Kite aerial photogrammetry: 1: image capture and cover preview.

With consents in place low level image capture of the Earith Bulwark in Sept 2014 recorded a surface comprising ragwort, thistle and nettle at full growth, it was not successful as a model when processed: the surface was a lumpy mess of vegetation cover. A re-visit in late winter (on right below) to take advantage of a north easterly flow caught the ground cover at low growth and the surface model as as close as possible to a DGM without getting into serious grass cutting.

Earith Bulwark Sept 2014 & Feb 2015The 2 ortho-images  are generated using down-sampled RX1  images 96 (left) and 32  (right). Low angled sunlight is an absolute must for the low lying features of the earthwork to be captured, the February noon cover making the most of the higher intensity and duration of winter light.

_DSC1436Flying height restriction makes it difficult to frame the whole monument, overhead powerlines meant the upwind cover is very limited.

_DSC1296

06022015 Ortho3The output resolution is commensurate with 1:200 scale mapping: Ortho Extract

At the coast the wind was belting along at Bft6/7 and 30 miles inland it was still a handful. Of late I have found the combination of cold air (4C) and Bft4 suits the little Didak Explorer 1.6 well, it’s not a light wind kite but it is stable (after some trimming- I lengthened the outer shrouds by 1cm to get the keels to ‘belly’ out) and only starts to heel in Bft 5.WindspeedThe flight was between 12 and 2: just at peak flow. I picked the Didak Explorer 16 in preference to the more obvious choice of the ‘big wind’ PFK Nighthawk for 3 reasons,

  1. Reach:I wanted a long reach over the subject, the 30 degree flying angle is ideal.
  2. Stability: I’m finding the little flowform less aggressive than the PFK in adapting to gust and lull in the top end of Bft5. The red delta constantly hunts the prime wind vector in a series of tracking moves whereas the flowform pulses and heels in a predictable and less violent manner to deal with the flow variation. My safety box was bounded by powerlines on 3 sides: I had to be certain no sudden drift would risk contact of the kite line with the wires. The PFK likes a lot of sky.
  3. Power to wind ratio: The small size of the 1.6 matches the top end of Bft 4. It goes against the logic that bigger kites are more stable but avoiding an overblown state is best balanced by a reduction in sail area, this creates problems as the natural movement of smaller kites is jerky, the small Explorer 16 has a tendency to rock about but copes with gust well and gives plenty of warning by heeling across the wind well before its overblown.

_MG_3562The diminutive Explorer 1.6 flying off a ground stake, the upwind powerlines behind it limited the cover possible. Walking the kite upwind required a constant eye over the shoulder.

Processing the surface model at the full image resolution takes patience, having got confirmation of the cover with the down sampled images the next step is to process at full resolution.

B

About billboyheritagesurvey

Heritage worker
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