A Feasibility Study on the Use of Manual Unmanned Aerial vehicles for Topographic Surveys

Alexander Avery

Context

This study looks at the feasibility of using an in-expensive Unmanned Aerial Vehicle for photogrammetric topographical surveys as well as different types of software to process it. To do this it looks at and compares two commercially available compact digital cameras; a Canon SX230 HS and a GoPro Hero 3. It explores the benefits and flaws within each camera and compares the accuracy of the produced DSMs. It will also compare the generated DSMs to similar products currently available from Ordnance Survey.

The research questions looked at where :

· Can a small low-cost UAV be able to produce highly accurate DSMs from a photogrammetric survey?

· Is the Canon SX230 more suited to being used in Photogrammetric surveys than a GoPro Hero 3?

· Does Agisoft Photoscan generate DSMs more consistently and with a higher accuracy than the Photosynth software system?

Methodology

Two sites where surveyed using the DJI Phantom; Traprain Law and Yellowcraig Beach. Around 300 images where collected using each camera at each site. These were then visually compared in order to assess the usability of each camera. The photos were also processed using Agisoft Photoscan and the Photosynth software systems. This generated a set of DSMs which were then compared and evaluated for their accuracy and consistency.

Results

A series of DSMs where created using combinations of different terrains and cameras. These were combined using ArcMap. These compound DSMs showed the level of consistency in the generation of DSMs using the different software systems.

The accuracy of the DSMs where found by comparing individual cells within the DSM to the known points on the ground using ArcMap.

Discussion

The Canon SX230’s internal GPS would allow for the camera to be used in directly Geo-Referenced surveys. Further work would be required to establish a methodology and assess its accuracy as it may be too low to use. An adaption of the Canon Hack Development Kit could allow for greater usability of the cameras zoom feature.

Conclusion

The Canon SX230 has been shown to have many advantages over the GoPro Hero 3, particularly in image quality and low levels of distortion in the images. This has led it to be able to produce more accurate and consistent DSMs than the GoPro Hero 3.

In comparison, the GoPro Hero 3’s highly distorted images have been shown to have a negative impact on the outputted DSMs. It may be possible to correct the images, for the fish-eye distortion however this may lead to an addition of noise to the DSM and decrease its accuracy.

The DJI Phantom combined with a low-cost compact digital camera was able to be successfully used as a low-altitude remote sensing platform. Though the Phantom suffers from low battery life and poor stability, similar to many other small UAVs (Eisenbess, 2009), the Canon SX230 was able to be adapted to compensate for its shortcomings.

The Photosynth software system was found to have major limitations with Z accuracy rarely being lower than 30cm. In comparison Agisoft Photoscan provided a more consistent and more accurate DSM generation.

References

Eisenbess, H. (2009). UAV Photogrammetry. PhD Thesis, Institute of Geodesy and Photogrammetry,

ETH Zurich, Zurich, Switzerland,

Remondino, F. Pizzo, S, D. Kersten, T, P. Troisi, S. (2012) Low-Cost and Open-Source Solutions for Automated Image Orientation – A Critical Overview. In: M. Ioannides et al. Ed. Progress in Cultural Heritage Preservation. Springer-Verlag Berlin Heidelberg. Pp 40-55.

Rosnell, T. Honkavaara, E. (2012) Point Cloud Generation from Aerial Image Data Acquired by a Quadcopter Type Micro Unmanned Aerial Vehicle and a Digital Still Camera. Sensors. 12. Pp 453-480.