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Raw Data Processing

     Raw Data Processing

      In the case of the Aerial Digital Sensors, we refers as raw data for the GPS and IMU data(RAW) and the non-assambled digital images(RAW). The further photogrammetric processing relies on those two!

     GPS/IMU

     The need of high-quality resolution images at engineering-scale accuracy, has determined in time, the apparition of the Global Positioning System (GPS) and Inertial Measurement Unit (IMU) systems attached to the aerial cameras.

     Even though, people hoped that aerial triangulation process will no longer be needed, in practice the integration of the GPS/inertial(IMU) in a combined AT provides the most flexible approach. This combination allows the control of the whole process by increasing the reliability of the system. Besides this, it gives the possibility for self-calibration of the camera, which is inevitable for highest photogrammetric accuracy demands. Additionally, the misalignment between IMU and camera can be estimated for each image block optimally and long term errors caused by constant shifts in the GPS trajectory are detected and corrected in the adjustment.

    Image Sensor - GPS - IMU systems

     Field measurement survey is time-consuming and expensive; using the GPS/IMU data reduces dramatically the number of the GCPs used in aerotriangulation.

     In some cases, where the combined AT can't be finalized successfully, because certain reasons, using highly accurate GPS/IMU is essential for the block geometry. In the case of direct georeferencing based only on GPS/IMU, a calibration field area must be set up and a normal AT process must be run in order to determine the Offsets (GPS shifts) and Boresights (angles shifts); we recommend it only for larger scales in certain conditions.

     Short processing steps:

     1. The lever arm components between the GPS antenna, the centre of the inertial measurement unit and the camera perspective centre are measured a priori and the appropriate translational offsets are already considered during GPS/inertial data processing.

     2. GPS/IMU data will be downloaded using specialized software, witch enables automatic checking of the integrity of data. The log file, proof of data integrity is kept for further queries

     3. The GPS/inertial data integration is based on a loosely coupled decentralized Kalman filter approach, where the differential GPS phase data processing is done first and the obtained DGPS positions and velocities serve as update information for the later inertial data processing

     4. The next step is merging the GPS and IMU data.The high absolute performance from GPS helps the inertial navigation system with accurate estimates on his current behavior. this stage we will calculate position attitude and velocity for complete trajectory.

     5. With the AT results of a calibration field we will determine the offsets for attitude and position. Using specialized software, we will apply these offsets for a given area

     6. The final export is done in a format(for example PAT-B) specific for the aerial triangulation software.

     Digital aerial images

     The sensor unit of digital cameras (DMC, Vexcel UCD, UCX, UCXp etc) consists of eight independent cameras, so-called cones. Four of them create the large format panchromatic image. The other four cones are responsible for the multi spectral channels:red, green blue and near infrared. The panchromatic part of the camera combines a set of 9 medium format CCD sensors into a large format panchromatic image. The multispectral channels are supported by four additional CCD sensors. Normally, digital cameras offers simultaneously sensing of high resolution panchromatic information and additional multispectral (red, green, blue and near infrared (NIR) information. The method to combine panchromatic and multispectral subimages is known as the "pansharpen" method.

  Multispectral and pancromatic data sets

     The files resulted from the digital sensor will follow a data flow managed by a dedicated software (specific to each camera manufacturer).

     Short processing steps:

     1. The MSU (aircraft storage unit) is verified to have CCNS file(s), along with the Level0 images and corresponding raw files.

     2. The MSU is connected to a computer and all Level0 files are downloaded to a HDD drive.

     3. Image segments are radiometrically corrected and rearranged for efficient stitching, resulting Level1 images

     4. Level2 stitched images are created, geometrically and radiometrically cleaned, with color channels held separately

     5. Final radiometric parameters are set in order to have images with homogenous and good appearance throughout the block.

     6. Using the parameters established previously the Level3 images are transformed into colour images using the information from the panchromatic and RGB files.