**Object-Based Image Analysis of VHR Satellite Imagery for Population Estimation in Informal Settlement Kibera-Nairobi, Kenya**

Tatjana Veljanovski1,2, Urša Kanjir1, Peter Pehani1,2, Krištof Oštir1,2 and Primož Kovačič<sup>3</sup> *1Scientific Research Centre of the Slovenian Academy of Sciences and Arts 2Space-SI – Centre of Excellence for Space Science and Technologies 3Map Kibera Trust 1,2Slovenia 3Kenya* 

### **1. Introduction**

Cities in Africa and developing countries in general are having a difficult time coping with the influx of people arriving every day. Informal settlements are growing, and governments are struggling to provide even the most fundamental services to their urban populations.

Kibera (edge region within the Nairobi) is the biggest informal settlement in Kenya, and one of the biggest in Africa. The population estimates vary between 170,000 and 1 million and are highly debatable. What is certain is that the area is large (roughly 2.5 km2), host at least hundreds of thousands people, is informal and self-organized, stricken by poverty, disease, population increase, environmental degradation, corruption, lack of security and - often overlooked but extremely important – lack of information which all contribute to lack of basic services such as access to safe water, sanitation, health care and formal education.

In Africa, but also in other continents, urban growth has reached alarming figures. Informal settlements formation has been associated with the rapid growth of urban population caused by rural immigration, triggered by difficult livelihood, civil wars and internal disturbances. The result of this very rapid and unplanned urban growth is that 30% to 60% of residents of most large cities in developing countries live in informal settlements (UNHSP, 2005). Nowadays, informal residential environments (slums) are an important component reflecting fast urban expansion in poor living conditions.

Densely populated urban areas in developing countries often lack any kind of data that would enable the monitoring systems. Monitoring systems joining spatial (location) and social data can be used for the monitoring, planning and management purposes. New methods of monitoring are required to generate adequate data to help link the location and socioeconomic data in urban systems to local policies and controlling actions. In the past, rapid urban growth was quite difficult to manage and regulate when processes were in progress. Available census data barely accounts for the reality, as in most cases, they

Object-Based Image Analysis of VHR Satellite Imagery for

were tested to approach the potential population scenario.

**2. Informal residential environments monitoring** 

Mason et al. (1998).

**3. Study area description** 

Population Estimation in Informal Settlement Kibera-Nairobi, Kenya 409

was analyzed through comparison of images taken on different dates, using contextual multi-level pixel based approach. The results of object-based analysis based on morphology attributes were further explored to estimate the potential population. There is a big discrepancy among estimations on Kibera population, thus different density parameters

The first, introductory chapter sets the informal residential settlement issue in the wider context of the remote sensing possibilities framework, highlighting the methodology of the study. Chapter 2 gives an overview of research and applications of informal residential environments monitoring. Chapter 3 reviews existing conditions in Kibera, Nairobi's informal residential settlement, bringing into perspective the historical development of the slum, and its current characteristics. Chapter 4 consists of a set of specific procedures performed at two spatial extents, to attain both aims of the study. Entire Kibera settlement was being reviewed, to map the general state and dynamics of housing (change detection) between years 2006 and 2009. Raila village was studied in detail using object-based analysis to derive precise map of the village land cover/use to derive population estimation models in a given situation. Chapter 5 collects the results of mapping and population estimations. Chapter 6 discusses the data and analyses involved in managing monitoring aspects of the

slums. The last chapter concludes the study with some suggestions for future work.

Although there is a strong need to obtain spatial information about informal settlements in order to increase living conditions for its residents and regarding the fact that remote sensing images offer a well suited data source, studies on informal settlements with VHR data are not frequent. Nevertheless, in Hoffman (2001), first results of detecting informal settlements from IKONOS data in Cape Town showed the principle feasibilities using object-oriented approach. The results were promising but seemed to be very dependent on the data. Later on Hoffman et al. (2006) showed that several adaptations were necessary to OBIA algorithm improvement when applying their extraction methods to the QuickBird scene. Automatic image analysis procedures for a rapid and reliable identification of refugee tents from IKONOS imagery over the Lukole refugee camp in Tanzania was made by Giada et al. (2002). Sliuzas and Kuffer (2008) analyzed the spatial heterogeneity of informal settlements using selected high resolution remote sensing based spatial indicators such as roof coverage densities and a lack of proper road network characterized by the irregular layout of settlements. Cooperation between KeyObs, UNOSAT, OCHA and Metria resulted in digitalization of VHR GeoEye satellite image of Afgooye corridor (Somalia) from 2009, where all temporary shelters were identified (UNHCR, 2010). Different methods to detect and monitor spatial behaviour of informal settlements were presented also by Lemma et al. (2005), Radnaabazar et al. (2004), Kuffer (2003), Sartori et al. (2002), Dare & Fraser (2001) and

Kibera is a division of Nairobi area, Kenya, within Langata constituency. Located southwest of the city centre of Nairobi, Kibera encompasses an area of 2.5 km2, accounting for less than percent of Nairobi's total area while containing more than 25% of its population. It is the

are based on figures extrapolated from old census, carried out in the 1970s or, if recent, they are obtained with poor accuracy, as informal settlements are difficult to survey (Sartori et al., 2002). More can now be done at least to monitor the extent and consequences of rapid urban growth. Where accurate maps of informal settlements and relevant census data completely lack, answers can be found using independent survey, derived from satellite or aerial technologies. Usage of satellite imagery nowadays enables rather quick answers to questions such as: where informal settlements are, what was the dynamics of their growth, how many people potentially live there, what basic services inhabitants need. Among the main issues to be addressed in informal settlements are the needs for potable water, waste evacuation, energy, education and health care facilities, and crime control. It is believed these actions can be planned based on quality mapping of the phenomena.

