Engineers, project managers, town planners, architects and all other professions involved in design, planning and construction need accurate spatial data prior to executing their projects. by altus strydom*
verifying design tolerances is the first step in ensuring a perfectly executed survey.
The next step is selecting the right techniques and equipment to achieve the desired measurements and final mapping parameters.
The role of the land or technical surveyor is critical in ensuring that spatial (survey) data collections are executed and supplied according to the predetermined design specifications. It’s a profession that comes with exacting responsibilities and one that is governed by legislation.
In this respect, the Geomatics Professions Act (No. 19 of 2013) clearly stipulates that a person must be registered with the South African Geomatics Council (SAGC) to perform survey work. Anyone who breaches this stipulation is committing a criminal offence.
It’s also essential to understand that there are various categories of surveyors: the correct discipline and survey category must match the specific project outcomes. Professional land surveyors are registered to perform all areas of survey, whereas technical surveyors are not permitted to perform any work related to cadastral surveys. The latter relates to the registration of property rights and property boundaries.
Furthermore, a qualified surveyor must have a registration number with SAGC and at least a BSc or NDip qualification. To avoid mishaps, please verify with SAGC or ask the South African Geomatics Institute (SAGI) to assist.
Although surveys take many forms, they can be split into three main categories, namely airborne, ground-based and subsurface services. The key aspect to consider is that the level of accuracy needed must be agreed upon upfront, since this will determine the type of survey instruments used, as well as the practice tolerances must always be clearly stated up front in the specifications.
Data imported from other sources like existing maps, satellite imagery and certain GPS devices can be used as a guideline, but not as the sole basis for precision mapping. For example, incorrectly applied aerial and GPS technology can generate terrain height measurements that can be out by 5 m or more when verified by a detailed ground-based survey.
Airborne surveys
Airborne services are invaluable for overall planning and encompass techniques that include:
- lidar surveys using medium-format cameras
- large-format photogrammetry
- orthophoto mapping
- 3D oblique 45-degree photography
- supply and processing of satellite imagery and digital terrain model data.
Lidar surveys are normally required for larger areas and the equipment used can be mounted on any vehicle (air, land, water). Overall, it’s difficult to achieve accuracies beyond 8 cm.
Today, it’s also possible to supply high- accuracy 3D plans surveyed with lidar equipment, as well as photogrammetry.
However, this is a very specialised area and there are many pitfalls for the inexperienced. The best approach is to employ a professional surveyor and 3D CAD specialist. Within the aerial spectrum, drones and their on-board systems are becoming increasingly sophisticated. At this stage, though, it should be noted that drone surveys are still application- specific, plus they cannot always capture all the detail required. For this reason, infill ground surveys are still necessary to verify all the data. However, as drone software developments advance, fewer control points will be needed.
Ground-based services
Ground-based services are wide-ranging, providing pinpoint precision when required. An example would be a tunnel project where alignments of a few centimetres may be specified. In this instance, the survey map will also bypass any geological anomalies identified during the initial exploratory drilling stage and accurate calculations are necessary where earth curvature plays a role. For other structures like bridges or high-rise buildings, it’s equally obvious that the accuracy of setting out control information is critical.
Below are some of the typical applications:
- 3D high-accuracy mobile corridor mapping for pipelines, powerlines, roads and tunnels – depending on the equipment used, the expected accuracy will range from 5 mm to 30 cm
- 3D terrestrial laser scanning, commonly used for building projects, with an accuracy range from 2 mm to 30 cm
- contours: topographical and engineering surveys and mapping; this is executed with standard equipment such as GPS, and total stations, with an expected accuracy of 2 cm to 30 cm
- cadastral, as-built and assets are also surveyed with standard equipment, with 2 cm to 30 cm accuracy
- underground services detection and mapping using ground-penetrating radar, with 2 cm to 2 m accuracy
- bathymetric surveys for the mapping of subsurface objects – here, the equipment employed includes echo sounders, telemetry and sound velocity tools, with an accuracy of 10 cm to 5 m.
Before accepting a project, a professional surveyor will need a clear description of the purpose and scope. On larger projects, it is essential to supply existing plans and data as part of the survey brief.
Achieving the relevant design specifications
This is among the most misunderstood areas of a survey brief and leads to the failure of many projects. Equipment and software today are designed for ease of use and ‘pressing buttons in a sequence’.
This means that laymen can use the instruments, but do they really understand what they are doing? A person once said to me: “Nothing lies better than a well-prepared plan.” If the project leader does not understand or specify the accuracies to be achieved, they will get quotations where persons use substandard equipment. A typical example is where cheaper GPS equipment is used to do high-accuracy GPS work. A R12 000 GPS cannot achieve the accuracy of a R50 000 to R150 000 set. The same principle applies for cameras and 3D scanners mounted on drones and vehicles. CAD software can produce a great-looking plan notwithstanding the incorrect instrument used.
The golden rule is to specify the accuracies to be achieved and/or accuracy tolerances of equipment to be used.
Survey control
The purpose of sur vey control is to ensure that the original design survey and setting-out after design are aligned. This aspect is quite often overlooked in the survey specification and can be a big cost factor in an RFQ for a survey.
Survey benchmarks are the backbone of any construction site for smaller or larger terrain. Benchmarks are fixed, or permanent, survey markers that are left on-site for future work on location. A minimum of three sturdy benchmarks in a safe location are required for any project, but more may be specified as agreed upon and required.
A project example is the Kusile power station: the as-built and design accuracy was sub-millimetre and benchmarks had to be checked monthly to determine the movement in mm during cold, hot, dry and wet conditions. The concrete base of these benchmarks was 25 m deep: these benchmarks need to last during the entire lifespan of the project.
Survey control is also very relevant where remote sensing techniques (satellite imagery, airborne and mobile mapping) are used.
Survey and mapping requirements
Each phase of a project has different survey or mapping requirements and a qualified surveyor can easily advise on which equipment to use to execute the project. This will range from aerial surveys for overall planning to ground-based surveys for specific planning. The above underscores the complexity and critical importance of the professional surveyor at every stage
Survey report
Once all the work has been completed, this needs to be documented in the survey report in order to supply information to the next professional that will work on the project. Here, the surveyor must clearly specify the full survey methodology, including how the coordinates were calculated.
If you are in doubt on any area, contact SAGI for free advice and help with information, formulating accuracies and writing of specifications.
*altus strydom is the chairperson: Northern Provinces at the South African Geomatics Institute (SAGI).
IMIESA November/December 2019 23
