Planimetric Accuracy Assessment of Digital Spatial Data for Large Scale Planimetric Mapping: The Case Study on Dire Dawa City, Ethiopia

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Addis Ababa University


This study assesses the planimetric (x, y) accuracy of digital spatial data used for mapping large scale planimetric maps in Dire Dawa city. These digital spatial data are Orthophoto and its line map (parcel and road center line) and Worldview-1 panchromatic satellite image. RTK GPS measurement technique was used as an independent source and point wise method were used to assess the planimetric accuracy the digital spatial data set. Due to unavailability of local CORS station which used as a reference during RTK GPS measurement second order GCPs which was established by E.G.I.A (Ethiopian Geospatial Information Agency) was used as a base station. But before conducting RTK GPS survey a 12 hour static GPS survey were conducted on selected two second order and one first order GCP established by E.G.I.A and the GPS data are processed with a tie to IGS CORS station found in Addis Abeba and two free online GPS data processing service provider that are AUSPOS and OPUS. For accuracy comparison OPUS processed coordinates result were taken. However, the positional accuracy of selected GCP has mean error of -0.77m and 2.4m on x and y direction respectively. Therefore due to discrepancy of coordinate obtained from E.G.I.A and processed coordinate planimetric accuracy of orthophoto was assessed twice for selected 20 points with reference to RTK survey coordinates computed from E.G.I.A GCP and computed coordinate from IGS station. The planimetric accuracy of orthophoto relative to GCP established by E.G.I.A has good accuracy than coordinate computed from current derived coordinate from IGS station. Based on this relative to E.G.I.A GCP the planimetric accuracy orthophoto for the selected 20 points has RMSE of 0.138m and 0.15m on X and Y direction respectively and with respect to computed coordinate the planimetric accuracy of orthophoto has RMSE of 0.67m and 2.498 on X and Y direction respectively. Thus to increase the significance and applicability of this study finding the remaining line and polygon feature of orthophoto and its line map and satellite image derived coordinate compared with RTK survey coordinate correction computed from reference E.G.I.A second order GCP. Therefore the planimetric accuracy of orthophoto for the selected parcels, line and points are computed separately relative to RTK survey results thus the RMSE of these feature lie between 0.138m-0.22m and 0.12m – 0.24m on x and y direction respectively and 95% confidence level lie between 0.37cm and 0.55 cm and the planimetric accuracy of digital line map (parcel) RMSE are 0.173m and 0.196m on x and y direction respectively and 0.45m with 95% confidence level. The digital line map (road center line) also assessed its planimetric accuracy relative to coordinates obtained from RTK GPS survey coordinates and has RMSE of 0.148m and 0.191m on x and y direction respectively and 0.41m at 95% confidence level. Similarly the planimetric accuracy of worldview-1 satellite image assessed on similar sample parcel and line (flood protection fence) feature with that of orthophoto. Thus planimetric coordinates extracted from selected sample feature from satellite image are compared with Coordinates obtained from RTK GPS survey on similar feature. Based on this parcel feature has an RMSE of 1.73m and 2.06m on x and y respectively and 4.67m at 95% confidence level. The planimetric accuracy of line feature (flood protection fence) has RMSE of 1.89m and 2.25m on x and y direction respectively and 5.09m at 95% confidence level. The planimetric accuracy assessment result obtained from orthophoto and its digital line map based national accuracy standard for legal cadastre 03/2015 and ASPRS international digital spatial data set accuracy standard are applicable for large scale urban cadastral map and . However the planimetric accuracy of worldview-1 satellite image is not applicable for large scale cadastral map production.



Orthophoto, Digital line map, satellite image, planimetric accuracy, RMSE