Understanding Urban Growth through Heat Islands Using Remotely Sensed Data: Yaounde Case Study, Cameroon

Studies in urban dynamics have focused on population growth, urban sprawl and activities expansion to determine, understand and study the phenomenon of urbanization in the world. The methods used in these studies have ex-plore remotely sense data from Global Positioning Systems (GPS), satellite imagery, aerial and ground photography for interpretation, analysis and ex-planation of urban land use and land cover evolution. This data is often combined with ground observations and other methods in social sciences to understand urban growth. In this article, data is tapped from temperature differences analysis in multi-date satellite images and combined with population statistics, human activities and infrastructure build up to explain the phenomenon of urbanization. The results of the findings will enrich our knowledge in Urban Geography on approaches and methods used in understanding urbanization and its problems. Drawing on case study material from Yaounde in Cameroon, this article examines how remote sensing techniques can help in understanding urban heat island in Yaounde and its negative outcomes on urban population, activity and the environment. As the study shows, urban growth has a direct relation with temperature increase and an inverse relation with vegetation change. Also, Increase in bare surfaces due to deforestation for development and settlement increases surface temperature and vice versa. Vegetation regulates surface temperature by absorbing energy from solar radiation and remission in the form of latent heat through the process of photosynthesis. Analysis of surface temperature increase through remote sensing techniques, urban land use evolution, determinants and implications of Yaounde heat islands constitute the focus of this paper. The implications of rise in surface temperature and relations with urban growth are important for decision making. This knowledge


Introduction
Sub Saharan African countries today are experiencing rapid rate of urbanization, Yaounde, the capital of Cameroon, is not an exception. The built-up space, activities and population of African towns (lagos in Nigeria, Abidjan in Ivory Coast, Cotonou in Benin, Nairobi in Kenya, Arusha in Tanzania and Yaounde in Cameroon amongst others) have increased (Ojuku and Mengue, 2017). Global temperatures in these cities are also increasing faced with population growth, human activities and climate change. Despite these rising temperatures, little is known about the link between urban growth, heat islands and application of remote sensing for its understanding. Studies carried out in urban geography to understand the expansion of our cities have focus on descriptive approaches, quantification and analysis of urban activities, mapping of land cover and land use change using aerial photographs and high-resolution satellite images. How urbanization can be understood through temperature differences involving the calculation of iron oxide indice in heat islands using satellite imagery is little known. Using multidate satellite images (ETM, TIRS, LC8, and TIRS), image processing softwares accompanied by ground truth verification, surface temperatures variations of the land can be determined to map out the urban build up space and other land uses. This information can help to enrich our knowledge of urbanization. The difference in temperature compared with field data (population statistics, urban activities and land cover/land use) can help to bring out the determinants of temperature variation for easy understanding of urban expansion. In Yaounde, unplanned urbanization characterized by haphazard construction resulting from poor implementation of urban plans and laws, corrupt practices by land governors, illegal occupation of risk prone areas (forbidden for construction), influences of multi-actors involved in urban space planning and management are some of the drivers identified in urban space mismanagement and disorder (Ojuku and Mengue, 2017;Ntemngweh, 2022;Choaumo, 2012).
The outcome of haphazard build up, land occupation and rise in urban temperature in Yaounde are many. Urban land occupation and exploitation has increased and expanded leading to destruction of forests, water resources, soil degradation, pollution and biodiversity reduction in urban wetlands and peripheries. The expansion of build up space has also resulted to social, economic and cultural tension, conflicts and problems. These newly occupied areas in the Yaounde urban space are witnessing aberrant weather pattern marked by exponential high surface temperatures and unpredictable rainfall that affect affects the town and population.

