13 JAN 2020
This article is an extract from Geospatial Engineering 2019 published by the Chartered Institution of Civil Engineering Surveyors, and is reproduced with permission.
Between the coastal belt of Guyana and the border with Brazil lies the road from Georgetown to Lethem, the critical north–south trade link for the smaller country. The Guyanese government intends to upgrade the Linden to Mabura Hill stretch of this road and design a new river crossing at Kurupukari.
Mott MacDonald contracted Malcolm Hughes Land Surveyors to carry out a topographic survey of the 120km stretch and complete a bathymetric survey at the site of the potential river crossing. The road being upgraded starts in the north, 103km south of Georgetown, while the proposed river crossing is further along the Georgetown–Lethem corridor, 108km by road beyond Mabura Hill.
These new infrastructure projects, with better transport links and all-weather roads, will improve traffic flow and support the national economy. The crossing is especially important because the only way to cross the 600m channel at present is on a private ferry, which operates at irregular intervals and only during daylight hours.
Malcolm Hughes staff planned a visit to Guyana, to include meetings with the Guyanese government, Mott MacDonald and representatives from local support firms as well as a review of the road and river crossing area. This planning stage was essential to appreciate fully the terrain and conditions in which surveyors would be working, as well as identifying the most appropriate survey methodology.
The nature of the landscape meant that we could not depend on modern techniques and technology. For instance, the use of drones and aerial LiDAR was considered, but because the specification for the survey included 50m either side of the road – into the jungle, under a heavy tree canopy – we reverted to traditional surveying methods, which are reliable in all situations.
A project plan was established to ensure the health, safety and welfare of our staff while in Guyana, and included the following measures:
Work began in early March 2019, with a pressing deadline to complete all surveying by early to mid May before the rainy season started. Accommodation was booked along the route. Securing the services of local people, including drivers, was another major factor in the success of the project.
Surveys in developing countries generally require primary control networks to be established. From this primary control, secondary networks can be established to allow traversing and detailed surveying of the areas required. The team consisted of a survey manager and four surveyors. A hydrographic surveyor joined the project to complete the river crossing survey. Primary control stations were established at Linden, Mile 35, Mile 58, Mabura Hill and Kurupukari. These points were supplied with dual-frequency Leica GPS receivers for up to 12 hours in two sessions over two days.
Most countries have implemented new geodetic control networks and continuously operating reference systems (CORS) enabled by global navigation satellite systems (GNSS); CORS allow surveyors to establish accurate control networks and record topographic detail quickly. Guyana has a new active station network and there are eight across the country, so the team was able to establish a strong network with the primary control. Receiver-independent exchange format data was downloaded from the cross-origin resource sharing network website for comparison with our observed data. All loop closures were found to be good and the network was adjusted successfully.
Secondary control was established between the primary control stations at 2.5km intervals, using feno markers – pieces of steel flat to the ground – in concrete for stability. These control points were occupied by dual-frequency Leica GPS receivers in a leapfrog traverse, an arrangement in which two stations are occupied by receivers concurrently for at least 20 minutes. This enabled the control to be linked between the primary stations and established a secondary network with closed loops that were adjusted successfully.
At Kurupukari, the secondary control consisted of static observations radiating from the main control point with occupation times of between 30 minutes and an hour. Temporary control markers were set up, with wooden pegs installed every 100m, to carry out the survey along the route. Construction survey control was then established along the entire route, and tied back to the more accurate higher-order control to maintain spatial and dimensional integrity.
Guyana uses a localised Provisional South American 1956 Datum for its national survey grid. Here we did rely on a more 21st-century approach: it was decided with Mott MacDonald that we would use the WGS 84 ellipsoid, the default global GPS coordinate output. Essentially, the project was viewed as being in its own grid system. Because there was no current geoid model available to convert the ellipsoidal to orthometric heights, it was also agreed that we would present data as WGS 84 ellipsoid heights.
Surveys in developing countries generally require primary control networks to be established
The Linden to Mabura Hill road runs through undulating topography in the watershed of the Essequibo and Demerara rivers. The road starts in Wismar and ends at Mabura Hill and there are two communities based on the road, called 47 Miles and 58 Miles. It is mainly a fair-weather road surfaced with laterite, and because of poor maintenance some sections can become impassable during the rainy season. The average width is around 20m and the total length of the road is about 122.5km. The stretch of road to be upgraded connects the township of Linden to Mabura Hill, a small logging community.
Detailed surveying was carried out along the road, including 50m either side into the jungle, using total stations and GPS. Heat and dust were the main challenges: although the road was not always busy, when a vehicle did pass it would create a mini dust storm that affected visibility. The jungle areas had to be controlled, and establishing protection zones and employing local assistants were key to keeping the team safe from unknown threats, including snakes.
The survey work at the site for the proposed river crossing was carried out by a team of two surveyors and a local assistant who doubled up as the driver. The task was to survey five transects at the existing ferry crossing, consisting of the main centreline of the proposed bridge together with two transects upstream and two downstream of this at 50m intervals. Another eight river transects and seven rock bars were surveyed downstream of the proposed crossing.
After sourcing a local boat and boatman, we began the river work. Because of the unusually low water levels and varying depths, each section was surveyed by a variety of methods, including an Ohmex Sonarmite transducer fixed to a GPS pole to echo sound the level of the deep areas from the boat. The rock bars were walked with GPS, while steep, dense banks on the sides of river and central islands made GPS observation impossible so traditional land survey methods were used. After the river work, we also surveyed a 1km stretch of the approach road either side of the crossing.
This was a challenging project. Staff worked away from home for nearly three months in an unknown environment in sometimes difficult conditions. The rains started early, in late April, but the team completed all the required work and returned safely in mid May.