Coastal California Digital Imagery

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The aerial imagery acquisition for Coastal California was flown to support the creation of digital ortho-imagery with a 30 cm pixel resolution. Fugro EarthData, Inc. acquired 296 flight lines in 37 lifts from August 26, 2010 through November 29, 2010; and August 23, 2011 (this lift was flown to minimize building lean). The lines were flown at an average of 9,490 feet above mean terrain, using the Leica ADS40-SH52 digital airborne sensor, serial numbers SH024, SH110, and SH022. | Type of Source Media: External hard drive

TerraSurv, Inc. under contract to Fugro EarthData, Inc. successfully established ground control for coastal California. A total of 241 ground control points were acquired; of which 75 were successfully located and used as ground control in the final block adjustments, and 126 were used as checkpoints. GPS was used to establish the control network. The horizontal datum was the North American Datum of 1983 (NAD83, NSRS2007). The vertical datum was the North American Vertical Datum of 1988 (NAVD88). | Source Geospatial Form: model | Type of Source Media: electronic mail system

All acquired ADS40-SH52 data went through a preliminary review to assure that complete coverage was obtained and that there were no gaps between flight lines before the flight crew left the project site. Once back in the office, the data was brought through a comprehensive quality check to ensure completeness, image quality and resolution. There are essentially three steps to this processing: 1) GPS/IMU Processing - Airborne GPS and IMU data was immediately processed using ground GPS base station data, which is available immediately after collection. 2) Raw ADS40-SH52 Image QC - A technician performed visual inspection of the raw ADS40-SH52 images on selected bands for each collected flight lines. This step ensures proper function of the sensor. 3) Verification of Coverage and Data Quality - The raw RGB images for each collected flight lines were rectified using existing DEM and the GPS/IMU solution in Fugro proprietary software. The technician visually reviewed all rectified images to ensure completeness and resolution of acquisition for all flight lines and to identify any data gaps, clouds, shadows and any un-predicted issues in project area. All issues that did not meet project specifications were rejected and recollected.

Once the ADS40-SH52 data was collected and accepted, the Aerotriangulation (AT) phase began. Following is a step-by-step description of the AT process: 1) AT was accomplished as a component of Fugro EarthDatas exclusive Pixel Factory process. The ground control, GPS, and IMU solution was ingested and tie points between strips were identified. Normally, only five tie points are needed between adjacent flight lines. We used automated tie point selection function in Pixel Factory which allowed us to increase the amount of tie points. 2) The technician performed AT process and applied the bundle adjustment result to the images of each AT block (consisting of multiple lifts or sorties). The results of the adjustment were verified through the generation of the full resolution panchromatic ortho chips over the ground control points for the data sortie. The ortho chips were inspected by the photogrammetric technician to identify any errors in the adjustment to ensure the accuracy meets project specification. The technician also generated and visually reviewed ortho strips covers across all flight lines to ensure edge matching between flight lines. Documentation of the methodology and AT report containing the RMSE and residual calculations of the tie and ground control points used in block adjustment was prepared and delivered to NOAA OCM.

The following section describes the Pixel Factory digital image production sequence. This workflow is unique to Fugro EarthData, Inc. and has been developed specifically for push-broom sensors like the ADS40-SH52. This is a mature, stable workflow and incorporates all production components into an integrated series of tools to accomplish elevation model development, ortho production, and finishing. 1) The digital elevation data was correlated using the stereoscopic CCDs that are part of the ADS40 camera system. Using several tools that are part of the Pixel Factory workflow, a digital surface model (DSM) was correlated at an appropriate post spacing for the final accuracy requirement. The Pixel Factory correlation algorithm computed the X,Y,Z value for each DSM post utilizing every stereo angle that was available. A series of DSM files were created for acquisition block, one for each stereo look angle. A mosaic was then created from the separate DSM files where the best vertical value for each posting was selected from all look angles compared against the aerotriangulation adjustment which is incorporated into the mosaic. 2) The digital imagery for each acquisition sortie was differentially rectified to produce 4-band ortho-imagery at the appropriate pixel resolution. The Pixel Factory used the cubic convolution algorithm in its processing to remove image displacement due to topographic relief, tip and tilt of the aircraft at the moment of acquisition. Each individual strip of imagery from each flight line was rectified and radiometrically processed. 3) In order to achieve maximum efficiency in data processing, the digital orthophoto technician produced quicklooks for automated seamline generation and radiometric processing of the imagery. Quicklooks are a rendition of each scale of imagery at 1/64 of the actual resolution of the orthophotography. The goal of the radiometric adjustment is to minimize tonal changes of ground features on adjacent strips of imagery giving a balanced look across the entire project. 4) The results of the radiometric adjustment and seamline generation were used to create a mosaic for each block. The seamlines were then reviewed to ensure that the aesthetic impact on features and the final product was minimized. The data for all blocks was then processed to create a uniform and seamless appearance for the entire area. 5) The final ortho-image tiles were processed to the required projection and datum and are clipped out using the approved tiling grid and naming convention. 6) Fugro EarthData provided NOAA OCM with the pilot ortho-imagery for selected portions of the overflight at full resolution for review and approval of tone (color) and contrast of the imagery. 7) Digital ortho-imagery was then quality controlled internally. Based on the feedback from QC procedures, Fugro EarthData performed final corrections to the orthoimage, depending on the nature of the artifacts to be corrected. Minor artifacts were corrected using Adobe Photoshop in an interactive editing session. 8) Final digital ortho-imagery tiles were written out into GeoTIFF deliverable format with internal corresponding georeferencing and copied to hard drives for delivery.