Approaches in Urban Studies
Application of remote sensing to determine heat islands for comprehension of urban growth is absent in many urbanization literatures. How city population, activities and heat Islands are related in urbanization is also not known. Heat islands and urban growth have however been investigated in isolation in urban studies. This is the case in Mauricie (Conde, 2010), Windsor (De Carolis, 2012).
Conventional methods of studying urban heat island have been through observation, collection and association of meteorological data for description of urban growth. Other authors have studied urban heat islands by use of statistical methods to determine energy budget through the testing of hypothesis preset; application of computer modeling techniques marked by energy flux to observe urban effects and surfaces; and adoption of urban modelling and measurement using the physical scaled models and fluxes measurement in different cities (Sobrino, Jimenez, and Paolini, 2004;Bechtel et al., 2012). Yet some authors have established the relationship between urban forms and their climate effects through field projects carried out by researchers and improved urban models through measurement programmes (Bechtel et al., 2012). In some studies, urban Heat Islands have been studied using several indices (Normalised Difference Vegetation Index, Normalised Difference Bareness Index, and Normalised Build up Index) to find the correlation with temperature (Chen et al., 2006). Others have used Local Climate Zones characterization schemes involving the scoring of a sky view factor, terrain roughness, building surface and impervious fractions, surface energy admittance, surface albedo and anthropogenic heat flux based on photography and Google Earth imagery observation to understand heat islands (Yvonne, 2015). This paper proposes the use of satellite images derived from sensors or aerial apparatus that detect radiation emitted from distance surfaces to determine urban heat islands. The resulting image is processed using Erdas Imagine and ArcGIS softwares to determine temperature variations.
This can be direct or indirect depending on the scale of measurement of surface temperature and sensor position. Even though many advances have been done in the methodology of study of urbanization and urban geography research, little is known in the temperature measure approach.

Material and Methods
In the methodology used for this paper, Finally, the actual temperature of the sensor (TEC) was converted into real surface temperature (RST) using the following equation: with the assumption of unitary emissivity and also using the constance of prelaunch (Zanter et al., 2016). With the heat islands determined for urban area, vegetation indice (NDVI) was then associated and direct field control missions carried out to determine the causes of the heat islands. These findings were imperative to understand urban growth and expansion from new perspectives. Generating knowledge on the evolution of urban heat Islands through multi-date satellite image processing help in explaining the phenomenon of urban expansion (Sundara et al., 2012).
Population data for Yaounde was collected from the National Census on Population and Housing (BUCREP) and exploited to find out the link between urban growth and temperature variation. The data was integrated on land used maps covering old and new quarters within the period of study. Data on urban activities and green spaces was also collected from ministerial departments, Yaounde City Council and NGOs operating on the environment. This data was process and analysis to corroborate the relationship between urban growth, activities and urban heat islands. The results of these data were displayed on the maps integrated in the paper.

Study Site
Yaounde, also known as "city of seven hills" is the political capital of Cameroon.
It is the second most populated town of Cameroon after Douala, with a total of 2,765,568 inhabitants in 2015 (BUCREP, 2010). Yaounde is the regional Head quarter of Mfoundi Division and the Centre Region. The town is located between longitude 11˚31'12 East and latitude 3˚52'12 North ( Figure 1). The area has a diverse relief comprising high plateau varying between 700 and 800 metres  of altitude crowned by round shape mountainous massifs that converge between 1000 and 1200 metres of altitude (Ojuku and Mengue, 2017). It is surrounded by Mefou-Akono in the West, Mefou-Afamba in the East, Mefou-Akono in the South and Lekié Division in the North. The town has a subequatorial climate with altitudes that moderate the temperature range. With deficits of rainfall, the climate has evolved with high temperatures resulting from the wanton destruction of its vegetation cover. This climate is characterized by alternation of two seasons (November-March for the long dry season, and June-August for the short dry season) and two rainy seasons (August-November for the long rainy season and April-June for the short rainy season). Average temperatures of 23.5˚C and rainfall of 1650 mm are recorded per year. The vegetation is of the intertropical type having a predominance of humid southern forest. Yaounde had a surface area of 14.800 hectares in 2002. Today the urban space has expanded to over 36,000 hectares (Tchindjang et al., 2020). The spatial distribution of heat Islands has changed with lands under development and population pressure warmer than other land use types.