The spatial resolution of space-borne remote sensing has improved to such extent that their products are comparable with the ones provided by aerial photography. Satellite images taken with very high resolution (VHR) sensors, i.e. resolution around and below 1 m, enable skilled user to identify and extract buildings, trees, narrow paths and other objects of comparable size. A side effect of higher resolution is larger quantity of data which require more storage capacities and processing costs. Detection of informal residential settlements from satellite imagery is especially challenging task due to the microstructure, merged/overlapping rooftops and irregular shapes of buildings in slum-like areas. High spatial resolution is essential to facilitate extraction of individual buildings that are characterized by small, densely packed shanties and other structures. Informal settlement Kibera is composed of varying sizes of houses, where roofs can be a combination of many different materials, and mainly unpaved road and path network. Typically this can produce a spectral response on satellite imagery that is difficult to interpret and makes it difficult for traditional classification strategies to differentiate across object class type.

Various approaches enable to extract data from imagery in urban environments. Simultaneously with expansion of VHR satellite systems an object-based image analysis (OBIA) was developed to answer new technological opportunities. OBIA approach works in similar way as human brain perceives nature/environment, namely (high detailed) image is segmented into homogeneous regions called segments or "image objects" (Benz et al., 2004), which are then classified into meaningful classes, following the specific context of the study.

#### **1.1 Objectives of the research**

Objective of the work perform was to help Map Kibera Trust initiative with satellite data processing. Studies on Kibera informal settlement had two aims: first, to derive detailed land use/cover map that can further supply population estimation, and second, to analyse the potential of VHR imagery for detecting changes and settlement growth in recent past.

Since object-based classification of VHR satellite data has been argued as the most appropriate method to obtain information from urban remote sensing applications, this approach was used to derive accurate land cover map. The study involved GeoEye and QuickBird satellite images acquired between 2006 and 2009. Object-based approach was used to determine detailed urban structure in informal settlements area. Urban expansion

are based on figures extrapolated from old census, carried out in the 1970s or, if recent, they are obtained with poor accuracy, as informal settlements are difficult to survey (Sartori et al., 2002). More can now be done at least to monitor the extent and consequences of rapid urban growth. Where accurate maps of informal settlements and relevant census data completely lack, answers can be found using independent survey, derived from satellite or aerial technologies. Usage of satellite imagery nowadays enables rather quick answers to questions such as: where informal settlements are, what was the dynamics of their growth, how many people potentially live there, what basic services inhabitants need. Among the main issues to be addressed in informal settlements are the needs for potable water, waste evacuation, energy, education and health care facilities, and crime control. It is believed these actions can be planned based on quality mapping of

The spatial resolution of space-borne remote sensing has improved to such extent that their products are comparable with the ones provided by aerial photography. Satellite images taken with very high resolution (VHR) sensors, i.e. resolution around and below 1 m, enable skilled user to identify and extract buildings, trees, narrow paths and other objects of comparable size. A side effect of higher resolution is larger quantity of data which require more storage capacities and processing costs. Detection of informal residential settlements from satellite imagery is especially challenging task due to the microstructure, merged/overlapping rooftops and irregular shapes of buildings in slum-like areas. High spatial resolution is essential to facilitate extraction of individual buildings that are characterized by small, densely packed shanties and other structures. Informal settlement Kibera is composed of varying sizes of houses, where roofs can be a combination of many different materials, and mainly unpaved road and path network. Typically this can produce a spectral response on satellite imagery that is difficult to interpret and makes it difficult for

Various approaches enable to extract data from imagery in urban environments. Simultaneously with expansion of VHR satellite systems an object-based image analysis (OBIA) was developed to answer new technological opportunities. OBIA approach works in similar way as human brain perceives nature/environment, namely (high detailed) image is segmented into homogeneous regions called segments or "image objects" (Benz et al., 2004), which are then classified into meaningful classes, following the specific

Objective of the work perform was to help Map Kibera Trust initiative with satellite data processing. Studies on Kibera informal settlement had two aims: first, to derive detailed land use/cover map that can further supply population estimation, and second, to analyse the potential of VHR imagery for detecting changes and settlement growth in recent past.

Since object-based classification of VHR satellite data has been argued as the most appropriate method to obtain information from urban remote sensing applications, this approach was used to derive accurate land cover map. The study involved GeoEye and QuickBird satellite images acquired between 2006 and 2009. Object-based approach was used to determine detailed urban structure in informal settlements area. Urban expansion

traditional classification strategies to differentiate across object class type.

the phenomena.

context of the study.

**1.1 Objectives of the research** 

was analyzed through comparison of images taken on different dates, using contextual multi-level pixel based approach. The results of object-based analysis based on morphology attributes were further explored to estimate the potential population. There is a big discrepancy among estimations on Kibera population, thus different density parameters were tested to approach the potential population scenario.

The first, introductory chapter sets the informal residential settlement issue in the wider context of the remote sensing possibilities framework, highlighting the methodology of the study. Chapter 2 gives an overview of research and applications of informal residential environments monitoring. Chapter 3 reviews existing conditions in Kibera, Nairobi's informal residential settlement, bringing into perspective the historical development of the slum, and its current characteristics. Chapter 4 consists of a set of specific procedures performed at two spatial extents, to attain both aims of the study. Entire Kibera settlement was being reviewed, to map the general state and dynamics of housing (change detection) between years 2006 and 2009. Raila village was studied in detail using object-based analysis to derive precise map of the village land cover/use to derive population estimation models in a given situation. Chapter 5 collects the results of mapping and population estimations. Chapter 6 discusses the data and analyses involved in managing monitoring aspects of the slums. The last chapter concludes the study with some suggestions for future work.
