The aerial imagery acquisition for Nassau County was flown to support the creation of digital orthophotography with a 6 inch pixel. The imagery was acquired in two sorties over two days with a total of 26 flight lines. The mission was flown at 4800 feet AMT using the Leica ADS40 sensor, SP3.
The airborne GPS data were processed and integrated with the IMU. The results were imported into the ISTAR system for use in the aerotriangulation. The ADS40 imagery was downloaded onto the EarthData server and brought over to the UNIX based ISTAR system. The ground control was used in conjunction with the processed ABGPS results for the aerotriangulation. The properly formatted ISTAR results were used for subsequent processing.
An initial surface and bridge location was provided to EarthData by VARGIS. The surface data was translated into ASCII xyz files. A bridge buffer routine was then run on the 3D bridges supplied by the VARGIS creating an apron around the bridges. Because there was insufficient data coverage to complete the ortho tiles it was necessary to generate and ISTAR DSM to supplement the provided surface. ISTAR digital surface modeling is based on an auto correlated pixel matching system within the ISTAR software. The auto correlated DSM surface represents the initial surface model. This surface is then exported from the UNIX format into a Microstation compatible format. The techniques used to create the ISTAR DSM auto correlated surface are similar to those used for LIDAR processing. EarthData has developed a unique method for processing DSM data to identify and remove elevation points falling on vegetation, buildings, and other aboveground structures. The algorithms for filtering data were utilized within EarthData's proprietary software and commercial software written by TerraSolid. This software suite of tools provides efficient processing for small to large-scale, projects and has been incorporated into ISO 9001 compliant production work flows. The following is a step-by-step breakdown of the process.
1. Using the DSM ISTAR data set created by EarthData, the technician performed a visual inspection of the data to verify that the flight lines met correctly. The technician also verified that there were no voids, and that the data covered the project limits. The technician then selected a series of areas from the dataset and inspected them where adjacent flight lines met. A process, which utilizes 3-D Analyst and EarthData's proprietary software was run to detect and color code the differences in elevation values and profiles. The technician reviewed this information and located the areas that contained systematic errors or distortions that were introduced by the auto correlation.
2. Systematic distortions highlighted in step 1 were removed and the data were re-inspected. Corrections and adjustments can involve the application of angular deflection or compensation for curvature of the ground surface that can be introduced by crossing from one type of land cover to another.
3.The data were checked against the control network to ensure that vertical requirements were maintained. The ISTAR DSM and the ASCII created from the VARGIS data were both merged together and imported back into the ISTAR UNIX environment.
The digital orthophotography was created with a 6 inch pixel. One complete sets of orthos were made covering the entire project area in Color Infrared. Once the processed merged DSM data was in the ISTAR system the initial radiometric adjustments were performed on the imagery for each flight line attempting to reach the best possible histogram. The rectification process was run using the processed DSM surface and the radiometrically balanced imagery on each flight line. A second set of radiometric adjustments were made and mosaic lines were placed. QA/QC was performed looking for smears and other indications of problems within the digital orthophoto creation process. The final imagery data set is removed from the ISTAR environment in a process called "packaging" where the individual tiles are created. The created tiles are reviewed again for anomalies and interactive radiometric adjustment applied where needed. The Color Infrared orthos utilized the red, green and blue bands.
The final product was untiled and uncompressed TIF and GeoTIFF format digital orthos with supporting TFW files. Final quality control for horizontal accuracy and image
quality was performed by New York State.