Tricky stitch

The Earith Bulwark. Oliver Cromwell’s engineer had some good things going for him when he set out the trace of the Earith Bulwark, he had a good knowlwedge of 17th Century defensive theory and a level site to set it out on.  The simple 4 bastion plan survives intact. At Earith the defences are deeper than at other Civil War sites, the outer defences show signs of a firing step, artillery platforms; a small ravelin and connecting walkways still join parts of the raised works to this day. The site is strategic as a defence of the river crossings to the East and West of the fort. Movement across the Fens in the 1640s would have been much more restricted than today and any attack (by land or water) on Cromwell’s home turf at Ely from the West would have been blocked here.  Some 360 years later I took a ‘block’ of KAP images of his work.

The fort is almost invisible on approach at ground level today but from the air you can appreciate its form. I have visitited the site and decided as a KAP subject the Bulwark meets the 3 conditions needed for KAP sucess:  1. an open space to fly in, 2. a Westerly wind places the camera over the subject  and 3. sunlight, clearly winter sun is ideal for a low lying feature like this. A quick check with the weather forecasters and off I went to chase the trace.The landscape at Earith is classic Fenland, flat, littered with overhead cables and few trees of any size. To get to my launch site is a bit of a scramble but I’m pleased to see the livestock have been moved to winter quarters. A westerly will give me a good downwind  swath clear of power-lines.

A stiff westerly running at 8 to 15 mph lofted the HQ2.0 and my 850g of rig and camera easily and I watched it bucket about, slowly feeding it line as it rose into a smooth flow. At gust the line sang and I took the precaution of placing an earth anchor for recovery should the wind build.

Exposure grief. The Sun is shining from the East, the wind blowing from the West. I’m pointing the camera down so I figure glare won’t be a problem….wrong! Fen winds are powerful, I use my smaller flowform, drogued and a light line, the idea is for the kite to survive the lull and ride the gust, as the kite climbs it feels like gust all the time as the line begins to howl. With the line singing the rig is bouncing around like crazy but I decide to wait for things to calm down, eventually I’m able to hitch the line to my harness and make a start…as the Sun begins to hide behind high level cloud.  The result of the combination of glare and camera movement produced a strip of awful exposures. The camera was set at ISO800, auto shutter and auto aperture, almost all the nadir shots were overexposed and to make things worse, in haste, I used 180degree  pans to get coverage where I could, which gives opposed polarsing between shots. On the ground I’m unaware of most of this, my video relay let me know when I had a steady view and let me work at a safe height with as much of the subject in view as possible. Before resorting to the unsharp mask for the weak images I overlay the image with a duplicate layer and set the display to multiply: this has 2 effects, the intensity and contrast are increased and there is some improvement in sharpness, an unwanted side effect is a shift in the white balance towards grey making mid tones get purplish.

Swath pattern. The kite was pulling hard and the light fading fast so I abandoned the idea of getting refular strips of imagery by walking the camera across the subject but  opted for the faster ‘pan and shoot’ method which produces a series of perspective ‘fans’,  As the HQ 2.0 flowform reaches its limit of lift it heels across the wind window (usually- sometimes it spins to the ground) and I used this as the basis of the sweep across the site, at each rise in wind speed I’d frame my shots and hope for the best. I considered the shoot over when the weak sun finally faded to flat overcast. I could see clearly the arrow shaped bastions and, unfortunately I seem to have more shots of them than anything else!

Stitching was straightforward for the most part but the South bastion only had very oblique cover so some hefty transforms were needed to bring it in. The batch suffered from some height variation and a very poor rate of sharp image capture, shooting in burst got me some results, 1 out of 6 is better than none! Colour matching is wrecked by the polarizing inversions and at least one blurred image has been pressed into service. Nadir cover has been achieved…just!

After some careful sifting found some better shots, with exposures boosted for intensity and colour saturation equalized…and a geometry check from a screen grab from Google Earth’s 2003 historic layer. The image clearly shows the blunting of the Southern edge (bottom left) of the earthwork by seasonal floods of the washes:I think I have the best I can manage with the shots I have; hopefully this is sharper, colour balanced ( it’s still pretty poor I grant you!) and perspective corrected!

About billboyheritagesurvey

Heritage worker
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One Response to Tricky stitch

  1. Beware of the doctor, whose wife sells cemetery plots, whose brother owns a granite quarry, and whose father deals in shovels. Hundefutter Mera Dog Hundefutter Mera Dog

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