marking the site out into a grid
would you explain '' marking the site out into a grid'' in this context?
On the afternoon of 30th August 1989, fire broke out at Uppark, a large eighteenth-century house in Sussex. The following morning, Uppark stood open to the sky. A sludge of wet charcoal covered the ground floor and basement, and in every room charred and fallen timbers lay amongst the smoke. It was a scene of utter devastation.
After the initial sense of shock, the days which followed the fire were filled with discoveries. Helped by volunteers, the National Trust’s archaeologists and conservators swung into action, first of all marking the site out into a grid and then salvaging everything down to the last door handle. The position of each fragment was recorded, and all the debris was stored in countless dustbins before being sifted and categorized.