Road rehabilitation and Sanral specifications
BackgroundWithout accurate geographical (survey) information no construction project, whether it be roads, civil works, buildings, plant, electricity or any other project, can proceed. All professionals need reliable data for planning, design, execution and as-built purposes. Historically the surveyor supplied this data by means of ground survey with conventional equipment and aerial survey methods. Over the past few years the construction industry has been introduced to airborne lidar mapping and terrestrial (ground based) lidar mapping. The latest development to be introduced is high resolution 3D mobile mapping. Survey work on most construction projects can be categorised as:
•Pre-construction – this is the planning and design phase
•Construction – this consists mostly of setting out quantities and confirmation of construction
•Post-construction – Supply of as-built plans and data
The methodology for pre-construction surveys is outlined below: SANRAL specification
Most road construction surveys need to adhere to SANRAL specifications, especially accuracies. These accuracies are summarised in Table 1: TABLE 1
FEATURE HORIZONTAL ACCURACY VERTICAL FIXING ACCURACY
Kerbing and edge of asphalt 50 mm 10 mm
Road surface (Seal / Asphalt / Concrete) 50 mm 10 mm
Storm water pipes 50 mm 15 mm
Lined Drains 50 mm 15 mm
Gravel road surface 100 mm 30 mm
General Topographic Detail of the road prism 100 mm 10 mm
Off the road prism 150 mm 50 mm
Detail Accuracy The position of all well-defined detail shall be plotted in relation to the co-ordinate grid so that the positional error of the detail shall not exceed 0,5 mm on the plan.
The most stringent specification is 10 mm and therefore equipment and work execution should be conducted to achieve these accuracies. Existing information
Prior to survey it is necessary to gather and investigate existing information including:
•Aerial or lidar surveys and data of survey
•Existing ground surveys if any
•Existing SANRAL benchmark data
•Existing SANRAL surveys
•Cadastral/Road reserve data
Benchmarks
An accurate benchmark system is the basis of any survey. The benchmark system ensures that the pre-construction, construction and asbuilt surveys are all on the same system.
The benchmark survey typically consists of:
•Confirmation of existing benchmarks x and y by GPS or total station and z by electronic levelling
•Building of new or re-building of existing benchmarks. SANRAL specifies that the existing benchmark system must be confirmed and used. This must be included in the survey quote.
•If existing information needs to be used, it must be confirmed by ground checks.
Strip survey
This must be tied in to the benchmark system and consists of two types of survey which are detailed below:
•Road prism which includes kerbing, edge of road, topographic detail of road prism and asphalt surface. This is high accuracy (10 mm) survey. These accuracies cannot be achieved by aerial survey and so ground survey methodologies are necessary. In the past, this was only done by total station equipment but recent technological developments have enabled this to be done by lidar scanning equipment, including mobile or terrestrial scanners.
•Road corridor which is the area between the edge of road and road reserve boundary. Ground survey methodology
The advantages of ground survey are that it is relatively cheap, especially on smaller projects, and that the surveyor is on the ground ensuring that very little detail is missed.
The biggest disadvantages of current ground survey methods are:
•Safety: roads are busy and extensive traffic control measures are necessary to ensure safety of personnel and road users. These measures slow down the survey process.
•Completion time: the survey process is always on the critical path because the engineer needs the design data.
•Cross sections at 20 m intervals: road surface data between cross sections is seldom recorded.
•For surveys outside the road reserve it is necessary to obtain access from property owners.
Mobile mapping solution In general the mobile mapper is used for road prism surveys but with its reach it can easily cover most of the road reserve of up to 50 m depending on the terrain. This reduces the ground survey part to infill surveys only. The most significant advantages of the mobile mapping option are:
•Independent tests have proved that certain mobile mapping systems can achieve accuracies that comply with the TMH11 specification of which the most stringent is 10 mm height accuracy.
