Nicholson, L. and Mertes, J. (2017) Thickness estimation of supraglacial debris above ice cliff exposures using a high resolution digital surface model derived from terrestrial photography. Journal of Glaciology. doi:10.1017/jog.2017.68
Years and years ago when I was doing my PhD we wanted to get an idea of how thick the layer of rubble and dust was over the Ngozumpa glacier (in the photo below – this is the biggest glacier in Nepal and the lower 12-15 kms of it are covered in rock debris/rubble). To find out more about debris-covered glaciers like the Ngozumpa, have a look here.
Its pretty unrewarding work to dig through this kind of rock debris overlying the ice, but the debris covered part of the glacier is studded with steep bare ice cliffs, above which the rock debris layer can be seen in a vertical section. So, to avoid digging holes, we hit on a good idea that involved Doug Benn running round the crest lines of these exposed cliffs with a surveying reflector while I surveyed his position from the glacier side moraine, using a theodolite to measure the dip and distance to the reflector and also the dip to the debris-ice interface beneath him. Using simple trigonometry, and imagining the ice cliff geometry to be simpler than it really is, we could use these measurements to get an estimate of the debris thickness visible above the cliffs.
The world moves on. Now simple photographs can be used with some very cool software (Structure from Motion – Multi View Stereo processing software to be precise) to relatively easily make digital 3D surface models. It occurred to me that the kind of measurements I did in my PhD could be done readily, and probably more accurately, from a high-resolution terrain model. In spring 2016, Anna Wirbel and I took a bunch of photos of ice cliffs to see how it would go.
The short answer is: it turned out rather nicely. Below is an oblique view of the surface model we made from our photos and the coloured dots forming lines along the ice cliff crests show the implied debris thickness (hd) in centimetres along the three ice cliffs we took pictures of. For an idea of scale the cliff peak furthest away is almost 45m high. The numbered red and yellow dots are ground control points that were accurately surveyed and used to scale the model correctly.
For the long answer you can read the paper – its freely available online 🙂
I’d really like to see if the lower resolution, but closer-to-target, imagery from UAVs, that are now almost regularly flown over debris-covered glaciers can also be used to perform similar debris thickness assessments at a glacier scale. I don’t have a UAV though …. so I hope someone else will do that with their UAV images!