Product: Georeferenced un-rectified stereo images, unedited rectified frames, and orthoimagery. Geographic Extent: Four partial counties in Connecticut, New York and Rhode Island, covering approximately 210 total square miles. Dataset Description: Long Island Sound CT Coastal 0.5-meter orthoimagery project called for the planning, acquisition, processing, and derivative products of imagery data to be collected at a ground sample distance (GSD) of 0.25 to 0.52 meters. Project specifications are based on the U.S. Geological Survey National Geospatial Program Base Orthoimagery Specification, Version 1.0. The data was developed based on a horizontal projection/datum of NAD83 (2011), State Plane Connecticut, meters and vertical datum of NAVD88 (GEOID12B), meters. Imagery data was delivered as 4-band 16-bit georeferenced un-rectified stereo images in .TIF format, 4-band 16-bit unedited rectified frames in .TIF format, and 0.5-meter 4-band (RGB-NIR) 16-bit orthoimagery tiles in GeoTIFF format. Surface data used in the orthorectification process was produced in .TIF format. Ground Conditions: Imagery was collected in spring of 2017, 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 horizontal accuracy guidelines, Quantum Spatial, Inc. established a total of 14 control points and 23 accuracy checkpoints were used to assess the vertical accuracy of the data.
This project will collect imagery that maximizes the potential to delineate coastal, estuarine, and marine features. The collected imagery will aid in the long-term management and provide a framework for conservation and management across the land-sea interface. These efforts will provide important and replicable data and an information framework for ecosystem-based coastal and marine conservation planning and implementation in the Long Island Sound that can also serve as a model for other estuaries. It was created to current imagery data for the U.S. Fish and Wildlife Service (FWS) and the cooperation with University of Rhode Island’s Environmental Data Center (RIEDC).
ground condition
None. However, users should be aware that temporal changes may have occurred since this data set was collected and that some parts of this data may no longer represent actual surface conditions. Users should not use this data for critical applications without a full awareness of its limitations. Acknowledgment of the U.S. Geological Survey would be appreciated for products derived from these data.
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 horizontal be computed for orthoimagery files. The required accuracy is: 1 meter RMSEx and RMSEy, derived according to NSSDA. The RMSEx and RMSEy were tested with 19 of 23 checkpoints. The checkpoints were distributed throughout the project area and were surveyed using GPS techniques. Values from the orthoimagery were measured for the x,y location of each check point. Horizontal positions interpolated from the orthoimagery were then compared to the horizontal values of the surveyed control points. Accuracy has been tested to meet 1 meter or better as defined by the National Standards for Spatial Data Accuracy (NSSDA).
Tested 0.207 meters RMSEx as defined by the National Standards for Spatial Data Accuracy (NSSDA). The RMSEx of the orthoimage was calculated using 19 checkpoints.
Tested 0.282 meters RMSEy as defined by the National Standards for Spatial Data Accuracy (NSSDA). The RMSEy of the orthoimage was calculated using 19 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 14 control points and 35 photo identifiable objects and pre-flight targeting was used to provide ground based photo control. Please see the survey report for more information.
Aerial imagery was acquired using DMC I large format cameras with a flight design that included a total of 184 exposures in 5 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 15,000 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.
Digital elevation data provided from the national elevation dataset for use in developing digital ortho imagery.
Imagery Acquisition: Digital aerial imagery was obtained using a large format Z/I DMC I equipped with Airborne GPS/IMU. A total of 5 flight lines with 184 frames were collected in the summer of 2017 in multi-spectral (RGB-IR) 12-bit format. The 0.5-meter imagery was acquired at an altitude above mean terrain of 16,000 feet to yield a pixel resolution suitable for photogrammetric mapping and orthophoto production. The imagery was collected under conditions free from clouds and cloud shadows, smoke, fog, haze, light streaks, snow, ice on water bodies, flooding, excessive soil moisture, and foliage. Imagery was collected during low tide windows, during low turbidity conditions, and little to no surface wind (less than 10 mph) or waves. In order to minimize shadow conditions, and solar glint/glare, imagery was obtained during the period of the day when the sun angle was between 25 and 55 degrees. The imagery consisted of blue, green, red, and infrared bands. Imagery for the photogrammetric mapping and digital orthophotos was captured according to the USGS Contract No. G16PC00016, Task Order Number: G17PD00636 regarding, snow, haze and cloud cover, and modified as appropriate to accommodate the requirements specific to DMC I technologies and as specified in this scope of work.
Control Point Collection: A total of 35 ground based photo control points were established throughout the project area using a combination of conventional and GPS survey methods in order to support softcopy aerotriangulation and photogrammetric mapping meeting the accuracies specified in this Scope of Work. Ground control collection followed requirements set forth in USGS Contract No. G16PC00016, Task Order Number: G17PD00636, and were modified as appropriate to accommodate the specifications related to ABGPS collection specific to these end photogrammetric mapping requirements. Please see the survey report for more information.
Aerotriangulation: Softcopy aerotriangulation was performed on this dataset. The airborne GPS/IMU data, GPS ground control, and image coordinate measurements were utilized to allow the direct computation of the exterior orientation parameters for each image frame to support the photogrammetric process and orthophoto production.
Surface Creation: This process involved the development of seamless topographic landform elevation dataset utilizing existing elevation data to support the production of digital orthophotography that meet or exceed required orthophoto horizontal accuracy. The topographic features included a grid of elevation points and may include break lines that define ridges, valleys, edge of water, transportation features and abrupt changes in elevation. The final DEM is suitable for orthophoto production only (not suitable for contour generation). The DEM is used to then generate a Triangulated Irregular Network (TIN) to support orthophoto production.
Orthoimagery 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 the NAD83 (2011) State Plane Connecticut, US survey feet. 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/DTM 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 a horizontal accuracy of 1 meter or less at 95% confidence level when compared to higher accuracy check points based on NSSDA testing standards.
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 quadruplet of numeric values, one for each color band. Void areas along the project boundary of the quarter quads have value of 0 (black). 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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