**5. Conclusions**

234 Studies on Environmental and Applied Geomorphology

In light of the above, this study computed two indices - GIUH and TI – to assess the suitability of using the published elevation products under consideration in hydrological and environmental modeling. A GIUH enables the determination of the hydrological response of a catchment to rainfall by taking into consideration its geomorphology. The runoff volumes generated at the outlet of a catchment and time-to-peak are dependent on the topography of the overland regions as well as the transmission surfaces. The reliability of these parameters (runoff & time-to-peak) is, therefore, dependent on how well a catchment's terrain (landform) is represented by the underlying elevation product (DEM). A GIUH was calculated using the three DEMs and their responses compared. Apart from changing the Horton statistics for the respective DEMs, all other variables, such as the rainfall intensity and a mean holding time of 5 hours, remained constant. Result of the GIUH analysis is shown in Figure 7. The figure shows that the direct runoff hydrographs for the Reference DEM and SRTM DEM are comparable with respect to the volume as well as the timing (i.e. time to peak). The ASTER DEM, however, shows a delay in the rising limb and a higher peak discharge. Considering that Horton statistics are the only variable in the analysis (with all others remaining constant), the behavior of ASTER can be attributed to its representation of the catchment's geomorphology. Although ratios obtained in the Horton analysis fall within acceptable ranges, the stream area ratio (Ra) obtained for the ASTER DEM (3.99), which strongly deviates from that of the other two DEMs, is believed to have

caused the noted delay in rising limb and higher peak discharge.

Fig. 7. Direct runoff hydrographs for the three DEMs

Figure 8 shows results of the TI analysis. Continuous Topographic Index data range obtained for each of the DEMs was classified into integer classes, and the percentage of pixels falling in each was determined and plotted. The graph shows a notable difference between the reference DEM and the two global DEMs. In order to attribute reasons for the results obtained, slope maps of the three DEMs were created and analyzed. Slope was chosen and analyzed due to the fact that TI is a function of the local slope angle acting on the pixel. The analysis revealed that, the distribution of slopes was quite different in all three DEMs. The reference DEM was found to have about 67% of its area having slopes up to 10, while for the ASTER and the SRTM DEMs, this slope class accounted for nearly 50% and slightly over 50% respectively. In the case of SRTM the radar reflective surface seems

In this study, two near-global DEMs - SRTM and ASTER – are compared and validated against a reference DEM for two sites in Ghana. The reference DEM used was generated using hypsographic and hydrographic data from a 1:50 000 topographical map produced by the SDG. DEM differencing, profiling, correlation plots, extraction of catchment area and drainage network, computation of Horton statistics and GIUH are some of the methods employed in the comparison.

Comparison of SRTM and ASTER Derived Digital Elevation Models

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Results obtained indicate that, for the two sites selected, both SRTM and ASTER GDEM meet their predefined vertical accuracy specifications of 16m and 20m respectively. This is in line with results of previous studies (Fujisada *et al*., 2005; Rodriquez *et al*., 2006). It was realized that SRTM has a higher vertical accuracy (in terms of RMSE) than ASTER GDEM for both sites. RMSEs ranged between 4.9 and 5.5 (site 1) and 14.5 and 18.8 (site 2) for SRTM and ASTER respectively. The vertical accuracy of both products, thus, increases (by a factor of 3) on flat and less complex terrain (i.e. site 1). Analyses conducted revealed that ASTER GDEM underestimates elevation (i.e. negatively biased), even if fill routines are applied. SRTM, on the other hand, overestimates elevation, which may be partly due to the fact that SRTM records the reflective surface and, thus, may be positively biased with respect to the bare earth when foliage is present. The underestimation of ASTER is more pronounced on flat and less complex terrain (site 1), and of a greater magnitude than the overestimation of SRTM. Results of horizontal profiling on site 1 showed that the elevation of ASTER GDEM is consistently lower than that of the other two. In areas that are heavily vegetated, the effect of the under- and overestimation of ASTER and SRTM respectively can be reduced by constructing an average DEM (i.e. ASTER+SRTM/2), which will have an absolute accuracy between that specified for the two global DEMs.

In the relative accuracy assessment, the Horton plot revealed that SRTM and ASTER DEMs have a similar geomorphological structure as compared to the Reference DEM. The main deviation is with respect to the stream area ratio (Ra) of ASTER. All other ratio values extracted are within the ranges as given for the various ratio's (Rodrguez-Iturbe, 1993). The calculated direct runoff hydrograph showed similar response for the Reference and SRTM DEMs. ASTER DEM, however, showed a delayed rising limb and a higher peak discharge. This might be due to the deviating area ratio obtained for ASTER DEM. The calculated Topographic Index for the DEMs showed a substantial difference between the global DEMs and the Reference DEM. This difference is to be attributed to the gentle slopes that prevail in the area (site 1) analyzed.

In summary, the study has revealed that SRTM is "closer" to the Reference DEM than ASTER, although both products are useful and are an excellent replacement for local 1:50 000 hypsographic data both in absolute and relative terms. The relative assessment further confirms that various surface processes can be appropriately studied when using these global elevation data sets, which is a great asset to geomorphologists. Here the relative assessment conducted is more focused to hydrological processes, one of the terrain processes important in geomomorpholgy.

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**10** 

*The Netherlands* 

**From Landscape Preservation to** 

**Experiences with Sustainable** 

Joks Janssen1 and Luuk Knippenberg2

**Landscape Governance: European** 

**Development of Protected Landscapes** 

*2Centre for International Development Issues, Radboud University Nijmegen,* 

Over the last two decades there has been a significant increase in the appreciation of the cultural landscape by the public and by politicians; this phenomenon is taking place in most European countries. In 1992 the importance of cultural landscapes was recognized on an international scale with their inclusion in the World Heritage Convention. Eight years later, in 2000, the Council of Europe adopted a European Landscape Convention (ELC) and presented it to member states for adoption. Through innovations such as the World Heritage Convention and the European Landscape Convention, cultural landscape has become increasingly central to matters of sustainability and place-making across both urban and rural realms. As a consequence, the thinking on protected areas has undergone a fundamental shift. Cultural landscapes are at the interface of nature and culture. Therefore, both natural and cultural resource conservation converge, creating opportunities for

In Europe, the approach to protecting landscapes has generally been one of 'designation', that is, drawing lines round areas valued by experts. The 'designation' approach, however, has come under criticism for a number of reasons, not least the growing realization that neither the ecologic and geomorphologic nor the axiological and aesthetic aspects of landscapes can be safeguarded in the long term on the basis of corralling stand-alone sites. Modern aesthetic, geomorphologic and ecologic objectives rely on a site-in-context approach based on a concern for visual, morphologic coherence and ecological connectivity across the wider countryside. Whereas protected areas were once planned against people, now it is recognised that they need to be planned with local people, and often for and by them as well. Instead of setting landscapes aside by 'designation', nature and landscape conservationists now look to develop linkages between strictly protected core areas and the areas around: economic links which benefit local people, and physical links, for instance via ecological corridors, to provide more space for species and natural processes. As a result, landscape conservation of continuously evolving landscapes is about the management of change – the landscape should not become frozen but kept alive (Bloemers et al., 2010).The

**1. Introduction** 

collaboration.

*1Land Use Planning Group, Wageningen University, Wageningen,* 

Zhang, W. and Montgomery, D, 1994, Digital elevation model grid size, landscape representation, and hydrologic simulations. *Water Resources Research*, 30, pp. 1019– 1028.
