Product: This orthoimagery data set includes 0.5 ft 16-bit 4-band (RGB-IR) digital orthoimage tiles in GeoTIFF format.
Geographic Extent: Eight counties in Connecticut, covering approximately 5241 total square miles.
Dataset Description: The USGS Connecticut Orthoimagery project called for the planning, acquisition, processing, and derivative products of imagery data to be collected at a ground sample distance (GSD) of 0.5 ft. Project specifications are based on the American Society of Photogrammetry and Remote Sensing (ASPRS) standards. The data were developed based on a horizontal projection/datum of NAD 1983 2011 StatePlane Connecticut FIPS 0600 Ft US, Foot US. Orthoimagery data were delivered in 23381 individual 2500 ft x 2500 ft tiles as 0.5 ft 16-bit 4-band (RGB-IR) orthophoto tiles in GeoTIFF format.
Ground Conditions: Imagery was collected in spring 2019, while no snow was on the ground and rivers were at or below normal levels. In order to post process the imagery data to meet task order specifications and meet ASPRS horizontal accuracy guidelines, Quantum Spatial, Inc. utilized a total of 168 ground control points to assess the horizontal accuracy of the data.These data depict geographic features on the surface of the earth. The dataset was created to current imagery data for Connecticut.
Ground condition
None. However, users should be aware that temporal changes may have occurred since this dataset was collected and that some parts of these data may no longer represent actual surface conditions. Users should not use these data for critical applications without a full awareness of their limitations. Acknowledgement of the organization providing these data to the public would be appreciated for products derived from these data.
Quantum Spatial, Inc.
Radiometry is verified by visual inspection of the digital orthophoto. Slight systematic radiometric differences may exist between adjacent orthoimage files; these are due primarily to differences in source image capture dates and sun angles along flight lines. These differences can be observed in an image's general lightness or darkness when it is compared to adjacent orthoimage file coverages. Tonal balancing may be performed over a group of images during the mosaicking process which may serve to lighten or darken adjacent images for better color tone matching.
All GeoTIFF tagged data and image file sizes are validated using commercial GIS software to ensure proper loading before being archived. This validation procedure ensures correct physical format and field values for tagged elements. Seamlines and tile edges are visually inspected. Seamline mismatches are corrected unless the overall displacement is less than one pixel.
Orthoimages are visually inspected for completeness to ensure that no gaps or image misplacements exist within and between adjacent images. These images are derived by mosaicking multiple images to ensure complete coverage. Source imagery is cloud free.
The project specifications require that only horizontal accuracy be computed for orthoimagery files. The required accuracy meets or exceeds ASPRS standards. The horizontal accuracy was tested with 20 checkpoints located throughout the project area. These checkpoints were not used in the calibration or post-processing of the imagery data. The checkpoints were distributed throughout the project area and were surveyed using GPS techniques. See survey report for additional survey methodologies. The x and y positions of where the checkpoints fell on the orthoimagery were collected. These values were then compared to the surveyed control point x and y values.
Tested 0.0993648 meters (0.33 feet) RMSEx. The RMSEx of the imagery was calculated using 20 independent checkpoints.
Tested 0.0414528 meters (0.14 feet) RMSEy. The RMSEy of the imagery was calculated using 20 independent checkpoints.
Tested 0.1075944 meters (0.35 feet) RMSEr. The RMSEr of the imagery was calculated using 20 independent checkpoints.
There is no vertical component for orthophotos.
Quantum Spatial performed a geodetic control survey in support of the digital orthophoto production project. A total of 168 points were used for QC testing. Please see the survey report for more information.
Aerial imagery was acquired using a UltraCam Eagle camera with a flight design that included a total of 116 flight lines. Aerial imagery was supplemented with the simultaneous acquisition of airborne GPS/IMU data, which captured the ground coordinate for the nadir point of each photograph. Aerial imagery was exposed at an altitude of 8650 feet above mean terrain.
Softcopy aerotriangulation was performed utilizing the airborne GPS/IMU data, GPS ground control and image coordinate measurements allowing the direct computation of the exterior orientation parameters for each image of the project.
A compilation of topographic land form elevation datasets developed using in-house LiDAR data and data from the National Elevation Dataset for use in developing digital ortho imagery.
Orthophoto Processing: Utilizing all four bands [blue (B), green (G), red (R), and infrared (IR)] digital orthorectification was performed using bilinear interpolation algorithms resulting in a spatial and radiometric transformation of the digital image from line/sample space into NAD 1983 2011 StatePlane Connecticut FIPS 0600 Ft US, Foot US. The interior and exterior orientation parameters from the aerotriangulation process were used to project each pixel into the ground coordinate system, while the ortho grade DEM was used to correct for relief displacement. Radiometric correction software and techniques were used to create orthophoto files that minimize the appearance of image seams and without loss of feature signature. Orthophotos are checked for geometric accuracy, image quality, and are tonally balanced to produce a uniform contrast and tone across the entire project. The individual overlapping orthophoto frames were mosaicked together. The ortho photos meet ASPRS horizontal accuracy standards.
Three band orthoimagery is organized in three color bands or channels which represent the red, green, and blue (RGB) portions of the spectrum. Four band orthoimagery is organized in four color bands or channels which represent the red, green, blue (RGB), and near infrared (IR) portions of the spectrum. Each image pixel is assigned a triplet or quadruplet of numeric values, one for each color band. Numeric values range from 0 to 255.
U.S. Department of the Interior, U.S. Geological Survey, 1996, Standards for Digital Orthophotos: Reston, VA.
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