Homogenizing coordinates through the use of active CORS in Ghana

Abstract

Abstract

In this study homogeneous three-dimensional (3D) coordinates of newly established active CORS have been determined based on the War Office datum in Ghana. In order to attain homogeneity, the coordinates of three primary control points GCS 121, GCS 305 and GCS 306 as established by the Gold Coast Surveys in 1926 were adjusted by least square approach (LSA) at 1mm standard deviation, keeping GCS 121 as error free station.  The adjusted coordinates of GCS 305 and GCS 306 were used as reference to compute positions of the LiSAGNet CORS in differential mode using 24hr data for 11 consecutive days. The GPS data covered DoY 71 to DoY 82 in 2020. Post processing was done using Topcon Tools v8.2.3 software. The final positions of the CORS computed by this approach indicate some differences with the officially published coordinates of the same CORS, confirming the suspicion of non-homogeneity in the source coordinates used in establishing the local CORS coordinates. Hence, a test network consisting of five CORS stations was designed and carried out to examine the consistencies and inhomogeneity in the officially published coordinates as opposed to those computed in this study. Using the officially published coordinates as reference input, the LISAG_ODA station was fixed by three different CORS resulting in average coordinates variability of 2.78m and 0.80m in Northing (N) and Easting (E) directions respectively. Substituting instead, the coordinates computed in this study as reference input, the LISAG_ODA station was fixed by same three different CORS resulting in coordinates variability of 0.002m and 0.006m in northings (N) and eastings (E) directions respectively. The analysis revealed inconsistencies and inhomogeneity in the officially published coordinates. It is thence recommended that, the officially published coordinates of the CORS be replaced by adjusted homogeneous and consistent values through the approach adopted in this study.

Keywords: CORS, Inhomogeneity, Coordinates Variability.