Regular Digital Camera as a Practical Geodetic Measurement Tool
The field of digital photography is undergoing a major technological progress in the last few years. This progress had led to the substantial improvement in the quality of regular digital cameras, and to an order of magnitude increase in their image resolution. This remarkable progress in the quality of digital photography in recent years was coupled with a significant price decline of regular digital cameras. As a result, land surveyors face for the first time an opportunity to adopt regular digital cameras as practical geodetic measurement tools. Using regular digital cameras on a daily basis for geodetic measurements may allow the unprecedented enhancement in the productivity of land surveying field teams. Moreover, it may also allow unparalleled improvement in the ability to control the quality of geodetic measurement projects, and to diminish the need for follow-up measurements in the field. Alas, adopting digital photography as a common tool for geodetic measurements introduces some key geodetic and practical challenges.
This paper overviews some of the theoretical and practical challenges that land surveyors face when embracing digital cameras for geodetic measurements. The paper introduces the geodetic challenge of calibrating a regular digital camera, reviews various methods for camera calibration, and discusses the advantages and drawbacks of each method through quantitative examples. In addition, the paper outlines the mathematical challenge of solving the photogrammetric model from a series of images taken from the ground level, and the anchoring of the model to a local or national geodetic grid in order to extract geodetic measurements from the images. Implementing the solution of the photogrammetric model by software allows turning a computer screen into a high precision theodolite-like tool. After obtaining the coordinates of a certain point in more than two images in which it appears, the software may calculate the point’s coordinates in the photogrammetric model. The software may also provide the point’s coordinates in any given geodetic grid, if the photogrammetric model is anchored to this geodetic grid through measured control points.
In contrast to aerial photogrammetry in which a small number of images may provide the coverage of a wide region, photography from the ground level is constrained by local obstructions. As a result, short-range photogrammetry usually requires a comparatively large number of images for the coverage of the measurement field. As a result, solving the photogrammetric model requires the sampling a large number of points in numerous images. This increases the complexity of the work, and thus challenges the practicality of using close range digital photogrammetry for the daily work of geodetic measurements. This paper reviews some advanced solutions based on image processing that allow overcoming these challenges, thus turning regular digital cameras into a practical geodetic measurement tool which provides geodetic measurements in a comparatively cheaper manner, and with higher quality compared to traditional land surveying techniques. Finally, this paper presents a practical experiment for creating a high-accuracy topographic map from a series of ground images taken by a regular camera Canon 600D DSLR, and compares the results with geodetic measurements made by a Total Station instrument with a standard geodetic accuracy level for producing 1:250 scale maps.