The creation of a Digital Elevation Model of the Astroni crater

5.2 The creation of a Digital Elevation Model of the Astroni

crater

Regular grid or raster Digital Elevation Models (DEMs) have become the basis for recent approaches to modelling earth surface's processes (Moore et al., 1991). A DEM is defined as a regular gridded matrix representation of the continuous variation of relief over space (Burrough, 1986). The value of DEMs derive from the extractive knowledge and information regarding terrain and its attributes, which can provide direct input to a range of environmental models. The extraction of information from DEMs can be divided into two classes of analysis. The first class of analysis, primary analysis, involves direct calculation from DEM raw elevation data, and includes the estimation of slope, aspect, profile and plain curvature, flow path-length and specific catchment area (Burrough, 1986). Secondary analysis involves calculation from a combination of first class analysis results, and can be used to characterise the spatial variability of specific processes occurring in the landscape. For example, the amount of incident solar energy can be estimated from the elevation, slope, aspect and shadowing of a surface (Dubayah & Rich, 1995).

The primary data for a DEM is based on terrain elevation observations that are generally derived from one of three sources: digitised contours; photogrammetric data capture (including aerial photography and digital satellite imagery); and ground surveying (McCullagh, 1988). Although the extrapolated data can then be stored in a variety of ways, two techniques in particular have been the most popular and best explored. These are the rectangular grid or elevation matrix structure, and the Triangulated Irregular Network (TIN) structure (Zienkiewicz, 1971; Lancaster & Salkauskas, 1986). Both are image representations that use a point model. The elevation matrix or rectangular grid is the most commonly used modelling construct for a DEM (Moore et al., 1991). This is because the data structure of a grid shares much similarity with the file structure of digital computers, in that both store elevations as a two- dimensional array. Owing to such similarity in storage structures, the topological relations between the data points are recorded implicitly, which streamlines both information processing and algorithm development. The TIN model provides a network of connected triangles with irregularly spaced observation points with x, y co-ordinates and z values. Its major advantage over an elevation matrix is its ability to generate more information in areas of complex relief, thus avoiding the problem of gathering a lot of redundant data from areas of simple relief (McCullagh, 1988). The more efficient data handling structure also allows a more rapid calculation time. The use of grid or plot based algorithms in the vegetation dynamics component of ASTROMOD favours the use of a grid-based structure for the Astroni crater DEM. This would signify a loss of information on the steepest slopes of the crater, but this

is a m in o r lim itation com pared to the com plexities involved in trying to in te g ra te a T IN based D EM w ith a raster based vegetation dynam ics m odel.

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6 1 t o 7 5 m .

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1 3 6 t o 1 5 0 m .

7 6 t o 9 0 m .

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1 5 1 t o 1 6 5 m .

9 1 t o 1 0 5 m .

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1 6 6 t o 1 8 0 m .

1 0 6 t o 1 2 0 m .

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1 8 1 t o 1 9 5 m .

1 2 1 t o 1 3 5 m .

H H 1 9 6 t o 2 1 0 m .

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Key:

< 1 6 m .

3 1 t o 4 5 m .

4 6 t o 6 0 m .

2 1 1 t o 2 2 5 m .

2 2 6 t o 2 4 0 m .

2 4 1 t o 2 5 5 m .

> 2 5 5 m .

Figure 5.1: Height above sea level DEM of the Astroni cra te r.

T h e n e x t stage in the dev elo p m en t o f a D EM for th e A stroni c ra ter was th e choice o f g rid size. T h e grid size o f a D E M has been show n to affect the to p o g rap h ic attrib u tes o f a la n d sca p e (H utchinson & D ow ling, 1991; Jenson, 1991; P an u sk a et al., 1991; Q uinn et a l , 1991; Z h an g & M ontgom ery, 1994). Z h an g a n d M ontgom ery (1994) found th a t grid sizes o f 30 m an d 90 m d id n o t accu rately d epict hillslo p e a n d ru n o ff g en eration processes in m oderately to steep g ra d ie n t topography. A 10 m g rid -size w as found to show sig n ific an t im p ro v em en ts over 30 m or coarser grid sizes, w hile finer grid sizes

also limits the resolution of the DEM. Decreasing the grid size beyond the resolution of the original survey data does not increase the accuracy of the land surface representation of the DEM, and potentially introduces interpolation errors (Zhang & Montgomery, 1994). For the Astroni crater, a 1:10,000 digitised contour map, combined with point maxima and minima height measurements, were used to extrapolate a 25 m grid DEM. The 25 m grid would allow sufficient accuracy in environmental modelling, and the combined survey data was deemed to have sufficient resolution to limit interpolation errors. A smaller grid size was not thought to be necessary since the vegetation dynamics model was already based on a 25 m. grid, and the extraction of higher resolution data from the 1:10,000 map would increase the chances of interpolation errors. Figure 5.1 shows a graphical representation of the DEM of the Astroni crater, with grid colours indicating the height above sea level.