[Qgis-user] Fwd: Georeferencer Plugin Help - QGIS

[nagyrobi_r at yahoo.com]

 Hei!

As far as i remember from the courses i took in remote sensing (And i don't find any reason to believe that QGIS would have a different theory):
 - The Residual error is calculated for every GCP( between the expected coordinate and the actual location). 
 - Then the total error is calculated by taking the RMS(Root mean square) of the residual errors. This would sum up the whole georeferencing. Some people take the worst residual to characterize the quality of the georeferencing. 
 - The RE for GCP points should be lower than half of the pixel size 
 - The georeferencing method changes the residual error, cause the estimation are based on the the georeferencing method. This might be observed as well if you toggle between for example 1st order polynomial and Helmert transformation.
 - A low RE or RMSE does not say anything about the absolute position of the raster. It just describes how well the raster is referenced to the GCP's.
 
Don't take my word for it though, i ask the community to correct me if i'm not saying it right.     On Monday, November 2, 2020, 10:18:30 PM GMT+1, Randal Hale <rjhale at northrivergeographic.com> wrote:  
 
   
Good afternoon QGIS People 
 
 
I received a question on RMSE errors in georeferencer and after talking to Ethan about it I realized I have no clue what I'm talking about. How would we go about calculating an RMSE for a rectified image in QGIS? 
 
 
I've cc'd ethan on this so he can chime in also. 
 
 
Thanks 
 
 
Randy
 
 
 
 -------- Forwarded Message -------- 
| Subject:  | Georeferencer Plugin Help - QGIS |
| Date:  | Fri, 2 Oct 2020 15:27:13 +0000 |
| From:  | Ethan Cissell <ecissell at bio.fsu.edu> |
| To:  | info at northrivergeographic.com <info at northrivergeographic.com> |

 
 
 Hi Randal,  
   I have a question about the usage and outputs of the Georeferencer Plugin in QGIS, and thought, given your expertise, you might be able to directly help, or point me in the right direction.  
   I am a Ph.D. candidate in Ecology and Evolution at Florida State University applying spatial techniques to study the dynamics of benthic cyanobacterial mats on coral reefs. This past summer, I photographically tracked individual cyanobacterial mats in which I had placed plastic stakes as ground control points for later image rectification. I am trying to essentially create image stacks, so that I can parameterize a generalized additive mixed effects model that accounts for the inherent spatial autocorrelation and includes a parameter with occupation at the previous time-point explicitly included to hopefully model the population dynamics of this system.  
   Thus, a quantitative understanding of the error in my alignment is absolutely critical to the downstream conclusions (especially for informing my resampling to larger grid size to account for error in my alignments).  I am using QGIS for obtaining all raw data from these photos (scaling, rectification, classification, resampling), but I am stuck at the rectification step. I am using the “Georeferencer” plugin to accomplish my alignments, but I have received mixed advice on how to interpret the residuals output on the GCP table in the Georeferencer window (screenshot below). The documentation for this plugin seems to indicate that the far right column (Residual) is actually a per GCP RMS error.  
   Can you tell me how to interpret these errors/how to obtain a total RMSE error for the image alignment (i.e. not just per GCP error, but total error for each image)? I have been told to sum all of the values from the residuals column and take the square root of that raw summation. Is this correct?  
   Many thanks in advance. I apologize for the lengthy email, but I wanted to establish the motivations for this project so that you would have more context for the goals of the alignment.  Any and all help here is greatly appreciated.  
   
   
   
   Best,  
   Ethan C Cissell  ——————————————  Ph.D. Candidate - McCoy Lab  National Science Foundation Graduate Research Fellow  Florida State University  _______________________________________________
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