The architect Sir John Soane prepared designs in 1793-95 for "a lazaretto at Chetney Hill
to provide the necessary warehouses ...". These drawings are now kept at Sir John Soane's
Museum in London, which holds his collection of drawings and antiquities, and his own
drawings and designs for his commissions (
www.soane.org). Soane's plans were not adopted.
Froggatt reports that James Wyatt was entrusted with the project and that his plans were
destroyed in fires, at parliament and at the Custom House.
The engineer Rennie was also engaged for the project. From Samuel Smiles, Lives of the
Engineers Smeaton and Rennie, London 1904:
"It was proposed to isolate this hill by a canal, provided with a lock; and Mr Rennie
was requested to prepare the requisite plans, which he did (in 1806), and the works
were executed at a heavy expense"
The first Ordnance Surveys were carried out in the 1780s and 1790s as part of preparations
for an expected war with France. Chitney Marsh was surveyed in 1797, and this is an extract
from the surveyor's field book (held at the British Library, on-line gallery of exhibitions,
Ordnance Survey drawings, Halstow):

The drawing is 6in to the mile. This extract shows a bend in Stangate Creek (left), and the
Shade (a smaller creek leading from it to bottom right). Part of Greenborough Marsh is left,
Barsoe Marsh bottom left, and Chitney Marsh from mid top to bottom right. Chitney Hill is the
flat area of grassland in the centre with saltings to the N and SE. Three counter walls
converge on the hill. The red object at the edge of the hill is the Old Chitney farmhouse. The
white patches on the hill might indicate pools on the summit, or might just be an unevenness
in the watercolour wash applied by the surveyor to his field drawing.
These surveys were published in 1801 as "an Entirely New and Accurate Survey of the
County of Kent with Part of the County of Essex" by Captain William Mudge, at a scale of
1in to the mile (on-line at map-co.co.uk). This extract shows the area from Chitney Hill
to Iwade:

Being to a smaller scale, this adds no extra detail. The hill itself is indicated by cross-hatching,
which is possibly deceptive since it's only a hill in relation to the saltings, relatively flat,
maybe 10-15ft above sea level. The on-line maps don't show any height, the farm track
nearby is 12ft.
These two maps show the state of the area before any work started.
Froggatt reports buildings being recorded on the 1819 OS 1in edition. This is not available
on-line.
The next map available on-line is the OS 1869 6in edition:

Chitney Hill is largely obscured by the lettering of the printed info. There's a well (below
the D of IWADE), a pond (between W and A), and a triangle that might mean a trig point
(between W and A above the pond). Near the shoreline in the south is a building that
Froggatt interprets as a farm building rather than a remaining part of the lasaret. This building
was still there in 1961, but not much longer. No on-line OS map, 1869 or later, shows the scar
seen on GoogleEarth (previous posting).
The striking difference to the earlier maps is the canal and the dock. I wonder what happened
to the vast amount of spoils - for example, were they distributed over the island? Were they
carted away? They were not used for a seawall round the island. How was the work done? By
shovel and wheelbarrow, or did they have mechanical diggers? Froggatt's appendix with costs
for the project includes coal for some £670. Does this point to the use of steam engines?
Presumably the canal would need pumping dry as work progressed.
The 1885 25in edition records a shingle hard across the canal from Old Chitney, still there in 1938.
The 1961 25in edition shows a causeway across the canal:

Froggatt includes a photo from an aerial survey done by the OS in 1947 revealing regular patterns of
foundations. The detail has not survived well, from photo to book illustration which was then scanned
for the on-line copy:

For comparison the same aspect from GoogleEarth:

These foundations straddle the scar (also visible on the 1947 photo)
The Notes on the GoogleEarth photo:
F F F F mark the corners of the foundations
P P mark pools recorded on the OS maps
W is the well
B is the site of the building seen on the OS maps from 1869 to 1961 (gone in 1967).