The following section describes the ISTAR digital image production sequence. This is a mature, stable workflow and incorporates all production components into an integrated series of tools to accomplish photogrammetric mensuration, aerotriangulation, elevation model development, ortho production and finishing. Production processes are fully documented in accordance with ISO9001 mandates.
The following is a step-by-step description of the ISTAR production workflow used to develop digital orthophotography to support the Pittsurgh, PA task:
1. The unprocessed ADS40 data and accompanying GPS and IMU data for one or more sorties is downloaded from the portable hard disks and checked to verify that no files are corrupted and that all data can be downloaded.
2. The GPS/IMU parameters for each sortie are optimized using the ground control points and the error calibration map. The horizontal and vertical positions of all ground control points in the block are observed in each channel (R,G,B, IR, S1 Pan ,S2 Pan and Nadir Pan).
3. Aerotriangulation is accomplished using the CLB tool that is a component of the ISTAR process. The ground control, GPS and IMU information is ingested and tie points between strips are identified. Normally, only three (3) tie points are needed between adjacent flight lines.
4. CLB produces a bundle adjustment for each data sortie. A digital file containing the RMSE computations of the adjustment is derived and available as a deliverable. The results of the adjustment are verified through the development of a sub-sampled panchromatic mosaic for the data sortie. The mosaic is corrected using the aerotriangulation points only. This mosaic is inspected by the photogrammetric technician to identify any gross errors in the adjustment as well as the identification of any voids or image quality problems.
5. Using several tools that are part of the ISTAR workflow, a digital surface model (DSM) is correlated at a post spacing of 2 meter depending on terrain and land cover. The ISTAR correlation algorithm computes the X,Y,Z value for each DSM post utilizing every stereo angle that is available. A series of DSM files are created for acquisition block, one for each stereo look angle. A mosaic is then created from the separate DSM files where the best vertical value for each posting is selected from all look angles compared against the aerotriangulation adjustment which is incorporated into the mosaic. EarthData will then edit the surface to the level required to support the orthophoto production.
6. The digital imagery for each acquisition sortie is differentially rectified to produce orthophotography for the Red, Green, and Blue channels at a resolution of 0.3-meter per pixel. As a part of the ISTAR workflow, the data from the color channels is edge sharpened using the nadir panchromatic channel. The orthophoto mosaics are inspected for accuracy issues that fall in the following categories:
Aerotriangulation relatedDSM related
Accuracy issues will be investigated through review and correction of the DSM or triangulation adjustment. Once the imagery has passed quality control review, final radiometric adjustments are performed to create a uniform overall appearance. The final 1500-meter X 1500-meter tiles are clipped out and the imagery is output in GeoTIFF format. GeoTIFF files will include the following GeoTIFF tags and keys:Model Tiepoint TagModel Pixel Scale TagGT Model Type Geo KeyGT Raster Type Geo KeyProjected CST Type Geo KeyPCS Citation Geo KeyProj Linear Units Geo Key
7. The completed digital orthophotos are checked for image quality. Minor artifacts will be corrected using Adobe Photoshop in an interactive editing session. Digital files are assigned final names derived from the southwest corner of each tile and are based on the National UTM Grid.
8. The project manager is responsible for coordinating and performing quality control on the project FGDC compliant metadata. File level metadata is provided in .xml format.