Countrywide Stereo-Image Matching for Updating Digital Surface Models in the Framework of the Swiss National Forest Inventory

Abstract

Surface models provide key knowledge of the 3-d structure of forests. Aerial stereo imagery acquired during routine mapping campaigns covering the whole of Switzerland (41,285 km 2 ), offers a potential data source to calculate digital surface models (DSMs). We present an automated workflow to generate a nationwide DSM with a resolution of 1 × 1 m based on photogrammetric image matching. A canopy height model (CHM) is derived in combination with an existing digital terrain model (DTM). ADS40/ADS80 summer images from 2007 to 2012 were used for stereo matching, with ground sample distances (GSD) of 0.25 m in lowlands and 0.5 m in high mountain areas. Two different image matching strategies for DSM calculation were applied: one optimized for single features such as trees and for abrupt changes in elevation such as steep rocks, and another optimized for homogeneous areas such as meadows or glaciers. The country was divided into 165,500 blocks, which were matched independently using an automated workflow. The completeness of successfully matched points was high, 97.9%. To test the accuracy of the derived DSM, two reference data sets were used: (1) topographic survey points (n = 198) and (2) stereo measurements (n = 195,784) within the framework of the Swiss National Forest Inventory (NFI), in order to distinguish various land cover types. An overall median accuracy of 0.04 m with a normalized median absolute deviation (NMAD) of 0.32 m was found using the topographic survey points. The agreement between the stereo measurements and the values of the DSM revealed acceptable NMAD values between 1.76 and 3.94 m for forested areas. A good correlation (Pearson's r = 0.83) was found between terrestrially OPEN ACCESSRemote Sens. 2015, 7 4344 measured tree height (n = 3109) and the height derived from the CHM. Optimized image matching strategies, an automatic workflow and acceptable computation time mean that the presented approach is suitable for operational usage at the nationwide extent. The CHM will be used to reduce estimation errors of different forest characteristics in the Swiss NFI and has high potential for change detection assessments, since an aerial stereo imagery update is available every six years.

The bedrock topography of Gries- and Findelengletscher

“…For the upper parts of the glacier, the DEM was therefore completed by using two partial DEMs referring to the year 2012 and 2013. These were made available within the frame of the Swiss National Forest Inventory (Ginzler and Hobi, 2015). The completion procedure is expected to introduce only marginal errors, as the surface elevation change for the affected areas is known to be below 0.2 m a −1 .…”

Wall-to-Wall Tree Type Mapping from Countrywide Airborne Remote Sensing Surveys

“…The VHM, as described in [30], and a shadow mask were used to mask out non-woodland areas for the final tree type map. The tree type was validated using independent NFI data from stereo-image interpretation.…”

Estimating vertical canopy cover using dense image-based point cloud data in four vegetation types in southern Sweden

“…A recent study demonstrated the successful generation of a nationwide digital surface model (DSM) in Switzerland by image matching countrywide coverage of digital aerial stereo-imagery (Ginzler and Hobi 2015). Using this national DSM, normalized with a digital terrain model (DTM) from ALS data, Waser et al (2015) produced a wall-to-wall forest cover map for Switzerland via a workflow including classification of areas into forest or non-forest based on vegetation height.…”