Have you ever wanted to purchase new imagery (or download free imagery from a government website) of your county or municipality? Here are just a few requirements you may want to think about before writing that big check for new data. Digital orthoimagery is the foundation for most public and private Geographic Information Systems (GIS). Orthoimagery refers to aerial and satellite imagery that has been geometrically corrected, or orthorectified, to correct for changes in the aircraft or satellite orientation and displacement in the imagery caused by changes in ground elevation. Orthorectification produces a uniform scale throughout the imagery, allowing the imagery to be used as a base map. This base map can be displayed in a GIS system and used to measure true distances and areas, or to collect and update new features in the imagery such as wetland changes, building footprints, land use patterns, crop types and transportation routes. In addition, existing GIS layers such as tax parcels, roads, and utility lines can be accurately overlain on the orthoimagery.
State and local governments may have a variety of concerns about acquiring orthoimagery such as the appropriate resolution or scale, best time of year to collect, accuracy standards, coordinate system and natural color versus false color infrared imagery.
It is important to select a mapping scale and resolution based upon the smallest objects that need to be identified. A three-inch resolution image (1”=50’ scale) is useful for identifying small features such as manhole covers or utility poles. Six-inch resolution imagery (1”=100’ scale) is a common resolution for urban areas, while one-foot resolution imagery (1”=200’ scale) is typically acquired in less developed and rural areas. Larger features such as road centerlines and buildings can be collected at many different resolutions.
The time of acquisition (time of year and time of day) for imagery can be critical. Imagery collected during leaf-off, snow-free conditions is essential for many local government applications where infrastructure that exists beneath tree canopies needs to be captured. Building and tree shadows as well as local tides in coastal areas may affect the time of day the imagery is acquired.
The horizontal accuracy of the orthoimagery dictates how well it will overlay with other GIS layers. Two common accuracy standards for county and municipal mapping applications are the American Society of Photogrammetry and Remote Sensing (ASPRS) and the National Standard for Spatial Data Accuracy (NSSDA) accuracy standards. The coordinate system and datum are also important considerations when acquiring orthoimagery. State Plane and Universal Transverse Mercator (UTM) are common coordinate systems and the North American Datum of 1983 (NAD 83) is a commonly used datum in the United States.
The number of spectral bands associated with an image refers to its spectral resolution. Most digital cameras have the traditional red, green, and blue bands associated with natural color imagery in addition to a fourth band, or near-infrared band, that is useful for identification of vegetation and wetlands. Commercial satellite imagery is usually acquired with four bands. The recently launched WorldView-3 satellite can collect up to 27 spectral bands!
The cost for these products depends on the level of detail and processing that is required. Be prepared to pay a little extra for increased horizontal accuracy and resolution. Six-inch resolution imagery is more expensive than one-foot resolution imagery. Additional costs can be expected for items such as removal of building lean, increased resolution of elevation data derived from the imagery, and stereo imagery.
A number of federal agencies have contract vehicles that provide access for governmental entities (local, state, or federal) to acquire geospatial services from the private sector. The U.S. Geological Survey offers the Geospatial Products and Service Contract (GPSC), while the National Oceanic and Atmospheric Administration has the Coastal Geospatial Services Contract (CGSC). Organizations interested in partnering with the U.S. Department of Agriculture Farm Services Agency’s National Agriculture Imagery Program (NAIP) can use their Indefinite Delivery Indefinite Quantity (IDIQ) contract.
A few federal agencies provide access to free imagery for most of the lower 48 states, including the U.S. Geological Survey’s National Map Viewer, the U.S. Department of Agriculture’s Geospatial Data Gateway and the National Oceanic and Atmospheric Administration’s Digital Coast Data Registry.