This is part one of a two-part discussion, in which we will provide an overview of the use of aerial photography, topographic mapping and photogrammetry in environmental enforcement actions. The visualization of spatial relationships of natural and man-made features can focus the scope of environmental investigation, and provide a simple, yet quantitative, historical record of changes in conditions on a site. Aerial photography has been used in environmental remote sensing since the early part of the 20th century. Aerial photos are valuable tools for environmental assessment because they provide objective, detailed documentation of surface conditions at a specific time. Furthermore, they can generally be obtained even in cases where access on the ground is denied to investigators. From aerial photos, precise quantitative information can be collected using photogrammetry. Such measurement and positional data can be produced in digital format for input into a Geographic Information System (GIS) for computerized analysis and display. Other information derived from aerial photographs requires specialized photointerpretive skills and experience. These include the recognition of vegetation mortality, oil-spill damage, and the ecological quality of water bodies. The location, extent and historical change of hazardous waste sites can be documented on topographic maps. These maps are often created from aerial photographs, and display the extent and location of real-world features by symbolizing them. The major advantage of maps over aerial photos is that maps can show things that are not visible from the air, while omitting unnecessary and distracting information. Because maps are derived products, they may contain bias in content and presentation, and they must be backed up by careful documentation and quality assurance protocols.