Photo Rectification and Classification Software
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Model Dev., 6, 837–848, 2013 Geoscientific www.geosci-model-dev.net/6/837/2013/ Geoscientific doi:10.5194/gmd-6-837-2013 Model Development Model Development © Author(s) 2013. CC Attribution 3.0 License. Discussions Open Access Open Access Hydrology and Hydrology and Earth System Earth System Sciences Sciences PRACTISE – Photo Rectification And ClassificaTIon SoftwarE Discussions Open Access Open Access (V.1.0) Ocean Science Ocean Science Discussions S. Harer¨ 1, M. Bernhardt1, J. G. Corripio2, and K. Schulz1,3 1Department of Geography, LMU Munich, Munich, Germany 2 Open Access meteoexploration.com, Innsbruck, Austria Open Access 3Institute of Water Management, Hydrology and Hydraulic Engineering (IWHW), University of Natural Resources and Life Solid Earth Sciences (BOKU), Vienna, Austria Solid Earth Discussions Correspondence to: S. Harer¨ ([email protected]) Received: 28 November 2012 – Published in Geosci. Model Dev. Discuss.: 18 January 2013 Open Access Open Access Revised: 2 May 2013 – Accepted: 16 May 2013 – Published: 22 June 2013 The Cryosphere The Cryosphere Discussions Abstract. Terrestrial photography is a cost-effective and 1 Introduction easy-to-use method for measuring and monitoring spatially distributed land surface variables. It can be used to con- tinuously investigate remote and often inaccessible terrain. Oblique terrestrial photography has become a more and more We focus on the observation of snow cover patterns in high frequently used observation method in various research dis- mountainous areas. The high temporal and spatial resolution ciplines, such as vegetation phenology (Richardson et al., of the photographs have various applications, for example 2007; Ahrends et al., 2008; Crimmins and Crimmins, 2008; validating spatially distributed snow hydrological models. Migliavacca et al., 2011), land cover studies (Clark and However, the analysis of a photograph requires a preceding Hardegree, 2005; Zier and Baker, 2006; Roush et al., 2007; georectification of the digital camera image. To accelerate Michel et al., 2010) and volcanology (Major et al., 2009). and simplify the analysis, we have developed the “Photo Rec- Here, we focus on glaciology and snow hydrology where, for tification And ClassificaTIon SoftwarE” (PRACTISE) that example, investigations of the snow albedo on glaciers were is available as a Matlab code. The routine requires a dig- realised by Corripio (2004), Rivera et al. (2008) and Dumont ital camera image, the camera location and its orientation, et al. (2009). For a comprehensive overview of snow fall as well as a digital elevation model (DEM) as input. If the interception of vegetation, glacier velocity and snow cover viewing orientation and position of the camera are not pre- mapping, we refer to Parajka et al. (2012). Terrestrial pho- cisely known, an optional optimisation routine using ground tography is used for these monitoring applications with an control points (GCPs) helps to identify the missing parame- increasing frequency. This has to be attributed to the ad- ters. PRACTISE also calculates a viewshed using the DEM vancements in digital photography and in off-grid power sup- and the camera position. The visible DEM pixels are utilised ply. In addition, it is related to the fact that field campaigns to georeference the photograph which is subsequently clas- and satellite-based remote sensing have limitations due to the sified. The resulting georeferenced and classified image can prevailing weather conditions and the complexity of moun- be directly compared to other georeferenced data and can be tainous terrain (Klemes, 1990). Terrestrial photography of- used within any geoinformation system. The Matlab routine fers an easy-to-use and inexpensive opportunity to monitor was tested using observations of the north-eastern slope of spatially distributed land surface characteristics, even in re- the Schneefernerkopf, Zugspitze, Germany. The results ob- mote areas. tained show that PRACTISE is a fast and user-friendly tool, With the increasing availability of cost-effective high- able to derive the microscale variability of snow cover extent resolution digital cameras and high-resolution digital eleva- in high alpine terrain, but can also easily be adapted to other tion models, new tools can be developed to observe and map land surface applications. the patterns of land surface variables such as the spatial dis- tribution of the snow cover in mountainous terrain. The main Published by Copernicus Publications on behalf of the European Geosciences Union. 838 S. Harer¨ et al.: PRACTISE – Photo Rectification And ClassificaTIon SoftwarE (V.1.0) Fig. 1. (a)Fig.The 1. (a)testThe site oftest PRACTISE site of PRACTISE is located isat locatedthe Schneefernerkopf at the Schneefernerkopf which is situated which in southernis situated Germany, in southern at the Germany, border to Austria (right frame). The DEM depicts the camera location and the field of view of the camera. (b) The installed digital camera system records hourly photographsat the border of to the Austria investigation (right area, frame). the north-eastern The DEM depicts slope of the the camera Schneefernerkopf location and summit the (upperfield of central view area).of the camera. (b) The installed digital camera system records hourly photographs of the investigation area, the north-eastern challengeslope for of spatially the Schneefernerkopf distributed monitoring, summit however, (upper central is the area).shed algorithm (Wang et al., 2000) was integrated that sim- georeferencing of a 2-D photograph to the 3-D reality. Tools, plifies and hastens the necessary visibility analysis by com- developed by Aschenwald et al. (2001) and Corripio (2004), puting the viewshed directly without the additional step of addressed this problem by projecting the DEM to the cam- using a geoinformation system, as is needed when using era image plane to establish a link between the photograph other georectification tools. PRACTISE also differs from ex- and the real world. Aschenwald et al. (2001) used a pho- isting software packages because it contains an automatic togrammetric approach that needs various ground control and a manual snow classification algorithm, and because points (GCPs) for the georectification process. This is, how- a batch mode is implemented, i.e. several images can be clas- ever, unfavourable in remote, mountainous terrain where the sified in one program evaluation. As stated above, the rou- derivation of GCPs can be time-consuming and costly. Ad- tines described here are optional and the user selects the rou- ditionally, the integrated optimisation procedure in their ap- tines depending on the task and the available data. If, for ex- proach only optimises the camera target position T , i.e. the ample, the exact camera location and orientation of a pho- centre position of the photograph, whereas all other param- tograph is known, the DDS optimisation with the need for eters remain fixed. It should be noted here that T is known additional GCPs can be omitted. By contrary, the DDS rou- as the principal point in photogrammetry. The georectifica- tine is absolutely necessary for the georectification procedure tion method applied in Corripio (2004) is based on an an- if the parameters are not precisely known. The strength of imation and rendering technique by Watt and Watt (1992). PRACTISE is that the new features form a flexible, fast and This method only needs one GCP (T ), but 13 camera param- user-friendly processing tool for analysing spatially and tem- eters have to be set. If these parameters are not accurately porally distributed land surface variables. A further strength measured, they have to be manually corrected by changing is that the Matlab source code is freely available, and even them in an iterative way, which is unfavourable if extensive though it is designed to classify the snow variability in moun- time series have to be processed. 18 tainous terrain, it can be easily adapted to other fields of re- The Photo Rectification And ClassificaTIon SoftwarE search, such as greenness indexes in phenology (Richardson (PRACTISE) introduced here is based on the approach of et al., 2007; Ahrends et al., 2008; Crimmins and Crimmins, Corripio (2004) but has been improved and extended by ad- 2008; Migliavacca et al., 2011). ditional model features. We use slightly different formula- tions for the calculation of the 3-D rotation and projection. Even more importantly, several new optional routines are im- 2 Data plemented in PRACTISE. This includes the dynamically di- mensioned search (DDS) algorithm (Tolson and Shoemaker, The test area for PRACTISE is located near the Zugspitze