[GRASS-user] Orthorectification of tilted digital camera images

Benjamin Ducke benducke at fastmail.fm
Mon Aug 17 01:54:34 PDT 2015

On Mon, Aug 17, 2015, at 08:43, Blumentrath, Stefan wrote:
> Hi Markus, Florian, and others,
> Thanks for your replies. I still did not solve this, so I am grateful for
> any hint.

Could you maybe apply a two-stage process where you first
correct (at least partially) the perspective distortion and then
do an orthorectification on the result? I don't think that the
GRASS georeferencer has a perspective/projective transform,
but QGIS does.

(In case anyone wants to implement the projective transform
in GRASS: I have made a plain C translation of the QGIS
algorithm available here: 
-- requires GNU Scienfic Library for the matrix algebra.)



> Now, I extracted the relevant data (XY-location containing the mapset
> (“reconyx”) with the image group (containing i.ortho.photo files), and
> camera file as well as the target location (“ETRS_32N”) with the “target”
> mapset containing the DEM (“dem_1m”) and two of my attempts for
> orthorectification (with and without INIT file active).  You can fetch
> the zip-file with the GRASS DB (22 MB) within the next 30 days from:
> https://www.dropbox.com/sh/swri19zn5m9gb7w/AACADlkqGZKO_JMS_YbLNXBXa?dl=0
> There you will also find the original image (“IMG_0015.jpg”) as it comes
> from the camera.
> As a next step I will take another test image with a more orthophoto-like
> perspective and no visible sky. I hope, that way I can check whether the
> issue is due to the chosen camera position / angle or the way I defined
> the exposure in the INIT file…
> I have a strong feeling that my problems - in one or the other way - have
> to do with camera angles…
> Unfortunately, it does not seem to be possible to specify only the
> exposure parameters one is sure about. i.photo.rectify uses either all or
> nothing to initialize the orthorectification algorithm…
> Cheers
> Stefan
> From: grass-user-bounces at lists.osgeo.org
> [mailto:grass-user-bounces at lists.osgeo.org] On Behalf Of Florian Mueller
> Sent: 15. august 2015 20:54
> To: grass-user grass-user (grass-user at lists.osgeo.org)
> Subject: Re: [GRASS-user] Orthorectification of tilted digital camera
> images
> Dear Stefan,
> Can you provide some example photos in a Dropbox (e.g.) folder?
> Best regards,
> Florian
> On Sat, Aug 15, 2015 at 7:53 PM, Markus Neteler
> <neteler at osgeo.org<mailto:neteler at osgeo.org>> wrote:
> On Tue, Aug 11, 2015 at 11:37 PM, Blumentrath, Stefan
> <Stefan.Blumentrath at nina.no<mailto:Stefan.Blumentrath at nina.no>> wrote:
> > Dear all,
> >
> > Although there is a lot of relevant information regarding the
> > orthorectification of tilted digital camera images in GRASS GIS online
> > (here:
> ...
> > Does up-slope direction of the photo or sky in the photo conflict with the
> > orthorectification algorithm?
> I have no idea... in general the algorithm is very robust but was
> really written for aerial photos.
> >
> > When it comes to the GCPs I am in principle quite confident that they are
> > placed OK. Or do you think that GCPs measured by a hand held GPS are too
> > imprecise?
> They should be fine.
> > However, depending on initial camera settings RMS something like 268, which
> > indicates that something is wrong here. If I do not use i.photo.init, the
> > image is placed much better (RMS around 10-20), but still pretty poor…
> About 10 years have passed that I tried the last time, memory is
> fading...
> Did you figure it out?
> ...
> > Btw: Some of the submodules of i.ortho.photo are not linked to the /bin
> > folder in GRASS 6.4.5svn, so I have to start them like this:
> > /usr/local/grass-6.4.5svn/etc/i.photo.init
> To my knowledge they are intentionally hidden in order to be run from
> the i.ortho.photo main menu (in GRASS GIS 6).
> Markus
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