Wednesday 1 July 2026
Every drill hole in a resource model starts at the collar. The collar coordinates define the surface position from which every downhole intercept, every lithological contact, and every assay interval is projected into three-dimensional space. When those coordinates are wrong, every metre of data below the surface is mislocated by the same amount.
A collar survey error of 2 metres on one hole may seem manageable. Across 200 holes in a drilling program, that same error shifts intercepts, distorts geological continuity, and can materially change the resource estimate. The JORC Code (2012) requires collar surveys to be "of sufficient accuracy" for resource estimation. When they are not, the Competent Person cannot sign off on the resource, and the data that took months to collect becomes difficult to defend.
Collar surveys are a surface discipline. They measure the XYZ position of the hole at the ground surface, using DGPS, RTK GPS, total stations, or traditional survey methods. This is distinct from downhole surveys, which measure the trajectory of the hole below ground. Both matter, but collar surveys are the foundation: get the starting point wrong and the downhole trajectory, however precise, projects from the wrong location.
These five habits protect the collar data that underpins the resource model.
1. Survey every collar with RTK GPS or DGPS
Handheld GPS units are convenient, portable, and accurate to ±3 to 5 metres under typical field conditions. For reconnaissance traverses and general navigation, that accuracy is adequate. For drill collar positions that feed into a resource model, it is not.
RTK GPS delivers horizontal accuracy of ±0.02 metres in good conditions, with vertical accuracy typically within ±0.05 metres. That is the difference between a collar position that survives an audit and one that introduces positional uncertainty large enough to undermine the resource estimate. Every collar in a resource drilling program should receive a survey-grade fix, whether by RTK GPS, DGPS, or total station. Handheld coordinates are placeholders, and placeholders should not enter the resource database as final positions.
2. Record collar coordinates before the rig moves
Once the drill rig pulls off a completed hole, the collar environment changes. Cuttings bury the collar peg, the drill pad is reshaped, and rehabilitation earthworks can obscure the original position entirely. Resurveying a collar weeks or months later, when the physical evidence has been disturbed, introduces uncertainty that compounds through the resource model.
The surveyor should record collar coordinates while the collar is still visible and the peg is intact. This means scheduling the survey as part of the drilling workflow, coordinating with the rig crew so the fix is taken before the rig moves to the next hole. Waiting until the end of the program to survey collars in bulk invites exactly the kind of positional drift that corrupts a block model.
3. Tie survey control to a permanent benchmark
Every collar survey should reference a local survey control network tied to a permanent benchmark, such as a state geodetic survey mark or a registered trigonometric station. The benchmark provides the fixed reference point that anchors the entire survey to a known coordinate system.
Without this tie, the survey can drift systematically across the grid. A 1 metre systematic shift applied to 200 collars does not average out or cancel: it moves every hole in the same direction and distorts the geological interpretation at block model scale. Permanent benchmarks are the insurance policy against systematic positional error, and establishing one at the start of a program is far cheaper than re-surveying an entire collar dataset after the drilling is complete.
4. Confirm the coordinate system before the first hole
MGA94 versus MGA2020 versus local grid versus WGS84: coordinate system mismatches are among the most common and most damaging errors in drilling programs, precisely because they are administrative rather than instrumental. The survey equipment may be functioning perfectly, but if the datum or projection does not match the project database, every collar position will be offset by a consistent amount, often tens of metres.
This error is caught by a single check: confirming the survey setup against the project database datum before the first hole is drilled. It is particularly important in multi-phase programs where different contractors or survey crews work across campaigns. Australia's transition from GDA94 to GDA2020 has made this check even more critical, as both datums remain in active use across different project databases. One mismatched datum can silently corrupt years of drilling data.
5. Re-survey 10 to 15 percent of collars at the end of the program
Select 10 to 15 percent of collars at random and re-survey them with independent equipment or a different survey crew. This QA/QC step tests the integrity of the entire collar dataset by checking a representative sample for systematic instrument drift, datum errors, and procedural lapses.
If the re-survey positions match the original coordinates within an acceptable tolerance, the dataset is defensible. If they do not, the discrepancy identifies the problem before the data enters the resource model. This is the same principle that drives duplicate assay programs in geochemistry: independent verification of a subset protects the credibility of the whole.
The bottom line
The collar is the starting point for every claim in the resource estimate. Every intercept, every grade, every geological contact in the model is projected from the collar position downward. Survey discipline at the surface is where resource model integrity begins.
Get the collar wrong and every metre below it is wrong too.
At Norwest Exploration and Mining Services, we place experienced geologists and field technicians who understand that the quality of a resource model starts at the collar peg. Whether you need a senior geologist to supervise a drilling program or a full field crew for a multi-rig campaign, visit our website to learn more.