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Office of the Chief Technology Officer (202) 727-5660 200 I Street SE, 5th Floor Washington DC 20003 US dcgis@dc.gov 8:30 am - 5 pm 20231002 ArcGIS Metadata 1.0 GIS Data Coordinator D.C. Office of the Chief Technology Officer (202) 727-5660 200 I Street SE, 5th Floor Washington DC 20003 US dcgis@dc.gov 8:30 am - 5 pm DC datasets can be downloaded from "http://opendata.dc.gov". Image Service Ortho_2019 2020-02-13 DC GIS 200 I Street SE, 5th floor, Washington, DC, 20003, US remote-sensing image <DIV STYLE="text-align:Left;"><DIV><DIV><P><SPAN>2019 Orthophoto - 3 inch resolution: This document describes the processes used to create the orthoimagery data produced for the District of Columbia from 2019 digital aerial photography. It was flown on April 23, 2019. The aerial imagery acquisition was flown to support the creation of 4-band digital orthophotography with a 3 inch/0.08 meter pixel resolution over the full project area covering the District of Columbia which is approximately 69 square miles. The contractor received waivers to fly in the Flight Restricted Zone (FRZ) and P-56 areas. The ortho imagery was submitted to DC OCTO in GeoTiff/TFW format tiles following the tile scheme provided by OCTO. MrSID and JPEG2000 compressed mosaics were delivered as well using a 50:1 compression ratio.METADATA CONTENT IS IN PROCESS OF VALIDATION AND SUBJECT TO CHANGE.</SPAN></P></DIV></DIV></DIV> This data is used for the planning and management of Washington, D.C. by local government agencies. http://dcgis.dc.gov GIS Data Coordinator D.C. Office of the Chief Technology Officer (202) 727-5660 200 I Street SE, 5th Floor Washington DC 20003 US dcgis@dc.gov 8:30 am - 5 pm Geographic Names Information System D.C. Washington D.C. District of Columbia United States of America (USA) Orthophotography Planning Imagery Orthos Aerial Photography imageryBaseMapsEarthCover ISO 19115 Topic Categories imageryBaseMapsEarthCover Orthophotography Planning Imagery Orthos Aerial Photography imageryBaseMapsEarthCover imageryBaseMapsEarthCover D.C. Washington D.C. District of Columbia United States of America (USA) <DIV STYLE="text-align:Left;"><DIV><DIV><P><SPAN>This work is licensed under a Creative Commons Attribution 4.0 International License.</SPAN></P></DIV></DIV></DIV> See access and use constraints information. Version 6.2 (Build 9200) ; Esri ArcGIS 10.7.0.10450 true -77.122373 -76.900716 39.001746 38.785481 1 -77.122373 -76.900716 39.001746 38.785481 Quality control procedures were implemented and documented at each step of the project to ensure that all services required by the contract are completed to specification; they include visual (manual) inspections, automated routines, and technical reviews. Aerial Data Acquisition QA/QC: 1. Flight Planning - The flight plan was based on the scope of work (SOW), DC OCTO provided boudnary, and flight restrictions. 2. Ground GPS Acquisition - The GPS equipment was assembled on a monument or a temporary established point and data was recorded. The data was then checked to ensure the PDOP is less than 3.0. 3. Data Acquisition Prior to full flight, the following steps was peformed: - Inspect storage and system components to ensure all units are operational and there is sufficient storage space - Select and confirm the lever arm coordinates - Load navigation system and perform system check - Perform 5 minute static alignment and record PDOP, GPS, and UTC start time - Ensure IMU is operational - Ensure all channels are operational, as applicable. After the pre-flight checks, the crew began flight line data recording: observe video display, POS status and mass memory screens; record UTC start/stop times, GPS data, ground speed, altitude, comments/concerns, lines, waypoints and times on the flight log. When the flight mission was completed, a 5 minute static alignment was performed followed by a systematic shutdown of the system. All collected data was downloaded for QC. Ground Control QA/QC: The Ground Control QA/QC strategy is based on reviewing the criteria described below: - Incidences of High PDOP - Poor network closure - Inadequate point location - Missing or omitted points - Damaged or missing panels The evaluation of the ground control is done through the following steps: - All planning, reconnaissance, field observations, post-processing, adjustments, and final report development will be performed under the direct supervision of a highly-experienced land surveyor. - Fixed height tripods will be used during the GPS survey to eliminate antenna height measurement error. - The field crews will perform processing and closure analysis of the data to eliminate field blunders and determine baselines, which do not fit the network or project tolerances and must be re-observed. - The final adjustment and processing of target locations will be coordinated, directed and completed by a single surveyor to ensure the overall consistency and integrity of the control network required to accurately map an area of this size. These efforts will also facilitate a smoother aerial triangulation process. IMU/GPS Processing QA/QC: Airborne GPS control is accomplished through the simultaneous observation of five or more satellites in the GPS constellation using the on-board receiver and one or more ground receivers (base stations) located over known control points that are in the vicinity of the project area. If accessible to non-airport personnel, the GPS occupation of a primary airport control station (PACS) is established prior to any airborne GPS collection. A GPS receiver is placed on a temporary marker using PK nails to define the location. The GPS station records at a one second interval for the duration of the airborne collection. Coordinates for this base station point is adjusted by the project surveyor and is tied into the project control network. Airborne GPS and IMU data is immediately processed using the airport GPS base station data. When necessary, combination of CORS stations and surveyor established GPS stations are used to ensure that a base station is operating within range of the aircraft at all times. If this occurs, it is included in the pre-planning. Once a decision has been made to fly, any GPS stations established for the project area is activated at a one second interval for the duration of the airborne collection. Aerotriangulation QA/QC: The Aerotriangulation QA/QC strategy is based on the criteria described in the table below: - Missing or corrupt ground control information - Camera calibration - GPS and IMU data integrity An initial bundle/block adjustment is developed for each data sortie. The accuracy of each