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    • CommentAuthormtravers
    • CommentTimeJul 11th 2008
     
    I would like to know if anyone has developed a project based on underwater stereo photos? I am a newbie to photogrammetry but really interested to see whether we can track changes in coral growth over time using this method. We have taken photos ca. 70 cm above the substrate and there are no surveyed ground control points but we have bars of known length within each photo. In the first instance we would like to generate as accurate a measure of x, y surface area but would like to progress into true 3d coverage if possible.

    My initial concern is that our photos are not clearly focused enough due to the difficult in keeping still underwater. I cant quite grasp whether I can just use the known distance function with maybe a few pass/tie points or whether we somehow must plig in a ground control point file somehow? Does ImageMaster or PI utilise automatic image recognition of control points or do they always have to be manually placed?

    Apologies for the vagaries and I am looking forward to the video tutorials that will be available soon to get me up to speed
    • CommentAuthorAndy Evans
    • CommentTimeJul 11th 2008
     
    I am not aware of any projects using underwater photos. I would imagine you might get some odd results....

    Regarding control, it depends if you just want a scale model or if you want to tie into your local coordinate system. If you just want scale, then to create the model you only need to identify pass/tie points to stitch the photos together, the scale only needs you to identify the 2 points measured on the ground.

    Picking the tie points is semi-automatic. In that you only need to idenitfy in one photo and the software should match the points in the other open overlapping images. If you have a number of stereo-pairs open, the software will attempt to mark the point you have picked in every open image.

    I hope that explains a little more for you.
    • CommentAuthormtravers
    • CommentTimeJul 15th 2008
     
    Thanks Andy

    Would you mind elaborating on the odd results you would expect that we may encounter?...

    A scale model is all we are chasing so it looks like I do not need to worry about the control points but need to have the 2 points measured on the ground which in each photo would be either the 10cm quadrat marks or the twin laser pointers in each photo which are exactly 10 cm apart also (problems with this due to the variable topography of the reef) but these should prove to be good tie-points.
    • CommentAuthorIanS
    • CommentTimeNov 21st 2008
     
    If you can get a reliable calibration of the camera you should be able to get decent measurements from the stereo pair by following usual photo taking rules. 1/3 of separation between images to object distance. It may be necesary to calibrate the camera in its undewater state. Take the images of the control board underwater. However I have no experience wether this would be necessary or not.
    • CommentAuthorpexejurer
    • CommentTimeDec 6th 2018
     
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