Digital Mapping Techniques: A Vital Tool for Updating Topographic Maps in Nigeria (A Case Study of Okigwe Local Government Area in Imo State)

Most topographic maps in Nigeria were produced over forty years ago and are now obsolete. The rate of development that has taken place since then is enormous and cannot be ignored. Lack of updated topographic maps in Nigeria as a whole and Okigwe L. G. A in particular has affected many good intended government developmental projects, thereby affecting the decision making processes and planning. The traditional method of data collection for map making is laborious and it takes a longer time to produce which renders the map obsolete in real sense by the time the map is published. There is need therefore to produce maps from data that can portray reality as faithfully as possible, so that the rapid changes such as infrastructural development are accommodated in the maps. Digital mapping techniques offer such useful data gathering tool. When remotely sensed data are used to produce a map, it takes a new form, encourage new uses, new users, and cartographic visualization can be possible. Additionally, adequate software image manipulation and analysis can also be achieved. This paper brings to the fore the effectiveness of digital mapping techniques in updating the topographic map of Okigwe L. G. A at 150, 000 scaleusing orthorectified SPOT 2.5m natural coloursatellite imagery through a low cost methodology. The software utilized wereArcGIS Desktop 10.1 suite, ENVI 4.5, Google Earth and Global Mapper 15. The methodology includedscanning, geo-referencingwith Root Mean Square error (RMS) of 0.000590169m, which was tolerable within the research specification (RMS 0.009m), image data validation, image visualization and interpretation, on-screen vectorisation and feature extraction, satellite image enhancement, image classification, contour interpolation from digital elevation model (DEM), field verification and completion, editing, cartographic embellishment, quality control and printing of the updated map. The updated topo map when overlaid on the old topo sheetdepicted that most parts of the study area had experienced tremendous development. For instance, the built-up area had a total increase of 9, 630.389Ha which is as a result of massive urban migration that has taken place over the years a common trend observed in most Nigerian cities. However, the farm land and forest land decreased by 3, 116.400Ha and 6, 410.934Ha respectively which could be attributed to the massive development that has taken place in the study area over the years. Another significant change was in the length of roads which was 716.565km in 1967 but increased to approximately 1, 229.674km in 2015. The interpretation of the satellite image and field verification solved doubts in the updating process and had an important contribution to the updated map. The results of the analysis showed that the use of digital mapping techniques for updating topo maps is fast and cost effective.