ASPECT AND SLOPE ANALYSIS: A CASE STUDY OF KETA MUNICIPAL DISTRICT IN THE VOLTA REGION OF GHANA
TABLE OF CONTENTS
ASPECT AND SLOPE ANALYSIS: A CASE STUDY OF KETA MUNICIPAL DISTRICT IN THE VOLTA REGION OF GHANA
Abstract
This study analyses the terrain characteristics of the Keta Municipal District using Digital Elevation Model (DEM) data to generate slope and aspect maps that describe the area’s topographic patterns. The DEM data were obtained from the United States Geological Survey Earth Explorer and processed using Quantum Geographic Information System. The slope and aspect were derived through spatial analysis tools that measure changes in elevation and slope orientation. The aspect values ranged from 0° to 358°, indicating the direction each land surface faces, while the slope values ranged from 0° to 27°, representing the degree of steepness across the district. The resulting aspect map shows a diverse range of slope orientations without a dominant directional trend, reflecting subtle variations in surface orientation and micro relief. Conversely, the slope map reveals that most of the district lies within the 0° to 5° range, indicating a largely flat terrain typical of coastal plains. Steeper slopes, reaching up to 27°, occur only in limited inland areas and along the sandbar separating the lagoon from the sea. These findings highlight the low lying nature of Keta, shaped primarily by coastal and lagoonal processes. The municipal’s gentle topography makes it suitable for agriculture, settlement, and infrastructure development, but also exposes it to risks such as flooding, erosion, and poor drainage. The integration of DEM-based terrain analysis provides valuable information for landuse planning, disaster management, and sustainable coastal development in the district.
Introduction
Keta Municipal, with Keta as the capital, is one of the fifteen administrative districts of the Volta Region. It was carved out of the Anlo district, which also comprises Akatsi and Keta districts. The district lies within longitude 0.30E and 1.050E and latitude 5.450N and 6.0050N. It is located east of the Volta estuary and about 160km east of Accra and off the Accra-Aflao main road. The municipal shares common borders with Akatsi district to the North, Ketu South district to the east, South Tongu district to the west and the Gulf of Guinea to the south. Out of the total surface area of 1,086km2, approximately 362km2 is about 30% covered by water bodies. The largest of these is the Keta Lagoon, which stretches about 12km at its widest section and 32km2 long. Hence, the remaining land area is only 72km, a situation which creates severe constraints on access to land for development in the district [1]. Historically Keta was also known as Quittah or Agudzeawo meaning Easterners in old Ewe and was assigned B27 as a postal mark [2]. The district is a low-lying coastal plain with the highest point of only 53 meters above sea level around Abor in the north. The lowest point is about 1 to 3.5 meters below sea level along the coast around Vodza, Keta and Kedzi townships. The main geographical belts that could be identified are the Narrow Coastal Strip, the Lagoon Basin of the middle belt, and the plains of the north. The main drainage basins are lagoons. The major lagoons include Keta, Angaw, Agbatsivi, Logui, Nuyi and Klomi. Into these basins drain some streams and tributaries of the Volta river. In recent years the volume of water in the lagoons has drastically reduced, leading to the emergence of several islands in the Keta, Angaw and Agbatsivi lagoons. The biggest among the islands are Seva and Dudu, which are partially inhabited [3]. The district is endowed with numerous water bodies and has a high potential for fisheries development. Among the available resources are the Atlantic coastline, lagoons, and creeks. Fishing is carried out in the sea, lagoons, and rivers. Several types of fishing gears are used for fishing in the sea [4]. For example, many people in Keta engage in lagoon fishing and harvest fish in commercial quantities for the local markets and other markets abroad [5]. In 1784, Fort Prinzenstein like most slave trade forts, was built by the sea’s edge. By 1907, the sea had retreated by about 600 ft. Since then, Keta has been subject to sustained erosion. The Bremen Factory and Coconut plantation, which were close to the high-water mark in 1907, had been swept away by the sea by 1924. The erosion has advanced as far as Queen Street and started to wear away the Fort [6]. Keta Lagoon is the largest lagoon in Ghana with a water area of 300 km2. This is in a larger wetland protected area of 1200 km2. It is a stopping point for many migratory birds and provides a breeding ground for sea turtles [7].
