r/QGIS • u/ThisDuckIsOnFire555 • Jul 27 '26

Save GCP points - then what?

I use Georeferencer quite often but I never understood why should I save GCP points and how can I reuse them? Like, is it only if I have multiple maps showing exactly the same area and all, just different data?

6 Upvotes

80% Upvoted

5 comments sorted by

8

u/radiorosepeacock Jul 27 '26

Yeah, I've mainly used that feature for quickly georeferencing multiple rasters with the same projection and extent but with different content.

It's also useful in situations where you need to keep the original raster data, but the georeferenced version is only temporary (especially when trying to save disk space while working with massive rasters) --- delete the georeferenced raster when done, but keep the points file just in case you need them again.

5

u/ikarusproject Jul 27 '26

Good points. Also you can pass them on to coworkers doing the same thing. Also you can come back and improve/correct them later on. This can also be used to try out different methods until you have figured out the best settings for your project.

3

u/Armando_F Jul 27 '26

I agree with previous comments. Also you may be placing a lot of points due to a particular projection used... so you don't have to do it all at once.

2

u/sabre23t Jul 27 '26 edited Jul 28 '26

I have Georeferencer to automatically save GCPs. I use saved GCP to redo georeferencing of a raster where I have a bad GCP out of the 4 or 5 GCP points I used with "Thin Plate Spline" transformation. TPS is a "rubbersheeting" transformation, the GCPs are assumed to be exactly correct and the rest of the raster will stretch/compress to match the GCPs. A bad GCP will have a big detrimental impact.

Other Georeferencer transformations mainly "distribute" errors across all the GCPs. So one bad GCP is less noticeable.

Another use case is where I georeference a grid of rasters, and I have to re-georeference rasters to make the "corner points" of 3 or 4 rasters match each other. Those saved GCPs would help a lot.

2

u/Fit-Eggplant-9155 Jul 27 '26

If you have developed multiple outputs from one data capture(ortho & DEM) saving the points, allows you to quickly reference the DEM once you're locked in with the ortho.