Connecticut 1990 Orthophoto is a raster mosaic of 1990 orthophotography for Connecticut. It represents a mosaic of 414 digital orthophoto quarter quads (DOQQ) produced by the U.S. Geological Survey (USGS). The date of the orthophotography is 1990. The raster mosaic is based on DOQQs that are gray-scale, leaf-off, 1:12,000 scale (1 inch = 1,000 feet), quarter-quadrangle (3.75 minutes of latitude by 3.75 minutes of longitude) input raster images with 1-meter ground resolution. Input raster DOQQs are in Tagged Image File Format (TIFF) format and named "ortho_1990_quad0078se.tif" after CT DEP quadrangle number 78, SE quadrant, for example. Each 7.5 minute quadrangle is broken into four quadrants: NE, NW, SE, and SW. Each of these 3.75 minute quarter quadrangles is referred to as a quarter quad.
	
Orthophotos combine the image characteristics of a photograph with the geometric qualities of a map. The primary orthophoto produced by the USGS is the digital orthophoto quarter quadrangle (DOQQ), a 1-meter ground resolution, quarter-quadrangle (3.75 minutes of latitude by 3.75 minutes of longitude) image cast on the Universal Transverse Mercator projection (UTM) on the North American Datum of 1983 (NAD83). The geographic extent of the DOQQ is equivalent to a quarter-quadrangle plus the overedge ranges from a minimum of 50 meters to a maximum of 300 meters corner points. The overedge is included to facilitate tonal matching for mosaicking and for the placement of the NAD83 and secondary datum corner ticks. The normal orientation of data is by lines (rows) and samples (columns). Each line contains a series of pixels ordered from west to east with the order of the lines from north to south. The radiometric image brightness values are stored as 256 gray levels, ranging from 0 to 255.

DOQs (or DOQQs) serve a variety of purposes, from interim maps to field references for Earth science investigations and analysis. The DOQ is useful as a layer of a geographic information system and as a tool for revision of USGS digital line graphs and topographic maps. A DOQ can be incorporated into any geographic information system (GIS) that can manipulate raster images. It can function as a cartographic base for displaying, generating, and modifying associated digital planimetric data. Other applications include vegetation and timber management, routing and habitat analysis, environmental impact assessments, emergency evacuation planning, flood analysis, soil erosion assessment, facility management, and ground-water and watershed analysis. The accuracy and detail provided by the DOQ allow users to evaluate their data for accuracy and completeness, make real-time modifications to their data, and create new spatial data.

The Connecticut Department of Environmental Protection converted the 1990 DOQQs produced by the USGS from UTM (meters) to the Connecticut State Plane Coordinate System North American Datum of 1983 (NAD83). Map units are feet, not meters. These DOQQs are in Tagged Image File Format (TIFF) format and named ortho_1990_quadxxxxYY.tif, where xxxx is a 4 digit quadrangle number beginning at 0001 and YY identifies the quarter quadrange (ne, nw, se, sw). The Connecticut Department of Environmental Protection subsequently created a statewide mosaic of these 1990 DOQQs.

An orthophoto or orthophotograph is an aerial photograph that has been geometrically corrected ("orthorectified") such that the scale of the photograph is uniform, meaning that the photo can be considered equivalent to a map. Unlike an aerial photograph, an orthophotograph can be used to measure true distances, because it is an accurate representation of the earth's surface, having been adjusted for topographic relief, lens distortion, and camera tilt.

For additional specific information on the production of digital orthophoto quadrangles, refer to Standards for Digital Orthophotos (National Mapping Program Technical Instructions) published by the U.S. Department of Interior, U.S. Geological Survey, National Mapping Division.

ground condition

None. No restrictions or legal prerequisites for accessing the data. The data is in the public domain and may be redistributed.

None. No restrictions or legal prerequisites for using the data after access is granted. The data is suitable for use at appropriate scale, and is not intended for maps printed at scales greater or more detailed than 1:12,000 scale (1 inch = 1,000 feet). Although this data set  has been used by the State of Connecticut, Department of Environmental Protection, no warranty, expressed or implied, is made by the State of Connecticut, Department of Environmental Protection as to the accuracy of the data and or related materials.  The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the State of Connecticut, Department of Environmental Protection in the use of these data or related materials. The user assumes the entire risk related to the use of these data. Once the data is distributed to the user, modifications made to the data by the user should be noted in the metadata. When printing this data on a map or using it in a software application, analysis, or report, please acknowledge the CT DEP and the USGS as the source for this information. For example, include the following data source description when printing this information on a map: 1990 Orthophotos - From USGS DOQQ. Source map scale is 1:12,000. Acknowledgement of the U.S. Geological Survey is appreciated in products derived from these data.

