DEVELOPMENT OF A NEW BUILT-UP INDEX FOR DELINEATING BUILT-UP LAND FEATURE IN SATELLITE IMAGERY: STUDY AREA BLANTYRE

Impervious surface area (ISA) is a key factor for monitoring urban environment and land development. Automatic mapping of impervious surfaces has attracted growing attention in recent years. Spectral built-up indices are considered promising to map ISA distribution due to their easy, parameter-free implementations. This study explores the potentials of impervious surface area extraction for mapping urban expansion from Landsat imagery using a study area of Blantyre city, Malawi. A new built-up index is proposed in this study. To evaluate its effectiveness, the results from this index was compared to the results from ISEI, NDBI, NBI and UI using the Landsat 8 OIL images of 2016 and 2020. Accuracy assessment was calculated on each and every index by using confusion matrix, and from the results, the overall accuracy for the new index in 2016 image was found to be 89% and from 2020 images is 80.45% and the overall kappa coefficient is 75.49% for the 2016 image and 60.6% for the 2020 image. The indices were also applied on Gaborone and Dodoma cities in Botswana and Tanzania in order to ensure the effectiveness of proposed index. And in additional to these, the area covered by the built-up on RGB image of Blantyre city and from the proposed index results of Blantyre were calculated in hectares and the results were closely related. Although the proposed index shows some good performance on Dodoma and Gaborone cities but the results from our study area are more superior than the results from these cities. Therefore, the proposed method is superior index based on ISA mapping in relatively compared with the other indices. However, to ensure its superiority the index must be tested in other areas other than Gaborone and Dodoma, and on other images captured by other sensors for instance worldview, sentinel- 2 etc.

To evaluate its effectiveness, the results from this index was compared to the results from ISEI, NDBI, NBI and UI using the Landsat 8 OIL images of 2016 and 2020. Accuracy assessment was calculated on each and every index by using confusion matrix, and from the results, the overall accuracy for the new index in 2016 image was found to be 89% and from 2020 images is 80.45% and the overall kappa coefficient is 75.49% for the 2016 image and 60.6% for the 2020 image. And in additional to these, the area covered by the built-up on RGB image of Blantyre city and from the proposed index results of Blantyre were calculated in hectares and the results were closely related. However, to ensure its superiority the index must be tested in other areas other than Gaborone and Dodoma, and on other images captured by other sensors for instance worldview, sentinel- 2 etc.

The study area of this research is Blantyre city. Blantyre is the largest commercial and industrial capital of Malawi, is in the shire highlands and geographical Centre of the southern region of the country Malawi and it lies 350 east of Greenwich Meridian and 15042l south of Equator. It is a transport communications mode with road, rail and air links to all part of the country and neighboring countries of Mozambique, Zimbabwe and Tanzania. It covers an area of 228km2. The city is classified as a National Urban Centre within the designed six hierarchical levels systems of urban centers of the country and is the regional administrative headquarters of the southern region.
It has varied topography ranging from an elevation of about 780 to 1,612 meters above sea level. There are three types of topographic features which have major implications for the development of the city: the hills, plateau and ridge and the natural drainage system.
The most conspicuous and dominant physical features are the numerous hills found in all parts of the city. The main hills are the headwaters of several rivers and streams that originate and radiate from the city, forming a natural drainage system with nine distinct catchment areas; Likhubula, Lunzu, Mombezi and Khombi, which drain the northern part of the city and Mudi, Chisombezi, Limbe, Luchenza and Mwampanzi draining the middle and southern parts. However, because of the hilly topography, these drainage channels are narrow and have steeps sides and gradient.

Traditional authority: Blantyre City

District: Blantyre

Coordinates: -15.7955, 35.04405