bundle/block is confirmed through an RMSE evaluation against the project ground control. The accuracy is verified through an iterative process where the adjustment is repeatedly run, while progressively increasing the constraints on the ground control. After the accuracy is verified, the technician applies the bundle adjustment result to the images of each AT block (consisting of multiple lifts or sorties). The results of the adjustment are verified through the generation of the full-resolution panchromatic orthophoto chips over the ground control points for the data sortie. The orthophoto chips are inspected by the photogrammetric technician to identify any errors in the adjustment to ensure the accuracy meets project specification. The technician also generates and visually reviews that orthophoto strips cover across all flight lines to ensure edge matching between flight lines. The adjustment/inspection process is repeated as bundle/block adjustment for adjoining sorties are complete and these small blocks are adjusted to build the overall bundle adjustment. Throughout the process, the accuracy of each adjustment is checked against the GPS ground control points. Imagery and Orthophoto QA/QC: The Orthophoto Rectification QA/QC strategy is based on the criteria described below: - Edge matching - Fit to ground control - Radiometric consistency - Insufficient coverage - Correct color band rendition Preliminary field data is reviewed to ensure that there are no gaps between flight lines before the flight crew leaves the project site. Data will be inspected for turbulence, and if it is present and affects the data quality, the line is rejected. A full visual review is conducted in the office to ensure that it is complete, uncorrupted, and that the entire project area has been covered without gaps between flight lines. The technician performs visual inspection of raw images on selected bands of each collected flight lines for completeness, this step also ensures proper sensor function of the sensor. The flight line trajectory files are reviewed to ensure completeness of acquisition for project flight lines, calibration lines, and cross flight lines. The raw RGB images for each collected flight lines are rectified using the DEM data and the GPS/IMU solution in Fugro proprietary software. The technician visually reviews all rectified images to ensure completeness of acquisition for all flight lines. The technician also uses these images to identify any gaps, clouds, shadows and any un-predicted issues in project area. The orthophoto production process incorporates the ability to develop a completed digital orthophoto mosaic of all or part of a data sortie at a greatly reduced resolution. “Quick look” generation permits the quick assessment of iterative adjustments to finalize the parameters that are applied to the data for radiometric corrections to the orthophoto while data limiting computer resources by only processing the imagery at full resolution once. Quick looks also enable the technician to assess the accuracy of the processed imagery as well as identifying areas of distortion that would necessitate regeneration of the DEM or aerotriangulation data. The imagery is visually checked for accuracy on the workstation screen, and its absolute accuracy is verified by overlaying and comparing the locations of the control that are visible on the image against a CAD file containing the point locations in vector form. The edge matching of adjacent strips of imagery is accomplished using a single color band from adjoining strips of imagery displaying each strip in alternating colors of red and cyan. In areas where the overlapping images are coincident, the imagery appears in a gray scale rendition while any offset is colored red or cyan. Any offsets are measured to confirm that the offset falls within the accuracy specification for the project. Using the parameters developed from the quick look, the finishing department radiometrically corrects the orthophotos prior to completing the mosaicking and clipping of the final tiles, then the files are returned to digital orthophoto production for mosaicking. The finishing department performs a 100% final visual check for orthophoto image quality prior to outputting the approach data to the designated media. True Orthophotography: Areas designated for true ortho capture photo capture are planned with a minimum 80 percent "endlap" between frames and 80 percent "sidelap" between flight lines. PCI Geomatics GXL and Focus tools are used in conjunction with internal proprietary techniques, and solutions to offer a comprehensive True Ortho product. The process utilizes a DSM extracted at the same resolution of the imagery to generate orthorectified imagery. With this approach, True Orthos are generated without voids, and the buildings are displayed without lean. The workflow is executed to produce the final products: • DSM Extraction • DSM Orthorectification • Mosaic Prep/Generation • Post Mosaic Editing GXL’s DEM Extraction workflow automatically scans an input directory for all valid stereo pairs, computes the epipolarized images and extracts the Digital Surface Model (DSM) for each stereo pair. Elevation values in a DSM are determined by matching points in a left and right epipolar input image using image correlation. After all DSMs in the batch are extracted, the GXL automatically merges them together to create a seamless multiview DSM. A “Pattern Suppression” algorithm helps to reduce or eliminate blunders in the output digital surface model, The DSM Orthorectification operation orthorectifies the aerial imagery using the derived DSM. Occlusion zones that are created during the ortho process are detected for each ortho image and filled by retrieving optimal visible pixels from the adjacent overlapping ortho images. A Mosaic Preparation processing module automatically normalizes the images, calculating color balancing and generating the seamlines (cutlines). Building footprints are used as a “cutline mask” forcing the mosaicker to avoid buildings, eliminating an extreme majority of known buildings. A Mosaic tool is used to adjust cutlines and perform color balancing. PCI Geomatica and internal proprietary tools and techniques, are used to conduct a comprehensive QA/QC while simultaneously editing the imagery for DSM, seamline, or mosaicking errors. 3 300000 0.080000 240000 0.080000 1 1 0 389400.000000 124200.000000 389400.000000 148200.000000 408600.000000 148200.000000 408600.000000 124200.000000 399000.000000 136200.000000 EPSG 4.5(3.0.1) Band_1 255.000000 0.000000 8 Band_2 255.000000 0.000000 8 Band_3 255.000000 0.000000 8 Band_4 255.000000 0.000000 8