In plane English, accuracy describes how accurately a point on the map is located compared to the actual location of that point on the ground. Technically, accuracy of orthophotography is described in terms of its root mean square error (RMSE) (the higher the error, the lower the accuracy). This is very important for properly locating features on the ground, and for overlaying or creating other accurate data in a GIS system.

The 2005 orthophotography will have the following accuracy specifications that meet or exceed well accepted standards for accuracy:

A DEM is a model of the earth’s surface which is used to remove distortions in the aerial photography caused by changes in land elevation (valleys and ridges). The DEM may include treetops, rooftops, and tops of towers, telephone poles, and other natural or manmade features; or it may include the ground surface if there is no vegetative ground cover. The DEM allows photography taken of a 3-dimensional surface to be rectified to an accurate 2-dimensional photo-map. The DEM used for ortho-rectification of the 2005 orthophotography will be delivered as part of the project.

Orthophotography is part of the basemap in a geographic information system (GIS). It is used like a photograph as a visual reference, and since it has the qualities of a map it is used to generate other important mapping data. Mapping can be used for emergency response planning and modeling, response, as well as recovery. Law enforcement can use it for mapping trends, thereby improving performance, and public health agencies can use it for syndromic surveillance. And because the basemap is shared with other local, state and federal agencies, the GIS systems can be used for other applications as well, such as property management, Census, tax assessment, flood mapping, planning, and economic development. By having different agencies reference the same basemap, data developed independently can “fit together.”