Vertical Accuracy Assessment of 20-metre SPOT DEM using Ground Control Points from Lagos and FCT, Nigeria
Digital Elevation Models (DEMs) are one of the most commonly used datasets for representing variations in terrain
elevation. Technological advancements in satellite remote sensing have led to the proliferation of digital elevation
datasets with near-global coverage such as the SPOT (Satellites Pour L’Observation de la Terre) DEM. SPOT DEM,
which has a spatial resolution of 20m, has a stated absolute vertical accuracy of 10-20m. In 2012, the Office of the
Surveyor General of the Federation (OSGOF) in Nigeria acquired SPOT DEM version 1.0 for use in topographic
mapping. As such, it is necessary to conduct localised assessments of the DEM to validate the stated accuracies. In
line with this, the aim of this study is to conduct an accuracy assessment of the 20m SPOT DEM in Nigeria using
Ground Control Points (GCPs) acquired from sites in Lagos State and the Federal Capital Territory (FCT). In addition,
the DEM’s suitability for topographic mapping is assessed in line with international standards. The DEM for both
sites acquired from OSGoF and the GCPs were harmonised in WGS84 datum within ArcGIS software. Subsequently,
heights were extracted from the DEMs with the GCP locations, for which the following accuracy parameters were
computed – height differences, standard deviation (SD) and root mean square error (RMSE). In the results, the
analysis of the height differences yielded the following – Lagos (SD: 3.367m; RMSE: 3.423m) and FCT (SD: 6.280m;
RMSE: 6.285m). This assessment proves that the SPOT DEM for both sites surpasses the stated absolute vertical
accuracy, and hence can be relied upon as a reliable elevation dataset. It was also shown that the DEM satisfies the
Class 2 accuracy standard by the American Society for Photogrammetry and Remote Sensing, and is suitable for
deployment in small and medium scale topographic maps. To fully exploit the advantages of the DEM, it is
recommended that OSGoF should deploy it for use in other applications beyond topographic mapping.
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