Record · Change
Subtract any two epochs.
A capture on its own is a photograph. Registered into the record — same frame, same control, every number carrying its unit and its ± — it becomes one term in a subtraction. Pick any two dates in a site's life and read what moved between them, with the uncertainty that belongs to a difference rather than the one that belongs to the totals.
- Record
- SPECIMEN SITE 04
- Epochs held
- 24
- First epoch
- 07 OCT 2025
- Latest epoch
- 21 APR 2026
- Frame
- ONE · VERSIONED · DATED
- Control
- 8 MARKS · ONE NETWORK
- Comparable pairs
- 276
The instrument
Pick the pair. The comparison already exists.
Every epoch here holds the same typed quantities, measured the same way, registered on the same control. That makes the chart below a chart of subtractions — twenty-eight of them across eight epochs, each one available the moment the later epoch lands, not assembled afterwards because somebody asked a question.
| Earlier epoch | E18 | E19 | E20 | E21 | E22 | E23 | E24 |
|---|---|---|---|---|---|---|---|
| E1703 MAR | |||||||
| E1810 MAR | — | ||||||
| E1917 MAR | — | — | |||||
| E2024 MAR | — | — | — | ||||
| E2131 MAR | — | — | — | — | |||
| E2207 APR | — | — | — | — | — | ||
| E2314 APR | — | — | — | — | — | — |
From
E17
03 MAR 2026
AERIAL · 1.8 CM DETAIL · 6 MARKS
VOLUMES ±2.8% ABSOLUTE
To
E24
21 APR 2026
AERIAL + GROUND 360 · 8 MARKS
VOLUMES ±2.2% ABSOLUTE
- Net cut to date118,400 → 201,400 m³
- +83,000m³ ± 2,573(3.1% of Δ)
- Stockpile SP-0412,400 → 5,100 m³
- −7,300m³ ± 307(4.2% of Δ)
- Steel erected0 → 402 t
- +402t ± 8(2.0% of Δ)
- Slabs poured
- +7 · exact
- Span
- 49 days
Counts are exact. Both epochs are registered on the same control, so the systematic error is common to the pair and cancels in the subtraction: the difference carries ±3.1% of the volume that moved, or the method’s noise floor over the changed region, whichever is larger. Absolute figures carry more, and say so.
The compounding record
Capture is repeatable. Last March is not.
Anyone can fly a site tomorrow. Nobody can fly it last March. Each epoch multiplies against every epoch before it, which is why the value of a record is never the newest capture on its own — it is the number of comparisons the newest capture has just created.
| 4 EPOCHSA month, captured weekly | 6PAIRS | |
|---|---|---|
| 12 EPOCHSA quarter | 66PAIRS | |
| 24 EPOCHSSix months | 276PAIRS | |
| 52 EPOCHSA year | 1,326PAIRS | |
| 156 EPOCHSThree years | 12,090PAIRS |
A site three years into its record answers questions a site three weeks into one cannot ask.
Nothing is overwritten: a corrected epoch is a new version, and the old reading stays readable.
Every epoch is registered to the same control network, so the pairs are comparable by construction.
The arithmetic is the arithmetic of counting — no claim is being made about it.
RECORD · NEVER EDITED · BOTH DATESThe honest half
Change below the error budget is not change.
Every method has a noise floor, and a week of work is sometimes smaller than it. The record keeps both figures — what moved, and what the instrument can resolve — and calls it change only where the first is larger than the second. The rest stays on the sheet as a reading.
Net cut, east basin
Surface differencing over the changed region · 3.1% of moved volume, floor 120 m³
+3,300 m³ ± 120
Reported as change
Stockpile SP-04
Volume against a fitted toe plane · 4.2% of moved volume, floor 90 m³
−800 m³ ± 90
Reported as change
Structural steel erected
Element count × schedule mass · 2.0%, floor 3 t
+61 t ± 3
Reported as change
Plinth settlement, building A
Ground control network, 12 marks · ±4 mm at 95%
−1.8 mm ± 4.0
Inside budget · held
Platform level, zone C
Surface height, graded plane · ±0.05 m vertical
+0.03 m ± 0.05
Inside budget · held
Indoors
The same station, occupied twice.
Aerial epochs difference surfaces. Interior epochs difference element states — what was hung, what was tested, what is now behind board. The station and the instrument height are fixed between visits, so the only thing that changes between two frames is the work.
| Element | E21 · 31 Mar | E24 · 21 Apr | On the record |
|---|---|---|---|
| Board, bay 07–09 | Not present | Fixed, 2 sheets | +18.4 m² ±0.3 |
| Cable tray CL | 2,410 mm AFFL ±8 | Concealed | Held at 2,410 ±8 |
| Sprinkler drop SPR-114 | Capped | Concealed | Head not yet fitted |
| Duct 600×400 | Hung, untested | Concealed | Pressure test 17 APR |
What is differenced
Surfaces, elements, quantities, dates.
Epoch-to-epoch is not one feature. It is what the record does to every measurement it holds, in whatever unit that measurement was taken.
Surfaces and volumes
Cut, fill, stockpile and platform level, differenced over the region that actually changed rather than across the whole site — so the tolerance belongs to the volume that moved, not to a cumulative total that dwarfs it.
Element state by trade
Hung, fixed, tested, boarded, accepted. Every element carries a state per epoch, so interior progress is a count of state changes with dates on them — not a percentage somebody typed into a cell on Friday.
The as-built programme
First-seen and last-seen epoch for every element is a start and a finish date, measured. An as-built programme falls out of the record without anyone writing one, which is what a delay analysis needs years later.
What was known, when
Epochs carry two stamps: when the work happened, and when the record learned of it. A question can be asked as of a date and answered as of that date — including a date before the answer changed.
The gaps in the epoch itself
Where a capture did not see a zone well enough to meet its error budget, the record says so and no difference is computed there. A stated gap is worth more than a confident number over an unseen corner.
The value of a record is not its newest epoch. It is every epoch the newest one can be subtracted from.