Archaeologists often obtain information from aerial photographs. Crop marks can indicate sites. From these photographs, you can see if the crops are growing high or low. If they're growing high there's probably a ditch beneath. If it's low there's probably a wall or a structure.
Shadow marks show shadows on banks and ditches in the early mornings and late evenings.
From the ground these might look like bumps on the floor, but from a plane you can see patterns, that give you more clues about the site your looking for. Once the site has been located it is prepared before the dig can begin. Archaeologists have to walk around and get an idea of how big it is and the type of soil. This helps archaeologists find out how many people they might need. Archaeologists need to survey the site before, during and after the excavation.