[Qgis-user] Fwd: Georeferencer Plugin Help - QGIS
Randal Hale
rjhale at northrivergeographic.com
Mon Nov 2 13:18:19 PST 2020
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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