Vegetation Cover and Build up Space
Analysis of satellite images covering the Mfoundi area shows that Yaounde was much covered by forest and green spaces prior to the 1990s. Much of Mfoundi and its seven subdivisions was less populated and urbanized. The buildup space was less during this period and gradually increased over time in a well-defined process to invade forest lands ( Figure 2). As seen on this Figures 2(a)-(c), the occupation of forest usually began by land transfer (>80% cases by purchase) from indigenes and deforestation for agriculture introduction. This was followed by construction of houses, population installation and establishment of commercial ventures such as schools, health services, poultry farms, building material stores which attracted other   Figure 3 and Figure 4).
The neighbourhoods of this area were covered by much forest and farmlands.
One observes a relationship between the vegetation loss in Yaounde since 1999, a continuous buildup and expansion of the town and variation in surface temperature. This loss of vegetation has been attributed to diverse socio-economic

Population Increase and Development
Apart from the evolution in vegetation and build up space, the population of Yaounde has also changed with implications on activities and services that influence surface temperature. According to population census data, the popula- had an impact on land use looking at the situation near that period (Table 1).
It is observed from Table 1   high population concentration are marked by many heats generating development activities and services (Figure 7). They include commercial buildings and activities (market places, banks, hotels, bakeries and restaurants, super markets, industries, transport agencies, motor parks and garages, weathering and construction places), public services (ministries and embassies with their buildings, equipment and parking), leisure grounds such as stadia and their parking lots and pavements, Schools, health and religious centres. The large number of service vehicles in these service places also produce a lot of heat that influences surface temperatures. As seen on Figure 7, development activities, equipment and infrastructures are many in the areas of high temperature and lower in the peripheries having more green spaces (vegetation, agriculture). Yaounde 1 district area has the highest number of activities. It is the area of the city that has a high concentration of activities. Yaounde 1 is closely followed by Yaounde 4 and 5 municipalities which are considered as the Central Business District of the town. Next, we have the Yaounde 6, 3 and 2 district areas. It is in these parts of the town where much of the dynamic Anglophone communities, Bamileke groups, populations of Northern Cameroon origin and other recent migrants live (Figure 9). These populations are very dynamic and industrious. They have implanted many business ventures, cottage industries and development activities that have modernized these municipalities and eliminated the remnants of vegetation and green spaces that were found in these parts of Yaounde city. Many schools, health services, hotels, petrol pumping stations, motor garages and parks, snacks, markets and drinking parlours, cultural halls have been established by these populations in Yaounde 6, 3 and 2. These installations generate a lot of heat. The establishments are densely concentrated, and their surfaces paved which cumulate much heat that increases ambient temperature especially in the dry season. Studies on Urban Heat Islands in South East Asia have shown similar conclusions. According to Ranagalage et al. (2017), Colombo which is the main commercial metropolitan in Sri Lanka, have had higher temperature increments and thermal discomforts inside the city with continuous development of the area. Finally, we have Yaounde 7 area which is being colonized rapidly by population, settlement and activities. It is in this area that many of the hilly landscapes, green spaces, agricultural institutions, farmlands and vegetation thrives. The hilly nature of the land act as an impediment to rapid urban build up (Ojuku & Mengue, 2017;Tchindjang et al., 2020). Here remnants of vegetation are being colonized by human activities (agriculture and habitats) are very visible (Figure 8 and Figure 9). This is the consequence of rapid population, infrastructure and activity growth ( Figure 9).
In 2000, the average high temperature for Yaounde urban core (heat islands) was 26.85˚C against 13.53˚C for the peripheries (Figure 6). Mvan, Ahala, Mendong, Mvog Mbetsi, Mbankolo, Emana, Ngousso, Messeng and Awae were the core quarters of Yaounde and heat Islands from 1999 to 2001. They had an average temperature of 26.85˚C. These quarters had high population concentration, dense buildings, equipment and infrastructures. Today Minkoameyos Olembe and Nkolbisong in the west, Minka Afanayoa, Mbalig, Simbock and Mendong (Figure 9) in the south, and Awae and Odza in the East that were dominated by vegetation, farmlands and isolated settlements have also been urbanized. They are the new urban heat islands that have added in the former Yaounde urban space. In the year 2000, the average surface temperature in these new quarters was 13.53˚C. As compared to 24.7˚C in the traditional core quarters of Yaounde (urban space) at that time. The temperatures for these new urban areas formerly covered by vegetation and green spaces have increased to 15.87˚C. Today the new quarters are very congested and marked by high density of houses, public and private buildings, equipment (schools, health structures, hotels, food and super markets, and business enterprises, churches and petrol stations amongst others) and infrastructure (roads, parking lots, playgrounds, airports, railway stations, marketing spaces amongst others). One observes a net relationship between high temperature distribution and high concentration of buildings, infrastructures and services that make up the urban space ( Figure 6). Quarters located at the fringes of Yaounde today such as Mbankolo and Nemayos in the south, Nsimalen and Mfou in the East, Okola and Zamagwe in the west, and Nkoumetou in the North are potential future urban heat islands of Yaounde, judging from the rapid rate of urban expansion into them. These next generation of quarters have much green spaces and forest with lower surface temperatures. In future, when these areas will be urbanized, temperatures might likely increase leading to the birth of new heat islands. Urban growth, temperature increase and development of heat island is not new in the world. Acceleration of rhythm of urbanization in the world since 1980s, have opened the way for studies on urban heat islands in the Paris and Montreal metropolitan cities (Anquez and Harlem, 2011), Nancy (Leconte, 2014), Filiatreault (2015).
In Yaounde, the extension of buildup surfaces between 1999 and 2015 have translated into a relative extension of heat surfaces in the town ( Figure 6) as is the situation in other cities in the world. Computed differences in the temperature of quarters in Mfoundi Division between 1999 and 2015 shows that Mendong and Simbock located in the southern part of Yaounde that were not urbanized have experienced the highest temperature increase (7˚C) in 15 years. The factors of this temperature rise are many as will be seen in the section that follows.

