Index > 3 Characterisation of radioactively contaminated sites >

3.5.6 Techniques for determining location of survey data points

Contents Reference coordinate system Traditional positioning techniques Global positioning system (GPS) Microwave ranging systems Ultrasound ranging systems Advanced surveying techniques: laser ranging

The accurate location of measurements made during a survey is important for a number of reasons:

  • To ensure that no parts of the survey area have been missed.
  • To allow areas of contamination to be relocated at a later date.
  • To allow the data to be accurately topographical plotted and presented.

This allows the radiological collected data to be properly coordinated and analysed. Reproducible knowledge of the coordinates of measurements may negate the need to repeat work in the future and may provide a standard of quality assurance which is more readily accepted by stakeholders (e.g., regulators). Consideration should be given to whether a locally defined frame of reference is acceptable (for instance measuring locations relative to buildings or roads) or whether (perhaps if extensive site demolition is envisaged) a more permanent frame of reference (e.g., latitude and longitude) will be required.

The positioning techniques vary in both sophistication and performance and have different fields of application. Traditional surveying techniques require trained personnel and can be slow.

Modern technology has provided methods which can assist considerably in the characterisation process. Reference coordinate system

Reference coordinate systems are established at the site:

  • To facilitate selection of measurement and sampling locations;
  • To provide a mechanism for referencing a measurement to a specific location so that the same survey point can be relocated.

A survey reference coordinate system consists of a grid of intersecting lines, referenced to a fixed site location or benchmark. Typically, the lines are arranged in a perpendicular pattern, dividing the survey location into squares or blocks of equal area; however, other types of patterns (e.g., three-dimensional, polar) have been used.

The reference coordinate system used for a particular survey should provide a level of reproducibility consistent with the objectives of the survey. For example, a commercially available global positioning system will locate a position within tens of meters, while a differential global positioning system (DGPS), see the section below, provides precision on the order of a few centimetres. On the other hand, a metal bar can be driven into the ground to provide a long-term reference point for establishing a local reference coordinate system.

Reference coordinate system patterns on horizontal surfaces are usually identified numerically on one axis and alphabetically on the other axis or in distances in different compass directions from the grid origin. Grids on vertical surfaces may include a third designator, indicating position relative to floor or ground level. Overhead measurement and sampling locations (e.g., ceiling and overhead beams) are referenced to corresponding floor grids.

For surveys of Class 1 and Class 2 areas, basic grid patterns at 1 to 2 meter intervals on structure surfaces and at 10 to 20 meter intervals of land areas may be sufficient to identify survey locations with a reasonable level of effort, while not being prohibitive in cost or difficulty of installation. Gridding of Class 3 areas may also be necessary to facilitate referencing of survey locations to a common system or origin but, for practical purposes, may typically be at larger intervals – e.g., 5 to 10 meters for large structural surfaces and 20 to 50 meters for land areas.

Figure 3.9 Example of a grid system for survey of site grounds using compass directions
Figure 3.9 Example of a grid system for survey of site grounds using compass directions

Reference coordinate systems on structure surfaces are usually marked by chalk line or paint along the entire grid line or at line intersections. Land area reference coordinate systems are usually marked by wooden or metal stakes, driven into the surface at reference line intersections. The selection of an appropriate marker depends on the characteristics and routine uses of the surface. Where surfaces prevent installation of stakes, the reference line intersection can be marked by painting.

Three basic coordinate systems are used for identifying points on a reference coordinate system.

  • References grid locations using numbers on the vertical axis and letters on the horizontal axis;
  • Reference grid locations using reference distances from the 0,0 point using the compass directions N (north), S (south), E (east), and W (west), see example in Figure 3.9;
  • Reference grid locations using reference distances along and to the R (right) or L (left) of the baseline.

