Satellite imagery and aerial photographs play an important role in general mapping, as well as GIS data acquisition and visualization. First, they help provide a solid visual effect. Many people are more able to put spatial concepts into perspective when they see photos. In addition, the secondary and perhaps more vital role is to provide a basis for gathering spatial information. Examples of this are features such as roads, vegetation, and water. Before this information can be gathered in a manner that is useful for a mapping or GIS system, the image data must be prepared in a way that removes distortion. This process is called orthorectification. Without this process, you wouldn't be able to make direct and accurate measurements of distances, angles, positions, and areas.