ProfPWL Lyne, Head of Department, School of Bioresource Engineering and Environmental Hydrology, University of Natal, for his guidance and support during this project. Mr Rob North, Chief Technician, School of Bioresource Engineering and Environmental Hydrology, University of Natal, for his assistance in building a camera mount.
INTRODUCTION
In all studies, the derived maps were used in the planning and implementation phase of the process of clearing the alien vegetation. However, they emphasized that their airborne survey system could be improved as there were several shortcomings or limitations of the system.
EXISTING AERIAL VIDEOGRAPHY SYSTEM
Pre-flight Planning
- Procedure
- Limitations
There were also procedural limitations regarding the determination of flight line positions. In the situation shown in Figure 1 as an example, the endpoints of the flight lines were calculated by simply extending the flight lines 5 km beyond where they cross the catchment boundary.
Image Acquisition
- Equipment
- Limitations
There were several other important features of the camera that made it ideal for integration into the airborne videography system. The bracket was designed to be placed and fixed in place of a removable floorboard between the pilot and front passenger seats in the aircraft.
Post-flight Image Processing and GIS Input
- Equipment
- Procedure
- Limitations
The GIS software package ArcView 3.0a was used to map the natural resource. It was noted that if rectified digital images of the area of interest could replace the topographic or orthophoto base maps in the GIS, the time required for mapping could be significantly reduced.
DIGITAL STILL CAMERAS: STATUS AND APPLICATIONS
Characteristics and Operation'
This means that the same number of digital pixels as there are photos in the burst are produced. Although some digital cameras allow manual triggering by the operator, digital frame camera exposure control is most commonly computerized.
Advantages
The low signal-to-noise ratio in the blue reduces the brightness of the images. Computer-based control also facilitates the performance of some real-time processing of the footage.
Disadvantages
Unstable internal orientation and focal plane fidelity of the cameras, designed more for the mass consumer market than for photogrammetry, are primary factors limiting the photogrammetric potential of large-area CCD digital cameras. In terms of focal plane flatness, any deviation from planarity of the chip results in the image being displaced.
Applications
- Vegetation monitoring and mapping
- Land use mapping, environmental analyses and resource database
- DEM and orthophoto generation
- Other
Positive Systems provides brief descriptions of a selection of the projects in which the ADAR systems have been used. King (1995) offered several possible explanations for this; ;)'':.J . concluded that the potential of the sensor used lay in its cheap purchase).
PRE-FLIGHT PLANNING IMPROVEMENTS
Program Inputs
- Camera and lens parameters
- Ground pixel size
- Catchment generate file
- Digital elevation model
- Overlap requirements
The choice of lens determines the focal length used in the calculation of flight height and flight bar spacing. For the program, the coordinates for the generated file must be in Gaussian coherent projection, in other words in meters north or south of the equator and meters east or west of the central meridian. If a digital elevation model of the area of interest is available, the user has the option to open the DEM and use the information it contains.
The geographic coordinate system was chosen [l<; i' (l'!0j, h the need to know the central meridian of the DEM and ensures that the DEM c regardless of the central meridian of the generation file.
Program Outputs
- Flight line spacing
- Flight strip way point coordinates
- Flying time summary
- Photograph/image positions
- GPS way point file
Calculate the Ya - Y coordinate for where the left edee of the slice intercepts the catcher boundary. Calculate the Yb - Y coordinate for where the right edge of the cut intersects the catchment boundary. Therefore calculate YcroS5 - the Y-coordinate for where the leading edge of the cut intercepts the boundary of the catchment.
The number of photographs to be taken, as shown in Figure 11, is determined during the calculation of the position of the photographs along the flight lines.
IMAGE ACQUISITION SYSTEM
The Sensor
- Cost-benefit analysis
It was decided that 30 surveys per year would be the largest number of surveys expected to be flown and processed per year. As you can see in Figure 16, the 9" photography system is the most cost-effective option for all areas larger than approximately 80 km2. The lower Rlha cost for this system for larger areas is due to the significantly larger size of 9" photography . compared to the other formats. The DCS460 could achieve lower R/ha costs for areas up to approximately 350 km2. The Hasselblad system would be a cost-effective alternative for areas up to 450 km2. However, for areas larger than approximately 500 km2, the 9" system would still be considered the best alternative, despite the apparently small Rlha cost difference between the three systems for areas from 500 to 1000 km2 in size.
Such savings were considered significant, especially considering that the total peripheral costs for the DCS460 and Hasselblad options for this area would be R56,000 and R44,800 respectively.
Camera Mount
However, the analysis shows the potential of using the Kodak DCS460 in combination with semi-automatic photogrammetry software for cheaply and quickly mapping more than just localized areas. Unfortunately, the high capital cost of the Kodak DCS460 camera meant that a camera could not be purchased for this project, and furthermore, no individual or organization with such a camera was willing to lend or rent the camera. However, it was possible to borrow a Kodak DCS420 camera which could be controlled using the same protocol and DLL as required by the DCS460 camera.
