Stockpile quantity survey services

Stockpile surveys using 3D laser total stations: background and applications

Industrial companies and groups often store bulk commodities in warehouses or outdoor yards as large stockpiles. Reliable estimates of the quantity of raw materials, semi-finished products and finished goods are needed for commercial and production planning, financial reporting, credit agreements, business forecasts, periodic inspections and monthly, quarterly or annual reports to the board of directors.

For stockpiles containing tens or hundreds of thousands of tonnes, weighing all material can require substantial time, labour and equipment, including loaders and trucks. ITVC’s 3D laser stockpile survey service provides an alternative that can reduce the time and cost of quantity assessment.

A stockpile survey determines the volume and mass of piled commodities. A 3D laser total station surveys the pile, and software developed by ITVC processes the measurements to determine volume. Bulk density is determined from samples collected using a vacuum deep sampler or other specialized sampling equipment.

Stockpile surveying is used in countries including Malaysia, Thailand, India, China, Indonesia, Australia and Vietnam.

EODR measurement system (3D laser theodolite)

• Motorized reflectorless laser theodolite with an extended measurement range
• Calibration chart software

Leica TPS 3D laser total station

Leica 3D laser theodolite: specifications stated in the original article

• Reflectorless survey range: 2,000 metres
• Prism range: 10,000 metres
• Distance and elevation measurement accuracy: 2 mm + 2 ppm/km, as stated in the original equipment description
• Laser spot size: 2 cm at a distance of 50 m
• Angular measurement accuracy: 0.0003 degrees
• Measurement speed: up to 30 points per minute
• Internal memory: 64 MB
• Electronic level compensator (automatic levelling correction)

Method references and scope of recognition

• The original article referred to ISO 7507 (BS 7723), Parts 4 and 5, for internal and external electro-optical distance ranging. These standards concern calibration of vertical cylindrical tanks; they do not, by themselves, establish a general standard for stockpile mass measurement.
• NATA accreditation applies to an organization and a defined scope of activities. Any claim that a specific stockpile survey service is accredited must be checked against the provider’s current scope; using a measurement method alone does not establish accreditation.
• Battery-powered laser equipment must be used in accordance with its laser safety classification, manufacturer instructions and site safety requirements. Activities that create fire or explosion risks must be controlled during the survey.

Reference sources: ISO 7507-5; NATA

Stockpile survey procedure

The method can be used for stockpiles of different shapes provided the entire surface can be observed and surveyed without obstruction. The field laser survey follows a measurement programme, with digital results stored in the instrument’s internal memory or memory card by a data logger or data collector. Recorded data include:

• Distance from the surveyed point to the instrument’s reference origin
• Angles of the laser beam relative to the horizontal and vertical
• Elevation of the surveyed point relative to a horizontal reference plane

The data are transferred to a computer and processed using software developed by ITVC. The software reconstructs the pile’s shape and calculates its volume. Average bulk density is determined in the field using representative samples obtained with a vacuum sampler or other specialized sampling equipment. Stockpile mass is calculated by multiplying volume by the measured bulk density.

Survey conditions and considerations

1. Determine bulk density in the field by testing representative samples taken from the surveyed stockpile. Equipment includes a vacuum sampler, a container of known volume and weighing scales. For material with non-uniform particle sizes, a truck body with a known geometry may be used. Material collected at different locations is loaded and weighed; its mass and the actual occupied volume are then used to calculate bulk density.
2. In this truck or container sampling approach, sampling may be limited to the surface layer. Moving material changes its packing and can introduce density error. Density checks at several locations and averaging the results help address this limitation.
3. Topographic Survey software reconstructs the stockpile from 3D laser total station measurements; Civil CAD software calculates the volume.
4. Stockpile mass equals the calculated volume multiplied by the average measured bulk density.
5. The customer declares information about the stockpiles and the boundaries between piles before the survey.
6. Commodity descriptions in the survey report are based on customer declarations and do not constitute ITVC certification of the material type, name or grade.
7. Material must not be moved or handled during the survey. If it is moved within the surveyed pile, the survey must be repeated. Normal cargo handling may resume after ITVC completes the survey.
8. Base layers are assumed to be flat.
9. Information on the stockpiled commodities and the boundaries between piles is supplied by the customer before the survey.
10. Outdoor stockpiles are not surveyed in rain.
11. The reported quantity represents the material present at the stated survey date and time; it does not establish the quantity at other times or under other conditions.

3D geometry of surveyed stockpiles

Accuracy of 3D laser stockpile quantity surveys

The original article reports indicative mass deviations of approximately ±1% for stockpiles with relatively uniform material and ±3% for less uniform material, based on the described method and customer feedback. These figures are not a guaranteed tolerance for every survey. Achievable uncertainty depends on visibility, the base model, sampling, bulk density, compaction and site conditions.

Characteristics and applications of the method

• TPS 3D laser theodolite measurements capture surveyed point positions, including distance, angle and elevation, and store them automatically. This reduces manual transcription and transfer errors; it does not eliminate all sources of human or measurement error.
• Professional survey and graphics software calculates the pile volume from the collected data. The result depends on measurement quality and the completeness of the surface and base model.
• Bulk density is determined from sampled material. Vacuum sampling can collect material at different depths where access allows; the sampling plan must address the representativeness of the samples.
• The method can assess stockpiles of scrap steel, pig iron, ores, clinker, bulk cement, sand, limestone, gypsum, clay, agricultural commodities such as cashew nuts, rice, wheat and maize, wood chips and coal.
• Stockpiles of many different shapes can be assessed when the survey conditions are met.
• Portable equipment and volume-based measurement can assess very large stockpiles of tens of thousands of tonnes without weighing all of the material, potentially saving time and cost.