Determinants of Urban Heat Islands 1) Population Growth
Yaounde is the second most populated and urbanized city in Cameroon after Douala. The population of the area has been increasing rapidly since 1950 and reached it first million in the year 2000. The 2021 estimate by the World Population Prospects shows that the population of Yaounde is about 4.1 million inhabitants, about 4.3% increase from 2020. Table 2 shows the changes in the population of Yaounde since 2000 indicating the total population for each year, the change within the years and the annual growth rate.  This Population growth is the engine of change in temperature resulting to urban heat islands. The more the growth of the population, the higher the demand for housing, infrastructure and services. Many houses have been constructed to meet up the increasing demand for houses ( Figure 10 and Figure   11). In the construction process the vegetation that regulates high surface temperatures is destroyed and not replaced. Many houses constructed do not have green spaces. The buildings have pavements, parking lots, asphalt and concrete use for their construction and creation of roads leading to them. The surfaces of these structures needed to accommodate the growing population and activities absorb and conserve rather than reflect heat, causing surface temperatures and overall ambient temperatures to rise.
Plate 1: Urbanization of Yaounde and material for construction.
As can be seen on Figure 10 and Figure  The urban impact is thus visible.
The urbanized quarters show a larger trend of surface warming than green cover lands (agricultural areas, woodlands, home gardens, public ornamented spaces).     The paved surfaces of these development infrastructures have a low albedo, absorb solar radiation as heat, are impermeable and marked by high water runoff redirected to streams rather than being absorbed by plants and soils to cool the areas through evapotranspiration or evaporation. Heat islands have developed with negative outcomes on the city.

Implications of Urban Heat Islands on Yaounde
Heat islands as an indicator of urban growth have many implications on the Yaounde Urban space. These outcomes are seen on health, nutrition and goods preservation.

