcgis
Currently I only have free course materials in English. Check: www.gisopencourseware.org and learn it with free materials without registration.
Si vous connaissez l'Anglais voiyez ici pour les materiaux libre acces: http:/www.gisopencourseware.org . Si vous voulez une formation sur demande svp me contacter.
this wasn't helpful in how to just import an image and set it to reference points.
That was not the aim of this video about image to image georeferencing. Check this video to georeference a map with entering gcp's: th-cam.com/video/hPLqy-NBgu8/w-d-xo.html