•Turnaround time of projects is reduced due to the speed of travel and less safety requirements.
•High resolution (10 megapixel) photography which reduces site visits.
•The survey of the road surface can be done from the safety of a vehicle. The vehicle is travelling at a speed of up to 70 km, minimising impact on traffic flow.
•The density of the points is well distributed, with typical densities of 1 000 pnts/m2 on the road surface.
•The survey platform enables the use of heavier equipment and certain mobile mapping systems are equipped with a laser sensor which has a useful range of up to 200 m.
•The scanner is mounted on a vehicle and can be lifted using a lifting platform to increase the angle of incident of the laser sensor. This increases the accuracy of the survey and enhances the ability of the scanner to penetrate through vegetation and grass.
•Mobile mapping was developed with roads in mind and the software is well suited for the creation of line maps, contour drawings, break line creation, intelligent point thinning and so on.
•A typical strip survey using the mobile mapper is a continuous point cloud with a high degree of accuracy. The military grade IMU ensures that the data set is homogeneous and the typical up-down effect of a registered terrestrial laser scan is largely eliminated.
The disadvantages of the mobile mapping options are:
•Infill surveys – infill surveys off the road prism are still necessary, especially in areas of dense vegetation. Fortunately traffic control measures are a minimum in this area.
As for deliverables, the data is available in any industry standard format and will be processed up to the level required by the engineer:
•Vector Data is available in 2 or 3D CAD format and exportable to AutoCAD, Microstation, Smallworld, Civil Designer, Modelmaker, Caddy or any other industry standard software.
•3D – Point clouds to be exported to AutoCAD or Microstation 3D format.
•Triangles are exported to GMS or TOT file standards
•Georeferenced high resolution photography with index of photography in CAD and Google Earth.
Terrestrial laser scanning
Detailed design of bridges and tunnels are required for most projects. Terrestrial laser scanning is used where the mobile mapper cannot reach or enter. Terrestrial laser scanning can also be used for the road prism survey because the accuracies are better than 10 mm.
The advantages of terrestrial laser scanning are as follows:
•Detail – high density data ensures that bridges, tunnels and structures can be accurately modelled.
•Reach – the scanners can reach areas where no survey prism can be held. Aerial lidar survey
Where re-routes are required or where access is not available yet, it might be necessary to use airborne lidar. (Note: It is essential that enough ground control and ground truthing surveys are executed to ensure the aerial model is on the ground.) The advantages of aerial lidar survey are as follows:
•It covers a wider strip than necessary and wider mapping is possible where re-routes are necessary.
•For record and information purposes, the photography is a valuable tool.
•It is necessary for overall planning and can be used for other purposes such as route planning, EIA studies, and stormwater design to name but a few. The disadvantages of aerial lidar survey are as follows:
•Accuracy – a rehabilitation survey requires high accuracies on the road prism, accuracies that cannot be met by airborne methods.
•Not all detail is covered – ground survey is still necessary.
•Mobilisation – it takes time to mobilise and weather can be a time factor.
•Aerial Surveys are often executed well ahead of construction and are often not tied to benchmarks close to the project area. As a result, the Aerial survey is often not on the same survey date as the subsequent surveys. It is the client`s responsibility to ensure that the Tender specification allows an Aerial survey that is tied to benchmarks on the ground. Specifications
When writing specifications the following should be taken into consideration: Surveyor
The work should be executed by a PLATO registered surveyor or under his/her supervision. The advantage is that the person is qualified and can be held responsible for incorrect work. Equipment
The equipment used should be suitable for the project. Ask for specifications of equipment to be used and confirm with suppliers that the equipment can achieve the accuracies claimed. This is essential with GPS and LIDAR equipment because there are literally hundreds of different types of equipment designed for different levels of accuracy and applications. Benchmarks
It must be understood that an accurate benchmark network is the backbone of every survey. The engineer must clearly specify at what density benchmarks should be erected and what accuracies need to be achieved. TMH11 covers this extensively.
