**2. What is monophotogrammetry?**

Since its appearance in the first half of the nineteenth century and until to the introduction of stereo pairs photography basically consisted of single oblique pictures. Most were produced on glass plates, films or other large-format capture mediums, making it possible to depict, and document detailed landscape features at very high resolutions and quality. Besides providing detailed views of historical landscapes, such single terrestrial photographs have the Using the Monoplotting Technique for Documenting and Analyzing Natural Hazard Events http://dx.doi.org/10.5772/intechopen.77321 109

climate change [6–10]. This makes managing natural hazards and mitigating their impact a

Learning from past events represents a very efficient way to expand our knowledge and improve the institutional infrastructure for tackling natural hazards in a sustainable manner [4, 12–14]. This also includes the systematic recording, documentation, and post-processing

Based on these considerations, many initiatives have been generated in recent decades for developing methods and protocols for the comprehensive and professional documentation of natural hazards and their marks (see, for instance, the international initiative documentation of mountain disasters (DOMODIS) [16] or the Interreg-Project DIS-ALP—Disaster and Information Systems of Alpine Regions [17]). When trying to build such a systematic register, many difficulties, and bottlenecks may arise concerning both ongoing and past events.

In the case of current disaster events, priority must be given to people rescue and the reestablishment of possibly damaged communication and traffic connections. During such actions, the signs of significant damage may be erased precluding the possibility of subsequent and detailed collection of information documenting the event. Similarly, when there are many simultaneous small events or when events occur in remote and poorly accessible areas and no special devices such as small aerial drones (micro-unmanned aerial vehicles [UAVs]) are available [18, 19], only terrestrial pictures or aerial oblique pictures taken by rescue or technical teams may be available. Nowadays, they are easy to shoot and fairly good in terms of

Significant past disastrous events dating back to before the first half of the twentieth century are often documented through very detailed terrestrial photographs. They are, however, not georeferenced, and as a result, are not readily suitable for reconstructing the precise location of the event and the damage caused [20, 21]. A great number of historical photographs thus exist that could provide important details on areas under the risk of natural hazards [22, 23]. To make such existing and potential photographic documentation available to extract localized information on natural hazardous events, a user-friendly georeferencing tool for oblique pictures is needed. In this chapter, we report on the main features, needs, and handling of the WSL monoplotting tool (hereafter also referred to as MPT\_2.0), a user-friendly software program we developed to orthorectify and georeference oblique pictures, and present some

Since its appearance in the first half of the nineteenth century and until to the introduction of stereo pairs photography basically consisted of single oblique pictures. Most were produced on glass plates, films or other large-format capture mediums, making it possible to depict, and document detailed landscape features at very high resolutions and quality. Besides providing detailed views of historical landscapes, such single terrestrial photographs have the

study cases related to past and present natural hazard events in Switzerland.

prerequisite for keeping mountainous areas suitable for inhabitation [11, 12].

of ongoing or past events [10, 15].

108 Natural Hazards - Risk Assessment and Vulnerability Reduction

quality even when using portable phones.

**2. What is monophotogrammetry?**

**Figure 1.** Main elements of the monoplotting system: Camera, image, and digital elevation model (DEM).

additional advantage of ease of interpretation as they represent people's everyday perception of the landscape [24]. Accurate georeferencing of such oblique pictures is, however, very difficult. This difficulty has so far prevented reliable quantitative geographical information from being obtained from such photographs.

To overcome this problem, the mono-photogrammetry or monoplotting idea was proposed in the early 1970s [25] wherein a photogrammetric system relates single unrectified, oblique terrestrial, or aerial images to a digital elevation model (DEM) of the corresponding landscape. As visualized in **Figure 1**, the basic idea is to relate to each other camera, picture, and DEM so that a ray from the camera center and going through a selected point in the picture will intersect the terrain DEM at the corresponding landscape point. Such a monoplotting system consists of the following main elements and input data:


Model: DTM) or may include vegetation, buildings, and other vertical objects (Digital Surface Model: DSM).

• A simple editor for defining and measuring features of particular interest (e.g., polygons,

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• Export–import routines allowing data exchange (e.g., CSV or shapefiles) between and from

The heart of the tool is the iterative process whereby camera calibration is achieved. This is done in order to precisely estimate and simulate the three intrinsic and the six extrinsic camera parameters. The three intrinsic parameters are the two-pixel coordinates (xc, yc) of the principal point (i.e., the image center), and the principal distance (perpendicular distance from the image to the projection center). The six extrinsic parameters are the three real-word coordinates of the lens pinhole and the three Euler rotation angles α (pan/z-axis), β (tilt/yaxis), and γ (roll/x-axis) of the camera (see [24, 33] for more details). The calibration routine generates a sequence of collinearity equations commonly used in photogrammetry [34] that estimates and approximates the unknown camera parameters [35] in order to progressively minimize the error of the camera model when applied to the input data. Once camera calibration is achieved, the tool implements a model of all the extrinsic and intrinsic camera parameters, which simulates the original camera setup when the picture was taken. The tool also

**Figure 2.** The MPT\_2.0 applied to the Sommascona flood event (see Example 2). A major strength of the 2.0 release is the option of opening and processing more oblique images or maps of the same area to obtain a complete view of the event.

lines, points, and heights) on the photographs.

conventional geographic information systems (GIS).

• The ability to handle native ERSI-shapefiles.

• **Control points (CPs)**, which are precisely and unambiguously identifiable locations (e.g., road and footpath intersections, rocky outcrops, and building corners) on both the image (pixel coordinates) and the landscape (real world coordinates, i.e., latitude, longitude, and altitude of the real-world coordinates). CPs should be at least four or more in number, preferably placed on the ground DTM, and possibly homogeneously distributed over the entire image. The corresponding real-world coordinates may be derived from georeferenced geographical information (e.g., orthophotos, maps, cadaster, DTM, and DSM) or can be directly surveyed in the field instrumentally (e.g., GPS).

Once calibrated in such a monoplotting system, single oblique pictures can also be interpreted three-dimensionally (3D). The main limitation of the monoplotting system is the fact that in mono-photogrammetry, only points on the terrain (DEM-surface) can be precisely located whereas stereo-photogrammetry enables the calculation of the position of any common pixel in the stereo pair.
