marking the site out into a grid
Hi
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.
Thank you

Question

marking the site out into a grid
Hi
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.
Thank you

Answer

The site [the various floor areas of the house] was labelled, in 2 dimensions, like a mathematical graph, so that the location at which various surviving objects were found could be defined by co-ordinates.  When the objects had been removed, the conservators would know in which part of the house they had been located, so an accurate picture could be constructed of the house before the fire.
The same technique is used by biologists in field work, using a grid of [usually] one metre square areas to define where plants or animals are found within a larger area.