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The polar vortex event in January 2019 brought extreme fluctuations in temperatures that cracked the entire length of this 250 foot stack.

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The crack affected both the brick liner and the steel outer casing. The unstable stack had to be monitored before and during demolition.

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The monitoring solution involved placing survey prisms onto the steel casing of the stack using a drone equipped with a ten-foot arm.

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Standard prisms would be too heavy for the drone, so GEO proposed microprisms fitted with magnetic bases.

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The photo above shows the arm of the drone loaded with a microprism. The magnetic base is facing outwards.

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Manual surveyors determined locations and angles for each of ten prisms. Drones then carried the prisms to the stack.

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The photo above gives some idea of the skill required by the drone pilots. A second drone was used to provide a cross-axis view.

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A microprism is visible in the crosshairs of the AMTS. The cracked steel and brickwork are clearly visible to the right.

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The stack was monitored continuously, with updates posted on the GeoCloud website every 30 minutes.