Positional Accuracy Validation of Base Map and Investigating its Impact on Road Projects: A Case of Addis Ababa City, Ethiopia

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Addis Ababa University


Positional accuracy is a topic that of renewed interest and concerns not only the distance between the geographic object and its actual or real location on the earth surface but also the accuracy with which map features are located. Collecting the highest quality independent ground-based data is a key factor in validating the useful location accuracy of digital geospatial data. This information simplifies geographic analysis and promotes better judgment. In this research the accuracy of the horizontal position of an orthophoto/line map had been investigated. The global positioning system and total station survey had been employed to access the accuracy of the horizontal coordinate of the line map. The horizontal coordinates of the terrestrial position of the points are observed using Sokkia Atlas GR5+ Differential Global Positioning System (GPS) device and the row data are post-processed using various processing tools such as Leica Geo-Office and Topcon Tools v.8.2.3 reference with to local GNSS reference stations. In this study, we evaluated the 2D positional accuracy of the line map by matching its horizontal coordinates with the associated ground control point observed as in-suit data. Comparing the digital line map coordinates to the corresponding in-situ ground control points, RMSE between the digital line map and the GPS/total station position of the twenty-five reference points is 0.813 m in x and 0.903m in y respectively. Furthermore, in comparison to GNSS/Total Station data acquired through LGO processing, the digital line map exhibited a horizontal precision of 2.103 m at a 95% confidence interval. This inaccuracy can potentially impact the drainage line or buildings that are constructed parallel to the road center line. Specifically, for the Saris site, it results in a 1.398 m deviation from the road design center line. This discrepancy has had a discernible effect on ground surveys and the design of road projects in Addis Ababa city, necessitating repeated right-of-way surveys and redesign efforts. Consequently, the precision of digital line maps falls short of the national error budget requirement (30cm) for Standard Mapping tasks under all circumstances. However, it may be suitable for imagining and small demanding tasks. In its current status, it is applicable to projects like master plans and general economic development plans.