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

[Randal Hale]

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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