A driveway profile needs two levels, one at the street boundary and one at the finished garage slab. A contour survey almost never states either at exactly the point you want.
Spot levels, contour lines and a datum are enough to derive both, and enough to be wrong by an amount that matters.
What the survey actually gives you
- Spot levels. Measured points, the most reliable thing on the plan.
- Contour lines. Joined points of equal height, interpolated between spot levels.
- A datum. What the levels are measured against, stated somewhere on the sheet.
- Kerb and invert levels. Often shown along the frontage, and directly useful.
Where you have a spot level, use it. A contour line is only an interpretation of the points either side of it.
Deriving the boundary level
The level you need is on the proposed driveway centreline at the property boundary. Work toward it in this order.
- Use a spot level at or near that point if one exists.
- Otherwise interpolate between the two nearest spot levels along the boundary, in proportion to distance.
- Failing that, interpolate between contour lines, which is least reliable and worth flagging as derived.
Deriving the garage level
The garage slab level usually comes from the architectural or engineering plan rather than the survey, as a finished floor level. Two adjustments commonly sit between that and the level the driveway meets.
- The slab thickness, where the stated level is a finished floor rather than the top of the driveway slab
- Any rebate or step at the garage door
Both are small numbers, and a 300 mm slab unaccounted for over a short driveway is a grade error, not a rounding error. The method is in boundary RL to garage RL.
Where the errors come from
| Source | How it shows up |
|---|---|
| Mixed datums | Survey and architectural plan measured against different references |
| Interpolating across a break | A retaining wall or batter between the two spot levels used |
| Kerb level read as boundary level | The crossing fall silently added to the driveway |
| Finished floor vs top of slab | Out by the slab thickness, consistently |
| Old survey | Levels predate earthworks, fill or a new kerb |
Three checks before you design
- Datum agreement. Confirm the survey and the building plan share a datum. If the sheet doesn't say, ask.
- Sanity-check the fall. Total rise divided by length should look plausible for the street. A 1 in 3 answer usually means a wrong level, not a steep block.
- Look for breaks between your points. Interpolation assumes an even slope, which a wall, batter or bench between two spot levels breaks.
Once both levels are settled the profile is arithmetic, and reading the long section checks the result.
Disclaimer
General information, not advice. Survey plans, datums and drafting conventions vary, and levels should be confirmed against the survey and the approved plans for the specific property. Where a level cannot be established reliably, have it surveyed rather than derived. Confirm requirements with the relevant council or a qualified certifier or surveyor.
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