JC010: Mud volcanoes and submarine canyons


 JC10



Photo mosaicking


I am sure you have heard of a mosaic before and you have perhaps even made mosaics from photographs in art class. Scientists use the same idea to help them understand more about the deep sea. To do this, ISIS link to page takes high-resolution (very detailed) photographs which allow the scientists to construct a ‘mosaic’ from them so that you can zoom in and out. This is a similar method to that used in making mapping websites like the new version of Multimap, Live Local and also Google Earth.

To take the photos, ISIS hovers 5m from the seabed (although this must be the same distance for each photo in a mosaic) and takes pictures, whilst stationary, at regular distances along a line, then goes back in a line beside the first one, taking pictures again, and again. Rather like this (the numbers refer to the order of the pictures):

A ‘PixelFly photograph’ of the seabed in the Nazaré Canyon and visual description of how
Isis takes the photographs in order. Note that the green arrows show the direction Isis moves,
but it ALWAYS faces the same direction,
so sometimes the ROV is reversing and sometimes going forward.


This screenshot (left) of the ROV control computer screen shows the position of Isis and RRS James Cook, whilst Isis takes pictures for a photomosaic.

Key:
Red line: RRS James Cook
Green plane: Isis
Yellow plane: Isis

The reason Isis is shown twice on the display is that there are two methods by which the ROV can be detected. In reality, the ROV is somewhere between the two.

With the ROV flying at 5m altitude above the seabed, the pictures that are taken are of an area of 46m2 and have a 50% overlap so that there are ‘reference points’ (the same part in two different pictures) that allow them to be attached together, using special software. This means that you can make a large mosaic of very detailed photographs, so that you can zoom in to it and see the seabed in great detail. This method of photographing the seafloor is better than using still images from video footage, because when the ROV stays still above the seabed, very accurate photos can be taken.

The ROV stays at 5m above the seabed because if it was any lower the photographed area would be very small and lots of photos would need to be taken to get an overview of the seabed. Fly any higher than 5m and the light from the camera flash does not penetrate to the seabed properly and the images are very dark. The features would also be too small to be seen.

Sarah ‘stitching’ the separate photos together to make
one big one – the photomosaic. This takes a lot of patience!

Andy Wheeler likens looking at the seabed to a farmer wanting to map his field: 
If a farmer wants to look at his field in detail, he can fly over it in a plane, or walk across it. If he flies across it, he can get an overall picture of the field, but not much fine detail. If he walks across it, looking down between his feet, he can see the field in great detail, but not the whole picture. Due to the difficulties in seeing through water, it is not an option to fly high above the seabed and take the photos from a distance anyway. Photomosaicking allows you to have the best of both worlds – you can zoom out to get the whole picture and you can zoom in for the fine detail, due to the high-resolution shots used.

Below are some video mosaics (they use video stills instead of photographs) by M. Kozachenko at University College Cork, of the Moira Mounds, off the SW coast of the Republic of Ireland. Do you think they would have looked this impressive if it was just one picture taken from higher up in the water?


For you to do:
Check out the PowerPoint activity (click on the icon to the right to download), where you can construct your own photomosaic of the seabed! The photos have already been taken for you and you just have to stitch them together.



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