3d photography as a measurement tool

The bass end of the Carolan harp (which was sometimes called the Rose Mooney harp) is very damaged, and there has been a lot of movement inside the bass joint. However it’s not possible to measure this movement from the outside, because of the later repairs with iron straps and canvas bandages completely covering this part of the harp.

I had an idea to try and make stereo pair photographs of this part of the harp, to see if I could use them to measure the amount of movement both downwards (towards the bass end of the soundbox) and backwards (towards the back of the harp).

My stereo pair makes a very lovely anaglyph for you to view with red-green 3D goggles, but I had less success trying to use photogrammetry apps to create a 3d model from these two images.

The camera was moved by 29mm (±1mm) between exposures, and the camera was about 43cm from the closest parts of the end of the soundbox. Other internal features can be used to calibrate a photogrammetric model, but I think these get a lot less reliable further from the centre of the camera’s field of view. My camera is not calibrated or rectified (straight lines on the real world show as curves on the image). Feel free to experiment with these two images and see if you can get a better result than I have.

I calculate that the flat central panel has moved away from the camera by 46mm. I estimate this is ±5mm – not super accurate but a lot better than anything we had before.

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