Comparison of the Landsat-7ETM+ and NigeriaSat-1 Imagery for the Revision of 1: 50000 Topographic Map of Onitsha Metropolis, Anambra State, Nigeria

Abstract

Topographic maps are needed almost in every aspect of public and private sector activities, including general engineering and construction works, economic and physical planning regulation, environmental management, general planning and as a base map for land use/land cover mapping. Most of the topographic maps available in Nigeria are outdated including that of Onitsha and its environs. Consequently, they cannot be used for the desired needs. This study focused on the use of Remote Sensing and Geographic Information System (GIS) in revising and analyzing 1:50000 topographic map of Onitsha metropolis. Two satellite images (Landsat-7 ETM+ and NigeriaSat-1) of the same date (2006) were used respectively to revise 1964 topographic map of Onitsha metropolis. The two imageries which have been georeferenced, in UTM Coordinate System of WGS 84 Zone 32 were sub mapped using the bounding coordinate of the study area. The NigeriaSat-1 and the digitized and georeferenced topographic map of the study area were co-registered to the 28.5m resolution of Landsat-7 ETM+ using ILWIS 3.3 software. The satellite imageries were classified and the classified images were vectorized in ArcGIS 9.3 Software and integrated with contour generated from SRTM data of 2000 to produce revised topographic map of Onitsha Metropolis (2006). The SRTM data of 2000 was used despite the difference in years (6years), because a sample survey revealed that the topography has not changed significantly between 2000 and 2006 within the study area. Pixel-based image analysis revealed that NigeriaSat-1 has better descriminability than landsat-7ETM+ based on the overall classification accuracy obtained from each image; 86.90% for NigeriaSat-1 and 85.77% for landsat-7ETM+. The NigeriaSat-1 was recommended to be used in revising Medium-Scaled topographic maps of Nigeria. The study recommends among others that Medium-scaled topographic map coverage of the entire country should be carried out without further delay. Furthermore, the study recommends that Nigeria should now adopt topographic map of scale 1:25000 as the base map for the whole country.

Keywords: Topographic Map, GIS, Remote Sensing, Satellite Imagery