All DOQ header data and image file sizes are validated by Tape Validation System (TVS) software prior to archiving in the Sales Data Base. This validation procedure assures correct physical format and field values for header record  elements. Logical relationships between header record elements are tested.

All DOQ images are visually inspected for completeness to ensure that no gaps or image misplacement exists in the image area or in overedge coverage. The DOQ images may be derived by mosaicking multiple images, in order to insure complete coverage. All DOQs are cloud free. Some clouds may, very infrequently, be encountered only in the overedge coverage. Source photography is leaf-off in deciduous vegetation regions. Void areas may exist, having a radiometric value of zero and appearing black. These voids are areas for which no photographic source is available or are the result of image transformation from other planimetric systems to the UTM projection. In the latter case, the void sliver areas are on the outside edges of the overedge area. The data set field content of each DOQ header record element is validated to assure completeness prior to archiving in the Sales Data Base.

During photographic reproduction of the source photography, limited analog dodging is performed to improve image quality. Analog dodging consists of holding back light from certain areas of the sensitized photographic material to avoid overexposure. The diapositive is inspected to insure clarity and radiometric uniformity. Diapositive image brightness values are collected with a minimum of image quality manipulation. Image brightness values may deviate from brightness values of the original imagery due to image value interpolation during the scanning and rectification processes. Radiometry is verified by visually inspecting the DOQs to the original unrectified image to determine if the DOQ has the same image quality or better image quality than the original unrectified input image. Slight systematic and radiometric differences can be detected between adjacent DOQ files due primarily to differences in source photography capture dates and Sun angles of aerial photography along flight lines. These differences can be observed in an image's general lightness or darkness when compared to adjacent DOQ file coverages.

The DOQ horizontal positional accuracy and the assurance of that accuracy depend, in part, on the accuracy of the data inputs to the rectification process. These inputs consist of the digital elevation model (DEM), aerotriangulation control and methods, the photo source camera calibration, scanner calibration, and aerial photographs that meet National Aerial Photography Program (NAPP) standards. Field control is acquired by third order class 1 or better survey methods sufficiently spaced to meet National Map Accuracy Standards for 1:12,000-scale (1 inch = 1,000 feet) products. Aerial cameras have current certification from the USGS, National Mapping Division, Optical Science Laboratory. Test calibration scans are performed on all source photography scanners.
		
		Horizontal positional accuracy is determined by the Orthophoto Accuracy (ORACC) software program for DOQ data produced by the National Mapping Division. The program determines the accuracy by finding the line and sample coordinates of the passpoints in the DOQ and fitting these to their ground coordinates to develop a root mean square error (RMSE). From 4 to 9 points are checked. As a further accuracy test, the image line and sample coordinates of the DEM corners are transformed and compared with the actual X,Y DEM corner values to determine if they are within the RMSE. Additional information on this testing procedure can be found in U.S. Department of the Interior, U.S. Geological Survey, 1993, Technical Instructions, ORACC Users Manual (draft): Reston, VA. Adjacent DOQ's, when displayed together in a common planimetric coordinate system, may exhibit slight positional discrepancies across common DOQ boundaries. Linear features, such as streets, may not be continuous. These edge mismatches, however, still conform to positional horizontal accuracy within the NMAS.
		
		Field investigations to validate DOQ positional accuracy reliabilty are periodically conducted by the USGS, National Mapping Division, Geometronics Standards Section. DOQ's produced by cooperators and contractors use similarly approved RMSE test procedures.

The vertical accuracy of the verified USGS format DEM is equivalent to or better than a USGS level-1 or -2 DEM.

Connecticut 1990 Orthophoto

Although this data set  has been used by the State of Connecticut, Department of Environmental Protection, no warranty, expressed or implied, is made by the State of Connecticut, Department of Environmental Protection as to the accuracy of the data and or related materials.  The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the State of Connecticut, Department of Environmental Protection in the use of these data or related materials. The user assumes the entire risk related to the use of these data. Once the data is distributed to the user, modifications made to the data by the user should be noted in the metadata.

Geographic information sytem (GIS), computer-aided drawing or other mapping software is necessary to display, view and access the information.

The data distributor does not provide custom GIS analysis or mapping services. Data is available in a standard format and may be converted to other formats, projections, coordinate systems, or selected for specific geographic regions by the party receiving the data.

None. No restrictions or legal prerequisites for accessing the metadata.

None. No restrictions or legal prerequisites for using the metadata after access is granted.