Digital Elevation Model Method Research Articles

Terrain representation impact on periurban catchment morphological properties

Modelling the hydrological behaviour of suburban catchments requires an estimation of environmental features, including land use and hydrographic networks. Suburban areas display a highly heterogeneous composition and encompass many anthropogenic elements that affect water flow paths, such as ditches, sewers, culverts and embankments. The geographical data available, either raster or vector data, may be of various origins and resolutions. Urban databases often offer very detailed data for sewer networks and 3D streets, yet the data covering rural zones may be coarser. This study is intended to highlight the sensitivity of geographical data as well as the data discretisation method used on the essential features of a periurban catchment, i.e. the catchment border and the drainage network. Three methods are implemented for this purpose. The first is the DEM (for digital elevation model) treatment method, which has traditionally been applied in the field of catchment hydrology. The second is based on urban database analysis and focuses on vector data, i.e. polygons and segments. The third method is a TIN (or triangular irregular network), which provides a consistent description of flow directions from an accurate representation of slope. It is assumed herein that the width function is representative of the catchment’s hydrological response. The periurban Chezine catchment, located within the Nantes metropolitan area in western France, serves as the case study. The determination of both the main morphological features and the hydrological response of a suburban catchment varies significantly according to the discretization method employed, especially on upstream rural areas. Vector- and TIN-based methods allow representing the higher drainage density of urban areas, and consequently reveal the impact of these areas on the width function, since the DEM method fails. TINs seem to be more appropriate to take streets into account, because it allows a finer representation of topographical discontinuities. These results may help future developments of distributed hydrological models on periurban areas.

Vectorization of contour lines from scanned topographic maps

In this paper we propose a general method for contour recognition and DEM (Digital Elevation Models) generation from commercially available printed topographic maps. Automatic contour recognition of a scanned topographic map is a difficult problem due to the presence of closely spaced or broken contours, overlapping data layers and complexities arising from textured background etc. Beginning with a scanned map our approach utilizes a multistage process which broadly includes contour identification, cleaning and vectorization. While the identification and vectorization steps are based on calibrating existing image processing techniques, the cleaning step uses a modified geodesic distance based approach. The proposed technique was tested on contour regions of varying complexity scanned from a Survey of India toposheet of scale 1:50,000, scanned at 300 DPI. It was found that the proposed technique resulted in clean segmentation, crisp contour borders in the case of regular contour regions. However, differing levels of manual interventions were required to vectorize complex contour regions represented in the printed map.

DEM-Based Analysis of Earthquake-Induced Shallow Landslide Susceptibility

This paper describes a simple method of Digital Elevation Model (DEM)-based earthquake-induced shallow landslide susceptibility analysis. Considering topographic effects in amplification of earthquake ground motion, Uchida et al. (2004) have developed a topographical parameter based empirical description of landslide susceptibility during an earthquake. In this research, the method proposed by Uchida et al. (2004) was utilized in raster GIS and shallow landslide susceptibility analysis was performed in the study area of Nishiyama Town of Kashiwazaki City, Niigata prefecture, Japan. The correlation of shallow landslides generated by the Niigataken Chuetsu-oki Earthquake in 2007 with landslide susceptibility values suggests that the method proposed by Uchida et al. (2004) can be around 80% accurate in delineating the probable locations of earthquake-induced landslides. By calibrating landslide data and landslide susceptibility values in a small site within the study area, a final landslide susceptibility map was prepared for the whole study area. The resultant susceptibility map is very useful for regional scale planning.

Seventeen years of the “La Clapière” landslide evolution analysed from ortho-rectified aerial photographs

We present a new method of digital elevation model (DEM) calculation from aerial photographs and apply this method to the La Clapière landslide in the Mercantour Massif in eastern France. The method uses new techniques of image correlation and of restitution of camera parameters. The calculated DEMs are used to ortho-rectify three aerial photographs acquired between 1983 and 1999. Measurements, made from these ortho-rectified images, show that the average velocity of the landslide was of 1.7 cm/day, with the rate of the headscarp retreat of 4.1 cm/day, and the toe of the landslide advancing at 1.7 cm/day, between 1983 and 1999. Within the landslide, the velocity field is relatively homogeneous, although their directions are locally heterogeneous. Scree slopes and scarps develop in an heterogeneous way within the landslide. Because the headscarp moves faster than the front of the landslide, material spreads laterally at the scree slopes, and/or is eroded at the front of the landslide by the river, and/or is accumulated within the landslide, on a low slope area located above a competent bar called Iglière bar.

Assessment of Dem Quality for Characterizing Surface Roughness Using Close Range Digital Photogrammetry

This paper presents a procedure for assessing the quality of a digital elevation model (DEM) which has been applied to the output of a normalized cross correlation based stereomatching algorithm. Using semimetric photography of natural gravel river bed surfaces acquired in the field, digital photogrammetry was used to extract DEMs automatically for use in characterizing surface roughness properties.The procedure for assessing DEM quality involves examination of (i) ortho‐images, to provide a qualitative check on stereomatching performance; (ii) DEM collection statistics which quantify the percentage of correctly matched pixels as a function of those interpolated; and (iii) height differences between check points, measured using independent field survey, and corresponding DEM points. The concepts of precision, accuracy and reliability are defined in the context of DEM quality assessment and methods are outlined which can be used to assess these variables. The assessment is conducted for two adjacent stereopairs with similar characteristics, considering the effects of both DEM collection parameters and different lens models upon DEM quality.Results show that digital photogrammetry, in conjunction with independent field survey, can be used successfully for extracting high resolution, small scale DEMs from natural gravel surfaces. Components (i) and (ii) of the quality assessment suggest the need to optimize DEM collection parameters, although the effects of not using a properly specified lens model were minimal at this scale. Method (iii) showed that increasing stereomatching success does not necessarily lead to more accurately estimated DEM points. However, the use of method (iii) remained difficult because of the scale of the photogrammetric application being used; check point positioning within the photogrammetric co‐ordinate system was only possible to ±10 mm which, for a gravel bed surface, was associated with elevation variance of a similar, sometimes greater, magnitude. The next stage of this research will require the use of higher quality check data, possibly from laser profiling.