Strip survey
Ground survey – It must be clearly stated at what density and accuracy the topographical grid should be surveyed. It is of little use to specify 1 m, 0,5 m or 0,25 m contours because that merely determines the level of interpolation.
A useful guideline is as follows:
•0,25 m contours: survey grid of 5 m (400 pts per Ha) to 10 m (100 pts per Ha) depending on terrain
•0,5 m contours: survey grid of 10 m (100 pts per Ha) to 20 m (25 pts per Ha)
•1 m contours: survey grid of 20 m or more depending on terrain.
There are many factors that influence mobile surveys and accuracies are a combination of:
•GPS Post Processing accuracies
•Lasersensor range accuracies
•IMU angular accuracies
•IMU drift rates
•IMU initialization quality
•System calibration accuracy
•Survey tie-point interval and accuracy Typical specifications for a 3D high definition mapping system are outlined in Table 2.
TABLE 2 Equipment specifications
Laser Scanner: Standard Pulse Rate and Range 200 KHz at 200m
Range Accuracy (at mid-range) 10mm at 150m
Precision / Repeatability 5mm
Field of View Full 360 Tender Specification
Angular Resolution 0.009
Angular Accuracy 0.001
Beam Footprint (Gaussian definition) 7mm on exit
Beam Divergence 0.3mrad
Environmental Rating IP64
Eye Safe Class 1 – eye safe
Online Waveform analysis Increased Accuracy on Ground shots Inertial Measurement Unit (IMU):
Update Rate 250Hz
Velocity Accuracy 0.005m/s
Pitch and Roll Accuracy 0.004
True Heading Accuracy 0.01
Gyro Bias 0.03per hour
Random Drift 0.005per hour GPS:
Horizontal 1cm + 1PPM
Vertical 1cm + 2PPM Ground control
Benchmark system check levelled to a 2mm per Km closure. Ground control
Benchmark system check levelled to a 2 mm per mm closure.
Pre-marks or lidar control (similar to photo control points) at regular intervals to be specified per project but not more than 500 m apart. These should be connected to the BM system and check levelled to a 2 mm/km closure. This is not necessary for unpaved road surfaces. It is a client`s right to ask for Quality Control reports and check values. Terrestrial scanning
The benchmark system should be sufficient to cover overlaps of set-ups but not more than 200 m apart. The benchmark system must be check levelled to a 2 mm/km closure.
With regard to the scanner it is important to specify what accuracies are needed and to insist that the scanner, make, model and specifications be part of the project proposal. Airborne lidar surveys
An airborne LIDAR system also consists of a scanner, IMU and GPS. Due to the accumulative error in these three instruments, it is very difficult if not impossible to get better than 10 cm accuracies which is outside most SANRAL specifications. It is essential that a benchmark system is surveyed on site and that the survey is connected to the benchmark system. The ideal is that the benchmarks are pre-marked and surveyed to a height accuracy of better than 33% of the specified survey accuracy. For example: for 10 cm accurate ground points the benchmarks need to be surveyed to 3 to 4 cm accuracy. After data has been supplied, it is advisable to do a “ground truthing survey” such as at least one cross section at each benchmark. Photography (Air or ground survey)
A big advantage of scanning whether it is aerial, mobile or terrestrial is that there are normally cameras attached to the system Ask for sample photography to ensure that you get the quality needed. It is of little use if you want to capture road furniture and you cannot zoom in to identify street names Conclusion
Clearly there is no ultimate survey methodology and every project needs be judged according to its requirements. Mobile and terrestrial scanning are “new tools in the toolbox” and certainly assist surveyors, engineers and planners in executing their tasks more effectively. The challenge therefore is whether surveyors, engineers and planners can adapt to these new technologies. Contact SA Strydom, Global Geomatics, Tel 011 763- 7173, global@geoinfo.co.za