1.1
1.2 Map of Digital Elevation Model
Figure 2. Digital Elevation Model of Keta Municipal District
2. Methodology
2.1 Data source
The primary dataset used for this analysis was a Digital Elevation Model (DEM) of the Keta Municipal District. The DEM was obtained from the Shuttle Radar Topography Mission (SRTM) with a spatial resolution of 30 meters, accessed through the USGS Earth Explorer platform. The dataset was selected for its accuracy, coverage, and compatibility with open-source GIS software. Additional base map data, including administrative boundaries, coastline, and water bodies, were integrated from OpenStreetMap for spatial reference and map presentation.
2.2 Software and tools
All spatial analyses were conducted using QGIS version 3.44.1, an open-source Geographic Information System. The following QGIS tools and plugins were used:
Raster Analysis Tools for slope and aspect generation.
GDAL (Geospatial Data Abstraction Library) for raster calculations and reprojection.
Print Layout Manager for map composition and export.
Coordinate Reference System (CRS): The project was set to WGS 84 / UTM Zone 31N to ensure spatial consistency and metric-based analysis.
2.3 Data preparation
The DEM was first clipped to the exact boundary of Keta Municipal District using the ‘Clip Raster by Mask Layer tool in QGIS. This ensured that only the area of interest was analyzed. The raster data were inspected for anomalies, and no significant elevation gaps were detected. The clipped DEM was then saved as a GeoTIFF file for further analysis.
2.4 Derivation of slope
The slope map was generated using the slope function found under Raster → analysis → slope. The tool calculates the rate of elevation change for each cell within the DEM, expressed in degrees. The resulting slope values ranged from 0° to 27°, indicating a predominately flat terrain with a few gentle slopes. The output raster was symbolised using a grayscale colour ramp, where darker shades represented flatter areas, and lighter shades indicated steeper slopes.
Figure 3. slope of Keta Municipal District
2.5 Derivation of aspect
The aspect map was produced using the aspect tool under Raster → analysis → aspect. Aspect measures the compass direction that each slope surface faces, calculated clockwise from 0° north through 358°.
The aspect raster was visualised using a multi-colour hue symbology, where each colour represents a distinct orientation. This colour representation helps identify the variation in slope directions across the district.
2.6 Map Composition and visualisation
Both the slope and aspect maps were formatted in the QGIS Print Layout Manager. Each map included essential cartographic elements such as :
Title, legend, and scale bar
North arrow
Source information
Appropriate map extent and layout orientation
Colour ramps were selected to enhance visual interpretation, grayscale for slope and rainbow hue for aspect. Maps were exported in high-resolution jpg format.
2.7 Data Interpretation
The maps were analysed visually to interpret spatial variations. The slope map helped to identify flat zones and slightly elevated areas, while the aspect map highlighted directional variations in the surface orientation. Together, these outputs provided insight into the topographic structure and geomorphological features of the Keta Municipal District.
3. Results and discussion
3.1 Results
3.1.1 Aspect analysis
The aspect map of Keta Municipal District (Figure 4) shows the directional orientation of slopes across the area, ranging from 0° to 358°. These values represent the compass directions that surfaces face, with 0° indicating north-facing slopes and 180° representing south-facing ones. The visualisation displays a multicoloured mosaic pattern, indicating a wide variation in aspect orientations across the district. Despite this variation, no dominant aspect direction is observed, suggesting that the terrain lacks a strong structural alignment. Most parts of the district appear relatively uniform, reflecting minimal differences in slope orientation. Flat and low-lying areas, especially around the Keta Lagoon and the coastal strip, exhibit no clear aspect since these surfaces are nearly horizontal. The distribution of colours in the aspect map indicates a terrain characterised by micro-topographic variation rather than distinct ridges or valleys.
3.1.2 Slope analysis
The slope map (Figure 3) displays values ranging from 0° to 27°, representing very gentle to moderately steep terrain. Most of the district falls within the 0° to 5° range, confirming that Keta is predominantly flat. These flat zones are found around the lagoon margins, coastal sandbar, and inland plains, which are characteristic of low-lying coastal environments. Areas with slopes exceeding 10° are limited and appear mainly on small elevated sections along the eastern barrier spit and a few inland patches. The highest slope value of approximately 27° occurs in isolated regions, likely due to small sand ridges or elevated terrain remnants. The gentle nature of the slopes indicates that gravitational erosion processes are minimal, and water movement across the surface is slow and diffuse.
