4. PRE-FLIGHT PLANNING IMPROVEMENTS
4.2 Program Outputs
4.2.4 Photograph/image positions
When considering how to calculate where each photograph or image should be taken, two alternatives were considered. Either the exact latitude and longitude of each photograph centre could be calculated and stored, or the position of the photograph centre could be represented as a distance from the end of the flight strip. In deciding which option to choose, the complexity of the processing required and the intended in-flight setup had to be considered.
The former option was discarded since, firstly, the amount of programming required to calculate the exact latitude and longitude coordinates of each photo centre was considered to be far greater than simply calculating a distance to the end way point ofa flight line. Secondly, it was considered unnecessary to know the exact coordinates of a photograph's centre when it is very unlikely that a pilot is ever able to remain exactly on the planned flight line. Consequently, had this option been chosen, it would have meant that the current GPS position and the calculated photograph centre position would have had to be continuously compared, probably at one second intervals, to find the position where the distance between the GPS position and the calculated photograph position was a minimum. The camera would then be sent a signal to take a photograph. This would have required the writing of some efficient code to provide the necessary one second updates.
Furthermore, the use of this type of algorithm would have meant that the exact position where the photograph should be taken would have to be passed before it could be confirmed using the GPS data that the aircraft had, in fact, passed that point. It would therefore be less likely that the required overlap could be consistently achieved using this algorithm.
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However, the aircraft will always be at exactly the required distance from the end of the flight strip at some point. The in-flight processing would therefore entail the implementation of a simpler algorithm where the current GPS position is read and then the distance from this position to the known position of the end of the flight strip could be calculated. A procedure written in Fortran 77 was available for calculating the distance between two points whose geographic coordinates are known. If this calculated distance was equal to or less than the required distance, the camera would be fired. It was felt that this algorithm would be less complicated, more efficient and therefore more suitable for the application.
The algorithm used in calculating the photograph positions for a single flight line is depicted in the flowchart in Figure 12 on the following page. This algorithm is employed if a DEM coverage of the area has been previously opened. The algorithm to determine the maximum, minimum and average altitudes for each flight line is very similar to that described in section 4.1.4 for calculating the maximum, minimum and average catchment altitudes. The algorithm used to compute the required photograph spacing for each flight line is similar to the algorithm that is used to determine the flight line spacing. This algorithm was described in section 4.2.1.
For the situation where a DEM is not available, a photograph spacing based on the average catchment altitude is calculated and used to compute the distance of each photograph from the end of its respective flight strip. For this scenario it was decided not to check for sufficient forward overlap at the highest point in the catchment since this could result in the photograph spacing for the entire survey being based on a minimum overlap at the highest point in the survey area. This would significantly increase the number of photographs that had to be taken and thereby lead to possible image storage problems.
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NEXT FLIGHT LINE:
---> Read DEM and calculate minimum, maximum and average flight line altitudes _
_____ L ________ _
~
spacing ensuring minimum forward overlap ) IfDEM available:Calculate photograph isl
obtained at highest point along flight line_
B) [fno DEM available: Calculate forward overlap based on average catchment altitiude
I NEXT PHOTOGRAPH:
Calculate distance of photograph from end offlight line:
NO
The first photo centre is taken at 5km after the start weighpoint Thereafter,
New Photo Position = Previous Photo Position -Photo Spacing_
Calculate YphotoPos - the gauss Y-coordinate of the photograph centre
YES Add distance of photo from end of flight line to array of photo distances_
Figure 12: Flowchart of algorithm for calculating photograph positions
A photograph or image is included if it contains data of the survey area not present in a previous photograph or image. Examples of photographs or images that would be excluded and included are shown in Figures 13a and 13b on the following page, respectively. Referring to the Figures, ifYphotopos+(O.5xSWYavg-(Flap%xSWYavg/100))>Ylow+5000thenthephotograph or image is included. Recall that Ylow is the Y coordinate of a flight line's northern way point as described in section 4.2.2. A similar approach is employed for photographs or images at the southern end of the survey area and for flight lines flown from East to West and West to East.
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Figure Ba: Example of photograph or image that would be excluded
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Figure 13b: Example of photograph or image that would be included