What is Open Pit Mine Surveying?
Open pit mine surveying is the repeated measurement of a pit and everything moved in and out of it, so that production can be quantified, designs can be checked against reality and the ground can be shown to be behaving. It is not one task but a cycle, and the number it produces has to survive being questioned months later. This guide explains what that cycle consists of and where its figures come from.
What the survey is for
Four questions drive almost all mine survey work. How much material moved this period? How much is standing in the stockpiles? Is the pit being mined to the design? And is anything moving that should not be?
Each answer is a comparison between surfaces or between a surface and a design. That is why the survey cycle is repetitive by nature: a single survey tells you the shape of the ground, and only a second one tells you what changed.
The survey cycle
Capture. Most sites now fly a drone and process the images photogrammetrically, producing a surface model and an orthophoto. LiDAR — airborne or terrestrial — is used where vegetation or the required accuracy demands it. Total station and GNSS work continues for control, for check points and for detail that aerial capture cannot resolve.
Control. Ground control points tie the survey to the site coordinate system. Without them a photogrammetric block can be internally consistent and still sit a metre from where it belongs, which is invisible until it is compared with last month's survey.
Surface. The captured data becomes an elevation surface. What kind matters: a surface model that includes stockpiles and equipment is right for measuring a pile, and wrong for measuring ground.
Computation. Volumes, cross-sections, contours and progress comparisons come off that surface.
Reporting. The output goes to production, to finance and sometimes to a regulator — each of whom will use it to make a decision.
What actually gets measured
Pit advance. The difference between this survey and the last, reported as excavated volume. It is the number production is measured against.
Stockpile inventory. Volume above a reference base, converted to tonnage with a material density. This is the figure that most often gets audited.
Dump capacity and growth. The same computation in the other direction — how much has been placed and how much room remains.
As-built versus design. Whether the bench faces, berms and floor levels being cut match what was planned, and by how much they differ.
Movement monitoring. Repeated measurement of wall positions to detect displacement, which is a geotechnical question that the survey supplies the data for.
Why survey volumes and truck counts disagree
This is the single most common argument in a mine survey office, and usually neither side is wrong.
A survey volume is in-situ: rock as it sits in the ground. A haulage count is loose volume — the same rock after excavation, which occupies substantially more space. Converting between them needs a swell factor, and that factor is material-specific and rarely known precisely.
Moisture, compaction in the dump and material left on the floor all add to the gap. So does timing: a survey is an instant, while a shift count is a period. Stating clearly whether a figure is in-situ or loose eliminates most of the disagreement before it starts.
What makes a survey figure defensible
Three things, and none of them is precision. First, the reference: what was this measured against — a previous surface, a design, a level? Second, the method: which surface type, what interpolation, what boundary. Third, the uncertainty: what change is large enough to be distinguishable from survey noise.
A volume reported with its reference and its uncertainty can be defended in a meeting six months later. A volume reported as a single number cannot, however carefully it was measured.
Frequently asked questions
How often should an open pit be surveyed?
Monthly is the common cycle because it matches production reporting, but stockpiles are often measured more frequently and active faces may be surveyed weekly. The right interval is the one where the change between surveys is comfortably larger than the survey uncertainty.
Is drone photogrammetry accurate enough for mine surveying?
For volumes and progress on an open pit, generally yes, provided there is proper ground control. The limitation is not the camera but the control and the surface type: without control the block can shift bodily, and a surface model that includes vegetation or equipment will not give a clean ground volume.
What is the difference between a survey volume and a reported production tonnage?
A survey volume is in-situ and geometric. A production tonnage is usually derived from haulage or weightometer data and reflects loose material. Reconciling the two requires a swell factor and a density, and the difference between them is a normal part of mine accounting rather than an error.
Which surface should stockpiles be measured on?
A surface model that includes what is standing on the ground, because the pile itself is what you are measuring. A bare-earth model has, by definition, tried to remove it.
STREAM computes stockpile tonnage, cut/fill and period-to-period pit change on your own survey data, offline.
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