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Abstracts of papers on Terrain Analysis

HUTCHINSON, M.F. (1988): A new procedure for gridding elevation and stream line data with automatic removal of spurious pits. J. Hydrol., 106: 211-232.

A morphological approach to the interpolation of regular grid digital elevation models (DEMs) from surface specific elevation data points and selected stream lines is described. The approach has given rise to a computationally efficient interpolation procedure which couples the minimisation of a terrain specific roughness penalty with an automatic drainage enforcement algorithm. The drainage enforcement algorithm removes spurious sinks or pits yielding DEMs which may be used to advantage in hydrological process studies. The drainage enforcement algorithm has also been found to significantly increase the accuracy of DEMs interpolated from sparse, but well chosen, surface specific elevation data. Moreover, it facilitates the detection of errors in elevation data that would not be detected by more conventional statistical means and forms a sound physical basis for cartographic generalisation.

DAWES, W.R. and SHORT, D.L. (1994): The significance of topology for modelling the surface hydrology of fluvial landscapes. Water Resour. Res., 30(4): 1045-1055.

The realistic representation of terrain in hydrological models of fluvial landscapes poses problems particularly for representing convergence and divergence of lateral flows. In this work we consider the topology of contour representation of surfaces. The networks of ridge and drainage lines are linked at critical points: peaks, saddles, confluences, and simple ridge junctions. The properties of these points are given, and from these, three topologic rules are derives. These are used to develop an automatic procedure for generating a catchment boundary and a modified flow net. Use of the flow net increases speed by 2 orders of magnitude over earlier contour-based topographic analyses, and the use of topology increases speed, accuracy, and sophistication over a recent flow net technique. Topology is used to evaluate simplifications of natural surfaces and, in particular, shows the unsuitability of the common ridge-to-stream rectangular flow strip. Finally, implications for the structure of distributed catchment models and for designing and interpreting field experiments are considered.

BAND, L.E., VERTESSY, R.A. and LAMMERS, R. (1995): The effect of different terrain representations and resolution on simulated watershed processes. Zeitschrift für Geomorphologie, Suppl.-Bd. 101: 187-199.

The effects of digital elevation model (DEM) resolution on raster and vector terrain data are investigated for the computation of hydrological terrain-soil indices. Cumulative distribution functions of ln (a/tan b), where a is the area drained per unit contour length and b is the surface gradient, were computed by two different grid-based methods and a contour based algorithm and appear to vary in a regular manner with DEM resolution. This information is used in hydrological models for the distribution of soil moisture and saturation runoff production. Tests of the sensitivity of the computed differences in a TOPMODEL-based watershed model reveal greater effects of resolution in the grid-based methods than the contour-based methods. DEM resolution appears to have regular impacts on simulated hydrographs, with the greatest sensitivity in the grid-based methods and the least sensitivity in contour and slope-line-based flow-element methods. Topological problems are found in all approaches specifically in the representation of stream channel and the area immediately around the stream channel. Future work needs to be done, both in the development of digital terrain models that better capture hydrologically significant aspects of watershed topography and in testing the different methods of computing terrain-based variables.

A major problem in developing digital elevation models (DEMs) is the realistic representation of flow though complex terrain. A contour-based DEM, which uses natural flow lines and contours to define the element network in the model, is an elegant. solution in that the governing equations of flow can be reduced to one dimension. Previous methods of constructing contour-based element, networks require a high level of user involvement to produce the flow net. A new method for constructing contour-based element networks is presented that relies solely on contour data and does not rely on user-defined high points, saddle points, streamlines, or boundary files. Critical points such as high points and saddle points are automatically detected and a robust scheme is used to create flow lines. An improved method for calculating element attributes for the final DEM is also presented that takes into account the nonplanar nature of typical elements in contour-based networks. Furthermore, the inherent deficiencies of traditional flow line construction methods are discussed, and to an extent overcome, resulting in a more realistic model. Preparation time for DEM construction has been reduced, and the user is given greater control over the rules for calculating flow lines. The algorithm has been implemented on a typical desktop computer, and comparisons have been made with existing contour-based element network construction methods. Results show improved speed and robustness over traditional methods and emphasize the convenience of minimal data requirements.

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