Thus, despite the poorer spatial resolution and reduced area coverage of the DCS420 camera compared to the DCS460, the DCS420 represented an ideal replacement until: MBB was able to purchase a DCS460 camera.
Global Positioning System (GPS)
- Standard GPS errors
- Differential GPS
- Carrier-phase GPS
- Further considerations
- The Trimble Pro-XRS GPS
Real-time DGPS, on the other hand, involves transmitting the correction message from the base station to the hover station during flight. The effects of multipath and receiver noise errors can be reduced by the GPS receiver itself. Loss of lock on the GPS signal is a problem common with differential and carrier phase GPS.
The GGA sequences contain position information and information regarding the number of satellites in view of the GPS.
GPS Interface
To determine if DGPS is enabled, the 'GPS Quality Indicator' field is read in the $GPGGA string. The speed read from the $GPVTG string is used in the interpolation, since the GPS speed is unaffected by the lack of a differential signal. If there are not enough satellites in view of the GPS, most of the $GPGGA and $GPVTG fields, including the latitude, longitude, speed, and GPS quality indicator fields, become null and the strings are reduced to mostly commas.
To communicate with the camera and capture an image based on the GPS position, a camera control interface was developed independently of the GPS interface.
Camera Control Interface
The adapter number field specifies which SCSI host adapter number to use, while the SCSI device number field is camera related and configurable on the camera. After these calls completed successfully, the camera was ready to capture images by calling DCCaptureImage. Calls to DCCaptureImage should be based on the GPS position as described in the previous section.
The GPS and camera control interfaces were then combined into a single in-flight program with a common graphical user interface (GUI).
In-flight Graphical User Interface and Program
Alternatively, the user can select the next flight line to be flown by accessing the Spinedit component labeled 6 in Figure 22. Before flying a flight line, the user should enable button 5 to ensure there is sufficient memory remaining is on the flight line. PCMCIA memory card of the camera to store the suggested images for the flight line. If there is insufficient memory, the user will be prompted to replace the current card with a blank PCMCIA card before proceeding to the next flight line.
While flying the flight line, the field labeled 7 indicates the status of the DOPS signal.
SYSTEM VERIFICATION
Initial Qualitative Testing and Refinement
Second, there can be a delay between the call of DCCaptureImage and the camera actually taking a picture. The delay between calling DC Capture Image and the camera capturing an image could not be precisely determined. It was found that at 80 kmIh the camera did not trigger at every tree marker.
It is considered important that the pre-flight planning program be updated at a later date to alert the user to situations where the camera's continuous acquisition rate may be exceeded.
Quantitative Testing
Since the maximum error should be of interest in estimating the system's maximum ignition error, the maximum error for a single, instantaneous average position at each study beacon was also determined. Looking at Table 13, the relationship between maximum ignition error and ground speed will appear to be approximately linear. If the relationship is linear, then at 300 krn/h the maximum ignition error would be approximately 8.3 meters.
Adding to this the maximum error due to camera delay of 8.3 meters, the final trigger error would be about 16.6 meters.
Test Flight
For images captured at a ground pixel resolution of 0.5 m with the Kodak DCS420, this equates to a maximum forward overlap percentage reduction of 3.4%. This was deemed acceptable and could be taken into account in the pre-flight planning program by increasing the planned percentage of forward overlap. Once images of the area were acquired, it was necessary to quantitatively determine the potential spatial accuracy of the images.
No comparison was made with the spatial accuracy of the videographic system, since the same techniques could be applied to video images and similar accuracies would be obtained at the same image resolution.
Image Spatial Accuracy Assessment
- Rubbersheeting and mosaicing
- Aerotriangulation and bundle adjustment
- Camera calibration
- GCP survey
- Photogrammetric process
- Bundle block adjustment results
It was ensured that the grid was spread widely over the field of view of the camera. After capturing an image of the calibration grid, the image was opened in the Scion Image software package (Scion, 1998). This was partly due to the inaccuracy of the initial photo station coordinates provided.
The results of group block adjustment using manually defined image connection points are presented and discussed in the next section.
DISCUSSION AND CONCLUSION
The GPS had to be an integral part of the system and a crucial consideration was the spatial accuracy of the GPS. The documented accuracy of 2 to Srn from DGPS was considered appropriate for the purposes of the system. All components of the developed in-flight system could operate on their own battery power.
It is clear that most of the factors affecting the system in flight have been accounted for or can be accounted for through future system refinement.
Evaluation of textural information in airborne CIR digital camera images for forest stand leaf area index estimation. Proceedings of )SI North American Symposium on Small Format Aerial Photography, Cloquet, Minnesota, USA, October, p Hosted by American Society for Photograrnmetry and Remote Sensing, 5410 Grosvenor Lane, Suite 210, Bethesda, Maryland USA.
Proceedings of the International Symposium on Geomatics in the Era of Radarsat, Ottawa, Canada, maj, papir nr.75.
