I have bought a Pentax Optio A30 to use as my aerial camera. It is similar to a number of the latest generation of sub-compact 10Mpixel cameras and has good reviews, especially in terms of its image quality. It weighs about 160gm with battery and SD memory card and at 58mm,has one of the smallest widths I have found – important for fittinginto the Cularis body.
The highest resolution image is quoted as 3648 x 2736 pixels. Based on the handbook, a 4Gbyte SD memory card at the best quality jpeg setting, should give about 1200 images. The camera has a continuous mode where images are taken and downloaded to the SD card as fast as possible as long as the shutteris pressed. One option for using the camera in the UAV, is to set itrunning in continuous mode and then select 1 in n of the imagesproduced to make the photo-composite.
With a fully charged battery, I tested the camera in continuous mode while photographing a clock. The camera managed to capture 1613 images at full resolution in high quality mode in 23 minutes before filling the 4Gbyte SD card. The battery indicator showed about half full at the end of the run.Although the average interval was about 1.2 seconds, there were somegaps between images of up to 8 seconds and in other cases, 2sequential images showed the same time. Both results suggest thatthe timing is not constant, but is a function of the amount of datathat needs to be stored on the card. The review also states that thefile size varies a lot depending upon the image content, so this makematters worse.
I then tried taking photos at timed intervals. I could get down to about a 4 second interval with only an occasional missed image, although this may have been finger trouble. So if I want images taken at regular, known intervals, I will have to use a servo driven IR interface such as the PRISM.
Now to look at flying the camera. There are two limiting scenarios.
- Fly at 120m (400ft) altitude. The ground area covered with the A30 lens set to wide angle (40mm equivalent in 35mm film terms) would be 108m x 72m. This gives a pixel resolution of about 3cm per pixel or 3 times the target of 12cm per pixel. Assuming a 30% overlap between images in thedirection of flight, then the distance between images is 75m and themaximum speed of the plane taking an image every 4 seconds is19m/sec or about 68kph (about 40mph).
- Fly at 480m (1600ft) Now the ground area is 432m by 288m which gives the required 12cm resolution. The distance between images to give a 30% overlap would now be 302m
Comments and Questions.
- Assuming a 20-30min flight, the memory size and battery power for the camera are not limiting factors.
- For regular images at known times and or positions, the camera will have to be fitted with an IR trigger.
- What should I aim for as a cruising speed for my Multiplex Cularis as this will impact the number of photos taken during each flight?
- Is it better to fly lower and use the additional resolution in post processing of the images to end up with corrected 12cm resolution map?
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