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Fast Orthophoto Production using the Digital Sensor System
Hence with its quick processing times, ground resolutions in the
range of 0.05 to 1 m, and geometric accuracy at the resolution
level, the DSS is ideal for meeting the current market demands
for orthorectified colour and CIR imagery.
Given these capabilities and its low cost, the DSS opens the door to many applications that often involve small localized areas, corridors or irregular spot shots:
This section is dedicated to highlight the geometric accuracy of the DSS using a test flight over Southern Ontario, Canada. The objective of this flight is testing and validating the terrestrial calibration of the DSS and in-flight approaches and analysing its capability for DEM extraction and orthophoto generation. A subsection of this flight test was used for the analysis presented here. This section was flown over Ajax, Ontario, and consists of a total of 72 images taken from a flight altitude of about 1200 m Above Ground Level (AGL). This test area contains a total of 50 Ground Control Points (GCPs) surveyed using DGPS pseudokinematic approach. All ground control points have been used as check points, except for one point that was used for calibration and quality control purposes. The Southern Ontario flight trajectory is shown in Figure 2, while a summary of the test flight parameters is presented in Table 3. Note that only the Ajax part of this flight data is used for the analysis presented here.
 Figure 2: Southern Ontario Flight Trajectory The POS AV data was first processed using POSPac TM , and the exterior orientation parameters were computed using the terrestrially calibrated boresight angles. Then, the imagery, the POS-derived exterior orientation parameters, and the GCPs were introduced to ZI ImageStation (ISAT) for tie point collection. In this part, the image block was sub-divided into three areas, namely
Additionally, it allows analysing the effect of different terrains on map production using the DSS. Using only 1 GCP for the in-flight calibration, tie point accuracy and POS-derived exterior orientation parameters are very important, especially in the forested area, tie point quality can be degraded significantly due to the lack of unique features and the resulting calibration parameters can therefore be less accurate. A summary of the configuration of each area is listed in Table 4.
For each terrain type, the ISAT-automatically generated tie points which were imported to POSCal TM along with POS-derived exterior orientation. The boresight and camera parameters were re-calibrated using only 1 GCP (around the centre of each area) to refine the parameters that were calibrated using the terrestrial calibration. The use of one GCP in the airborne calibration is needed to absorb any datum shifts. After the in-flight calibration, the data was imported again to ISAT where the EO Analysis function (Madani and Mostafa, 2001) was used to double check the validity of the POSCal TM calibrated parameters and to analyse the performance of the DSS. In the EO analysis tool of ISAT, the ground coordinates of checkpoints are computed for each stereopair using the EO parameters of the two images and the image coordinates of the checkpoints. Then the computed checkpoint coordinates are then compared with the land-surveyed coordinates. The differences between the DSS-computed and the land-surveyed coordinates of the checkpoints are then summarized by ISAT together with their statistics. The results of the EO Analysis on the three terrain areas are briefly presented in Table 5. 
Examining the results presented in Table 5, the following observations can be made:
To demonstrate the DSS capability in producing orthophoto mosaics in timely fashion, the in-flight calibrated DSS data was imported directly to ERDAS IMAGINE OrthoBASE to perform DEM self-extraction. Using the image stereopairs, DEM was extracted with a 5 meter posting. This procedure was performed in fully automatic mode without any external reference (existing DEM) or manual interaction (pre-defined elevation points). Unlike normal mapping procedure, the DEM was brought to create orthomosaic for the test flight area without modification (review/edit), to demonstrate fast orthophoto production with minimum operator interaction. The extracted DEM and the corresponding orthomosaic from the test flight area are shown in Figure 3. Note that the orthomosaic illustrates the locations of GCPs only; it does not reflect the actual number of GCP available. A summary of the self-extracted DEM and the orthomosaic is presented in Table 6. Note that the EO parameters used for orthophoto generation was derived from flight calibration using only 1 GCP, to absorb any datum shifts.  Figure 3: Self-Extracted DEM (left), Orthomosaic (right)
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