In addition, a less frequently used reference system is the polar coordinate system, which measures distances along transects from a central point. Polar coordinate systems are particularly useful for survey designs to evaluate effects of stack emissions, where it may be desirable to have a higher density of samples collected near the stack and fewer samples with increasing distance from the stack. Traditional positioning techniques

Traditional radiological topographical survey methods can generally provide the accuracy required for site characterisation methods and can give good standards of quality assurance provided that they can be linked to a series of well-defined and permanent reference objects. However, such measurements may be time consuming and labour intensive and may limit the speed at which measurements can be made in the field. Good measurements may be possible using only limited equipment (e.g., tape measures, surveyor wheels, grid marking, marking areas of contamination detected on-site using spray paint etc.) but will require that personnel have adequate training. Global positioning system (GPS)

The United States Department of Defense operates a satellite-based system of absolute positioning (known as the global positioning system, GPS) which allows a low-cost hand-held device to give locations anywhere in the world to an absolute accuracy of about 10 m. The signals from the satellite are usually deliberately perturbed to improve the accuracy. By obtaining correction signals from one or more base-stations at known locations, positioning over a large area, such as a city or even continent, to an accuracy down to as little as 1 m is possible, with accuracy relative to a local datum of as little as 1 cm. Measurements may be taken from moving vehicles, and the equipment can be used to navigate to a series of waypoints or along a predetermined path. Such a system, called Differential GPS (DGPS) can be operated entirely by the user, either in real time or by post-processed corrections. DGPSs can record and retrieve location data with a precision in the centimetre range.

Alternatively, correction signals are provided by commercial organizations for a fee (e.g., Trimble™, Novatel™, Garmin™). The corrections can be broadcast over a local radio network, multiplexed with other signals or transmitted over satellite links. The overall result is a portable system that can be carried by a person or fitted to a vehicle and can provide accurate locations. GPS or DGPS requires, however, a clear view of the sky and cannot be used inside buildings or under dense tree cover, and may suffer from inaccuracies caused by reflections when used close to buildings. Positional data obtained from these measurements will be repeatable to an absolute frame of reference and so are of special value where major site engineering operations which would otherwise destroy reference objects are likely to take place.

DGPS can be used to provide position information on surface features in areas being surveyed, linking the survey results to previously published maps and aerial photographs. In addition, survey results may be positioned using the DGPS readings to accurately and precisely locate the results as well as the results of any subsequent analyses to these same maps or photographs. A process called way-pointing uses the DGPS to locate specific points and allows the user to find pre-determined locations and set up gridded locations for measurements based on location data that are tied into local or state coordinate systems.

Limitations on the use of DGPS are related to the number of satellite beacons available to the system. When three or fewer satellites are available the accuracy and precision of the location data will be reduced. There are short periods of time (usually less than one hour even on the worst days) when a limited number of satellites are overhead in the continental United States. Satellites may also be blocked by excess tree cover or tall buildings. Distance between the moving locator and the stationary base station may be several kilometres or may be limited to line-of-sight. This limitation can be mitigated through the strategic use of repeater stations to re-transmit the signal between the moving locator and the base station. Microwave ranging systems

Local microwave or sonar beacons and receivers may provide useful location data in small areas, tree-covered locales. Various other techniques are available for providing relative positions over distances of tens of kilometers using microwaves. By measuring the time delays for a transmitted signal to be returned from two or more transponders, locations accurate to a few meters can be obtained.

One example of a sonar-based system is the ultrasonic ranging and data system (USRADS). With a number of fixed beacons in place, a roving unit can be oriented and provide location data with similar accuracy and precision as the DGPS. If the beacons are located at known points, the resulting positions can be determined using simple calculations based on the known reference locations of the beacons.

The logistics of deploying the necessary number of beacons properly and the short range of the signals are the major limitations of the system. In addition, multi-pathing of signals within wooded areas can cause jumps in the positioning data.

In many cases, however, DGPS would now be the preferred technique because it is absolute and does not require the accurate placing of transponders and a clear signal path to them. Ultrasound ranging systems

For relatively small sites, such as a disused factory, inside or outside, and where good spatial resolution is required, positioning systems based on ultrasound time-of-flight measurements are available. Such systems can provide locations to better than 1 m over distances of the order of 100 m. It is necessary to place and accurately locate several ultrasound transducers around the area to be surveyed. Advanced surveying techniques: laser ranging

Modern surveying equipment includes fully automatic total stations which use a laser device to measure the range and angle from a base station to a prism located at a mobile survey point. The accuracy of this equipment is typically in the mm range over distances of up to several km. The laser ranging equipment will track the prism and so is of use in moving vehicles provided that a line of sight between base and prism survey point can be maintained. The equipment needs two or more reference objects to be available to establish the position of the station but otherwise can give results comparable to DGPS. Single-handed operation is often feasible and the equipment could be used for surveying large indoor areas as well as outdoor areas in the vicinity of buildings where DGPS may be unusable.