ABSTRACT
Current development has shown many countries improving and strengthening their national control networks using modern space geodetic system. This study carried out 3D readjustment of part of the Nigerian Primary Triangulation Network using GNSS data obtained from Office of the Surveyor General of the Federation (OSGoF). First, the geometric analysis of the existing Nigerian Primary Network were evaluated using Triangle Inequality theorem and on how well the triangles in the network are conditioned. The result revealed that the network fulfilled the condition of the theorem however, when subjected to how well-conditioned the triangles were within the network, it was discovered that 56% of the triangles met the requirement while 44% did not meet the geometric conditionality. Different processing strategies are capable of giving different coordinate solutions for same point. Using fifty-two (52) GNSS station observational campaigns carried out within the period of October, 2010 - April, 2011, the study performed comparative evaluation of three different GNSS post-processing strategies with respect to points of reference originally processed with BERNESSE software from OSGoF. These processing strategies include; reducing observational campaign observed from pairs of stations (baselines) and combining these baselines into a network (Approach 1), taking GPS observations observed simultaneously at all stations directly into a network adjustment where all the coordinates of the network are presents as unknowns (Approach 2) and lastly, processing the observations using Precise point Positioning techniques (Approach 3). Due to the dissimilar nature of positioning, Trimble Total Control software was used to process Approach 1 and 2 solution (Relative solution approach) while GNSS-lab tool (gLAB) was used to process Approach 3 (Stand-alone solution approach). The residual (differences) in the horizontal and vertical component were computed for all observations. Out of the three solutions, Approach 3 gave solutions that were closest to the points of reference, followed by Approach 1 and then Approach 2. Poor performance of Approach 2 was attributed to some restraining factors that considerably induced errors within its solutions. Improvement on the study will be on how to develop a standard approach for harmonizing GNSS solutions in the near future.