true. Map regression Map regression, comparing maps from different periods of the same area, can reveal past structures that were recorded but are no longer visible as surface features. Using modern maps to transcribe or re-project earlier maps can help to locate these features with contemporary survey controls and techniques. Aerial photography Aerial photography is a good tool for planning a survey. Remains of older buildings often show in fields as cropmarks; just below the topsoil, the remains may affect the growth of crops or grass. There should preferably be photographs of the same area at different times of the year, allowing the analyst to find the best time to see cropmarks. Previous work in the region If the indicator that started the process was not a record of previous work, the archaeologists will need to check if any work has been done prior to commencement of the pending project. As many older surveys and excavations were published in papers that are not widely available, this may be a difficult task. A common way to handle this is through a visit to the area, to check with local museums, historians and older people who might remember something about the former activities in a particular locale. Permissions It is usually a simple matter to gain permission to perform a cultural field survey, especially a non-intrusive one. If the area is privately owned, the local laws may or may not require the landowners' co-operation. Permission for an intrusive form of survey may be more difficult to acquire, due to the fear of destroying evidence or property values and the threat of lawsuit for said damages from the property owner. Intrusive vs. non-intrusive surveys In a non-intrusive survey, nothing is touched, just recorded. An accurate survey of the earthworks and other features can enable them to be interpreted without the need for excavation. An intrusive survey can mean different things. In some cases, all artifacts of archaeological value are collected. This is often the case if it is a rescue survey, but less common in a regular survey. Another form of intrusive research is bore holes. Small holes are drilled into the ground, most often with hand-powered bores. The contents are examined to determine the depths at which one might find cultural layers, and where one might expect to strike virgin soil. This can be valuable in determining the cost of an excavation