3.2 Discussion
The combined slope and aspect analyses provide important insight into the geomorphology and environmental characteristics of Keta Municipal District. The dominance of low slope values and weak aspect differentiation confirms that the district lies within a coastal lowland system shaped largely by marine and fluvial depositional processes. The region’s gentle gradient promotes sediment accumulation, the formation of lagoons, and the development of wetlands. From a landuse perspective, the generally flat terrain is advantageous for agriculture, infrastructure, and urban expansion. However, it also poses significant challenges related to drainage, flooding, and saltwater intrusion. During periods of heavy rainfall or tidal surges, the limited slope restricts runoff, increasing the risk of waterlogging and surface inundation. This is consistent with the frequent flooding events recorded in parts of the Keta area.The aspect distribution, while variable, reflects subtle differences in slope direction that can influence solar radiation, soil moisture, and vegetation growth. For example, north-facing and east-facing slopes may retain more moisture and support denser vegetation, whereas south-facing and west-facing slopes could experience higher evaporation rates. Although such variations are minor in Keta due to the overall flatness, they can still affect micro-environmental conditions and landuse suitability.
The results also underscore the need for integrated coastal zone management. Given the terrain’s susceptibility to flooding and erosion, development should prioritise elevated zones and adopt adaptive infrastructure. The slope and aspect maps provide baseline data that can guide drainage planning, coastal defense design, and environmental conservation efforts. The slope and aspect analyses reveal that Keta Municipal District’s landscape is predominantly flat, low-lying, and gently undulating, shaped by coastal geomorphic processes. While this terrain favours human settlement and agriculture, it also necessitates proactive management to reduce environmental risks and enhance long-term resilience.
4. Conclusion
The analysis of the slope and aspect of the Keta Municipal District provides a detailed understanding of the district’s topographic structure and geomorphological characteristics. Using a 30-meter resolution Digital Elevation Model (DEM) processed in QGIS, it was established that the district’s elevation and slope gradients are generally low, ranging from 0° to 27°, with most areas falling within the 0° to 5° interval. The aspect values range from 0° to 358°, representing diverse slope orientations, though no dominant direction was observed.
These results confirm that Keta is a low-lying coastal landscape characterized by gentle slopes, flat plains, and minor elevation variations. Such terrain conditions are strongly influenced by coastal and lagoonal depositional processes rather than tectonic or erosional forces. The gentle gradient supports agricultural activities, settlement expansion, and transportation infrastructure. However, it also increases the district’s vulnerability to flooding, tidal surges, erosion, and drainage inefficiencies.
Overall, the slope and aspect analysis has provided valuable baseline information for sustainable spatial planning and hazard management. These findings can serve as a reference for future environmental modelling, flood risk mapping, and landuse suitability assessments in the district.
5. Recommendations
Based on the findings of this study, the following recommendations are proposed;
The Keta Municipal Assembly and other Local authorities should incorporate slope and aspect information into spatial and urban planning frameworks to ensure development is guided by terrain suitability and environmental safety.
Given the predominance of low slopes, proper drainage systems and flood control structures should be established to reduce waterlogging and surface runoff accumulation, particularly in settlement areas.
Continuous monitoring and management of the coastal zone are essential to mitigate the effects of erosion and tidal surges. The slope and aspect maps can help identify zones prone to such risks.
Farmers should consider minor aspect variations when selecting crop types, as slope orientation may affect sunlight exposure and soil moisture, influencing agricultural productivity.
Future developments should prioritise slightly elevated areas above 5° slope to minimise the risk of inundation and improve drainage efficiency.
It is recommended that future studies integrate DEM-derived slope and aspect data with hydrological and landcover analyses to create a comprehensive terrain-based risk model for the district.
References
[1] https://mofa.gov.gh/site/directorates/63-district-directorates/district-volta/286-keta-municipal
[3] https://mofa.gov.gh/site/directorates/63-district-directorates/district-volta/286-keta-municipal
[4] https://mofa.gov.gh/site/directorates/63-district-directorates/district-volta/286-keta-municipal
[5] Ahmed, S., Isaac, S. (2016). Assessing the Effects of Indiscriminate Disposal of Waste: A Case Study of the Keta Lagoon in the Volta Region of Ghana. J Biodivers Endanger Species 4: 170. Doi: 10.4172/2332-2543.1000170.
[6] Akyeampong, Emmanuel Kwaku (2001). Between the Sea and the Lagoon. Oxford: James Currey. P.46. ISBN 0-85255-776-0.
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