1) Health Implications
Rising temperature and uncertainties in the city of Yaounde has been a major such living conditions many are exposed to mosquito bites and the resulting malaria which remains the most fatal disease in the tropics. Also, heat islands in Yaounde contribute to higher daytime temperature which reduced nighttime cooling and higher air pollution levels. This condition contributes to heat illnesses such as respiratory diseases, heat cramps, general discomfort and in some situations heat stroke and headaches (Giguere, 2009). The high concentration of building on the hilly terrain of many quarters of Yaounde makes water circulation difficult after heavy rainfall. Floods and pollution become a problem. Waste is highly lithered around homes, roads and water corridors that are a health hazardous ( Figure 16 and Figure 17).   Heatwaves are also becoming a health hazard. Average daytime temperature is rising, as seen on analysis of satellite images used for this study. Many people in densely settled places of Yaounde without green spaces are obliged to drink large quantities of water, shelter under buildings, ventilated houses and workplaces, consume cold drinks and take a bath several times per day to avert the situation.
Several people have been victims of heat cramps, sleep deprivation with risk of mortality (Besancenot, 2002).

2) Food and Nutrition
In Yaounde, most lands used for food production in suburban areas have been replaced by buildings, urban equipment and infrastructure. Urban agriculture is on a decline following the reduction of croplands. Food security is at stake as urban food production has reduced. This has negative implications on human health as hunger and poverty is increasing. Gardens in home surroundings are also suffering the effects of excessive heat. However, many Yaounde city dwellers exploit heath islands effects to preserve foodstuff such as maize, groundnuts, onions, beans and other cereals and tuber crops from perishing. These foodstuffs are dried on house roofs, pavements and other platforms during sunshine.

Discussion
The disappearance of vegetation and failure to invest in the creation of green spaces contravenes the eleven UN development goal. This objective target rehabilitation and planning of towns and other human establishments in a way that can offer employment opportunities, basic access to energy, housing and transport services, public spaces amongst others to the population. Target seven entitled access to green spaces and public places stipulates that by the year 2030 everybody especially women, children, physically disable persons and the aged should have an assured access to these places. This is a concern in Yaounde with its rapid population growth and pressure on resources and the environment. The  This will reduce urban head islands effects.
It was highlighted that the rapid growth of economic activities, domestic industries, and construction works were responsible for production of urban heat islands in Yaounde necessitating their control. These activities are vital for urban development but must however be carried out with consideration for environmental protection. In some countries to prevent the growth of heat Islands, green rooftops, construction of greyish road surfaces to replace asphalt or tarred surfaces have been done (Tomlinson et al., 2011). In Yaounde, it will be more profitable to invest on green space production. This is because the trees or plants can easily grow considering that the town is located in a tropical rain forest ecological zone where water is abundant and soils favourable for luxuriant growth.
Planting trees in public places, homes, school premises, streets, parks etc as is the current practice is good. While promoting the environment and climate change combats, this practice will help create jobs and income to the people promoting these activities. The trees will also absorb the pollutant gases and prevent soil erosion, promote ecotourism, embellish the town amongst others. This will make Yaounde sustainable and a better place to live in.

Conclusion
In this study, there was evidence that the temperature of Yaounde has increased exponentially over the years as a result of heat generated by human activities, solar radiation absorbed by paved surfaces and heat trapped from high concentration of buildings following difficulties of air circulation. The relief of Yaounde  (Mcsweeney et al., 2010). Green solutions need to be sorted to reduce the impacts of urbanization. It has also been discovered in this paper that a net correlation exists between temperature variation and urbanized areas. This knowledge is available thanks to data obtained due to advances in remote sensing. Contrary to the past when urbanization was determined based on population and activities expansion, Landsat images of different dates can be exploited to calculate surface temperature to identify heat islands in our cities including Yaounde. This temperature when correlated with urban activities and population data can help to comprehend the urbanization process.