**2.1 Aster GDEM**

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is an advanced multispectral imager that was launched onboard NASA's Terra spacecraft in December, 1999. ASTER has three spectral bands in the visible near-infrared (VNIR), six bands in the shortwave infrared (SWIR), and five bands in the thermal infrared (TIR) regions, with 15-, 30-, and 90-m ground resolution, respectively (Yamaguchi *et al.* 1998). The VNIR subsystem has one backward-viewing (27.70 off-nadir) instrument for stereoscopic observation in the along-track direction, making imagery acquired by this satellite suitable for DEM generation. Other important properties that make ASTER data suitable for DEM generation are the platform altitude (705 km) and its base-to-height ratio of 0.6 (Abrams 2000; Hirano *et al*. 2003).

The Ministry of Economy, Trade and Industry of Japan (METI) and the United States NASA recently released a global DEM (ASTER GDEM) derived from ASTER images acquired since its launch (1999) to the end of August, 2008. It covers land surfaces between 830 N and 830 S, comprising of 22 600 10 –by- 10 tiles. The GDEM is provided at a one arcsec resolution (30 m) and referenced to World Geodetic System (WGS) 1984. Elevations are computed with respect to the WGS 84 EGM96 geoid. The vertical accuracy of the DEM data generated from the Level-1A data is 20 m with 95% confidence without ground control point (GCP) correction for individual scenes (Fujisada *et al*., 2005).

METI and NASA acknowledges that Version 1 of the ASTER GDEM is "research grade" due to the presence of certain residual anomalies and artifacts in the data that may affect the accuracy of the product and hinder its effective utilization for certain applications. For this study, two 10 –by- 10 tiles in Ghana (see figure 1) were download from NASA's Warehouse Inventory Search Tool (WIST - https://wist.echo.nasa.gov/api/)

#### **2.2 SRTM DEM**

The SRTM (Werner, 2001; Rosen *et al*., 2001a), undertaken by NASA and the NGA, collected interferometric radar data which has been used by the Jet Propulsion Laboratory (JPL) to generate a near-global (80% of earth's land mass) DEM for latitudes smaller than 600. SRTM has been the first mission using space-borne interferometric SAR (InSAR). The SRTM mission has been a breakthrough in remote sensing of topography (van Zyl, 2001), producing the most complete, highest resolution DEM of the world (Farr *et al*., 2007). An extensive global assessment revealed that the data meets and exceeds the mission's 16m (90 percent) absolute height accuracy, often by a factor of two (Rodríguez *et al*., 2006). Since its release in 2005, the user community has embraced the availability of SRTM data, using the data in many operational and research settings.

SRTM data for this study was downloaded from the website of the Consultative Group on International Agricultural Research Consortium for Spatial Information (CGIAR-CSI http://srtm.csi.cgiar.org). Data available from this site has been upgraded to version 4, which was derived using new interpolation algorithms and better auxiliary DEMs. This version, thus, represent a significant improvement from previous ones.

Comparison of SRTM and ASTER Derived Digital Elevation Models

study sites and allows direct comparison with the 90 m SRTM data.

Table 2. Suitable grid resolution for the Reference DEM

statistics of the DEMs compared for both sites.

**3.2 Comparison of DEMs** 

**3.2.1 Accuracy of elevation values** 

(relative accuracy).

lines (km).

**3. Methodology 3.1 Data preparation** 

over Two Regions in Ghana – Implications for Hydrological and Environmental Modeling 225

2. *<sup>A</sup> <sup>S</sup>*

where "A" is the area of the study site (km2) and "L" is the cumulative length of all contour

Table 2 shows the implementation of equation (1) for both study sites and the resultant grid resolution. Based on these results, a cell size of 90 m was chosen in generating the reference DEM. The choice of a 90 m spatial resolution reflects the complexity of terrains for both

**Site Area (Km2) Length of contours (km) Grid resolution (m)** 

DEMs of the study sites were transformed into the same projection system – Universal Transverse Mercator (UTM) zone 30 north. WGS 1984 was selected as both datum and spheroid. Part of the Volta Lake falls within site 2. For this reason, a mask was prepared and used to mask out the lake area on all DEMs (ASTER, SRTM and Reference). The original 30 m resolution of the ASTER GDEM was resampled to 90 m to enable comparison with the other DEMs. After resampling, a low pass 3 x 3 filter was applied to all DEMs to remove possible outliers still remaining in the data. In practice, smooth models of topography and a small amount of smoothing of DEMs prior to geomorphometric analysis have proved more popular among geomorphometricians, although no single smoothing approach is absolutely superior for all datasets and study areas (Hengl and Evans, 2009). No misalignment between DEMs was observed, thus co-registration was not necessary. Table 3 presents summary

Two main approaches were used to compare and validate the elevation products against the reference. These are: (1) determining the accuracy of the *elevation values* of the products (absolute accuracy) and (2) determining the accuracy of terrain derivatives of the products

• **DEM differencing**: This was performed to derive elevation error maps. Root mean square error (RMSE), a common measure of quantifying vertical accuracy in DEMs, was

This was achieved by performing DEM differencing, profiling and correlation plots.

**1** 12100 21736.0 278.3

**2** 12100 61875.6 97.8

*L*

Δ = (1)
