Pre and post-intervention performance measures.

Some controls within the F.C.T., Abuja was selected to serve as both the base reference stations and observation stations as shown in Figure 4.7. The selection was to ensure even distribution (as much as practicable) and location of the controls for geoid modeling. Their status was determined during reconnaissance surveys to confirm existence and physical state of the pillars with pictorial evidence as attached in appendix 1.

The topography of the study area is uneven with orthometric height ranging from the flood plain of the Gurara River at an elevation of about 70m above Mean Sea Level in Abaji area to around N35 with height of about 940.96m and hence surface model will show variations as indicated by the various orders of the multi-networks selected as base and observation stations i.e.the land rises irregularly eastwards, northwards and northwestwards of the FCT.

139

Figure 4.6: Plan of some selected Multi - Networks controls 4.6.1 Geometry of Study Area Base Reference Stations

The base reference stations are made up of primary controls and were chosen to sorroud the FCT. FCT 260P is located on the ground while the remaining three are located on hill tops. A sketch of the geometry is shown in figure 4.7.

140

276P 260P

130P 162P

Figure 4.7: Sketch of Primary Base Reference Station controls

The primary controls are located around the perimeter of the study area and the observation points are distributed within the area of study but according to criteria that meets triangulation/control network scheme including strength analysis. The geodetic controls support the production of spatially accurate data for surveying and mapping arising from the permanency/stability of the controls. The geodetic controls ensure accurate representation of curved features on the earth surface on a flat paper as medium for map production in a chosen recognized projection system that manages distortions.

4.6.2 DGPS Observations Program

Schofield and Brench (2007) gave the five principles of surveying expertise as: working from whole to the part, economy of accuracy, independent control/check, data consistency and data safety /security.Based on these principles, the observations commenced by setting up dual frequency receiver V30 Pro Hi – Target DGPS on each end of the baselines (Primary controls) for continuous data logging. The baselines are arranged in such a way as to avoid extrapolation of positions within the F.C.T.i.e. observation positions are to be obtained by interpolation only. It is worthy to note that the primary triangulation stations forming the perimeter of the study area are of the highest quality in instrumentation, observations,

141

processing and stability due to the very stringent conditions and specifications of primary triangulation/controls establishment and position determination.

Static GPS consists of the combination of antenna, receivers and static mode of observation was adopted for a 2 hour session timespent on each of the rover stations. This duration of observation is generally accepted as adequate for resolving ambiguities and produce accurate results. Various studies have indicated that there are virtually no significant improvements in accuracy after 2 hours of observations. Barnes et al. (2003) listed constellation of satellites, multipath and tropospheric effects as the dominant parameters that affect the accuracy of GPS baselines but Eckl et al. (2001) had earlier pointed out that duration of observations is the most dominant of factors determining accuracy in baseline length.

After the completion of the observations, the data were downloaded and then converted to rinex data for post - processing.

4.6.3 Satellite Geometry

GPS surveys involvedconsideration of factors such as:- i) Number of satellites available

ii) Elevation or mask angle (usually set at ).The reason for this recommendation is to reduce the effects of systematic errors in GPS surveying namely tropospheric delay, ionospheric delay and multipath.

iii) Positional Dilution Of Precision (PDOP) iv) Obstructions to satellite visibility

v) Vertical Dilution of Precision (VDOP) which is very critical for vertical GPS surveys.

4.6.4. Vertical Dilution of Precision (VDOP)

This is used to describe weakness in geometry of satellites in the vertical direction because satellites are usually situated above the antenna hence affecting the ellipsoidal height. This

142

implies importance must be attached to times and areas in which GPS vertical surveys will take place to achieve a VDOP as low as possible. Caltrans Surveys Manual (2012) recommended a maximum VDOP of 4 (or 6 maximum) and minimum number of satellites observed simultaneously as 5 with 3 receivers using fixed antenna height tripods with dual frequency receivers. Precise ephemeris is applied to the computation. For this research where dual base stations were used on known primary control stations at any given time, the expectation of high accuracy and reliability is most desirable and expected.

4.6.5 Position Dilution of Precision (PDOP)

DOP is a measure that allows describing the influence of satellite geometry in accuracy of obtained measurements. Lower PDOP implies better satellite geometry and hence better position reliability. In general, Shruthi and Bindu (2016) have a list of DOP ranges and the interpretations.

DOP comprise Horizontal Position Dilution of Precision (PDOP), VerticalDilution of Precision VDOP, and TimeDilution of Precision (TDOP). These are measures of geometrical strength of a position determined by GPS. In the FieldGenius Technical Notes-GPS Terminology, it was stated that a DOP of between 4 and 6 can be considered as the threshold for good and poor geometry. Lower PDOP values denote better satellite geometry and thus a better reliability in positioning. A DOP value of 2.8 was achieved.