I think the sign of human activity that is the most visible from the furthest out into space is forestry clearcuts. They are much bigger than cities. Farms fad

I think the sign of human activity that is the most visible from the furthest out into space is forestry clearcuts. They are much bigger than cities. Farms fade and start to blend together and their perfect geometric shapes are harder to see. But clearcuts are more haphazard but spread over hundreds of kilometers, so even when you zoom out really far, it just looks like someone spilled a bunch of salt on the map.