N4 Collooney to Castlebaldwin Road Realignment Scheme
About This Project
Sligo County Council is planning for the re-alignment of approximately 14.7km of the N4 National Primary Route between Collooney and Castlebaldwin in Co. Sligo. Figure 1 below shows the proposed route of the new road. The first 2.6km is outlined and is an upgrade of the existing single carriageway to a Type 2 dual carriageway. The scheme veers west of the existing N4 road until crossing back over the existing N4 to the east side. The realigned road ties into the existing N4 south of Castlebaldwin village. The scheme will bypass Lackagh, Drumfin, and Castlebaldwin.
Figure 1 Proposed route of the new road
Murphy Surveys were successful in the tender for the topographical survey project. The team carried out the topographical survey using a multi-disciplined approach combining GNSS, terrestrial total station, laser scanning and Unmanned Aerial Vehicle (UAV) survey techniques.
The client required the topographical survey to provide the necessary information to facilitate the detailed design and preparation of contract documentation for the project. The survey was carried out in two phases.
Phase I required the survey of the total topographical survey area. The length of the scheme was 14.6km with a topographical survey area of 275 hectares. The survey included 5.4km of the existing N4, 200m of the existing N17 and 7.3km of existing regional and local roads. Approximately 12.3km of watercourses where surveyed, along with 18 different structures and bridges.
Phase II required the survey teams to return to site to survey the reinstated areas affected by the archaeological survey carried out on site.
A CCTV survey was also required in an area near the Collooney roundabout.
A primary network of eight primary stations and 18 secondary stations were installed and coordinated throughout the project area. Static GNSS observations where observed to form the primary and secondary control networks. Leica GNSS receivers where used to carry out this aspect of the survey. The network adjustment was carried out using STARNET software.
Vertical control for the Secondary Control stations was provided via three double level runs through these stations and holding three Primary Control stations fixed.
The N17 and N4 national roads were traversed and 3D laser scanned using the Trimble SX10 scanning total station. Hard shoulder closures were employed to facilitate the survey from the verges and hard shoulder. This meant no survey personnel were required to enter the carriageway to survey details at any time resulting in greater safety, no need for traffic management and no interruptions to traffic flow.
The regional and local roads and watercourses and topographical survey infill areas were surveyed using Leica total stations.
The 275 hectare survey area was flown by UAV. We used our Sensefly eeBe Plus fixed wing UAV with the single rear mounted motor as this equipment is ideal for large area acquisition such as this.
Infill survey details not visible from the UAV imagery were surveyed using Leica total stations.
The 12.3km of watercourses were surveyed using total station and RTK GNSS techniques. In addition to this our hydrographic departments observed 150 cross sections on specific reaches for flood analysis.
The high resolution orthoimagery acquired from the UAV survey was provided to the client as a background within the AutoCAD 2D and 3D drawing files. This orthoimagery provided a great source of information to the client over and above the surveyed details.
The combined approach of terrestrial laser scanning of the roads, GNSS and total station survey, hydrographic survey and UAV survey allowed for the combining of geospatial data from different survey methodologies into the deliverables required by the client in an efficient manner and a shortened timeframe. It reduced the need to get foot access to all lands within the survey area and minimised the impact of landowners.
The combined approach also helped to resolve issues, for example, a CCTV survey was carried out in the north area of the site and when a conflict of information arose regarding the distances between manholes supplied by the CCTV contractor and the topographical survey. The orthoimagery was used to confirm that the location of the manhole in the topographical survey was correct and the distance measured along the pipe by the CCTV crew was less accurate.
During Phase II of the project, when the survey teams returned to site to survey the reinstated areas after the archaeological survey we found the reinstated ground to be too soft to walk on. In order to prevent any delay to the survey (by waiting for the ground to dry out), the team launched the UAV in these areas and acquired new orthoimagery. This allowed us to extract the updated survey information without any risk to the survey teams.
Sligo County Council
Laser scanning, Topographic Survey, Hydrographic Survey and UAV Survey