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Levels, falls and datum: shooting levels cheaply and accurately

Every drainage decision, every step, every retaining wall and every paved surface depends on levels. Water hose level, string line, laser level and dumpy — what each costs, what each is good for, and how to set a datum you can return to.

⏱ 90 minIntermediate🔗 9 resources (8 free)

Most Perth blocks look flat. Almost none of them are. Two hundred to four hundred millimetres of fall across a 20 metre backyard is completely ordinary on the coastal plain, and it is invisible to the naked eye — right up until you lay a patio and discover the water runs back at the house.

On a renovation you are also inheriting somebody else’s levels. Fifteen years of mulch top-ups have raised the beds, the lawn has been sand-topped every spring and now sits proud of the path, and a previous owner paved the side access at whatever height suited the bricks. None of that shows in a photograph. This lesson is about measuring what is actually there, in numbers you can do arithmetic on, from a reference point you can come back to.

Why a professional does this before drawing anything

A levels survey is the first piece of hard data a designer gets, and everything downstream is arithmetic on top of it.

It is the only way to design drainage legally. In Western Australia you must keep your stormwater on your own lot — the City of Wanneroo states it as a statutory obligation under common law precedents and the Local Government Act 1995 to confine water runoff within your property boundaries. You cannot demonstrate that without levels.

Councils ask for levels in AHD. The Town of East Fremantle’s onsite drainage specification requires the site plan to show existing ground levels or contours in AHD and proposed levels of paved or concrete areas in AHD. If your project needs a development application, someone has to produce those numbers.

It tells you whether you can build what you want. A sunken firepit, a level lawn, a deck flush with the back door — each is a levels problem before it is a design problem. Fall you cannot lose is the most common reason a concept gets redrawn.

It finds the retaining early. The moment the survey shows you need to cut or fill more than about half a metre you are into approvals territory. Get that news in week one, not after the concept is signed off — see retaining walls approval.

The professional habit here is not the instrument. It is setting a datum first and closing every level run back to it.

The four ways to shoot levels

Method Cost Practical accuracy over 20 m People Best for Where it fails
Water level (garden hose plus two clear tube offcuts) Materials only Very good — the physics is exact, your reading of the meniscus is not; expect a few millimetres Two, or one with clamps Transferring a level around corners and through existing planting you cannot see across Air bubbles, and one leg sitting in full Perth sun while the other is in shade
String line and line level Cheapest option here Poor — the line sags and the vial is tiny; 10 to 20 mm out is normal One A garden bed edge or a single step riser Anything you intend to put on a drainage plan
Rotary laser and detector Hire by the weekend, or buy Good — a few millimetres, repeatably One Large open areas, and any job you are doing alone Cheap cross-line lasers are useless outdoors here — you need a rotary laser with a staff-mounted detector to work in Perth daylight
Automatic level (“dumpy”) and staff Hire by the weekend, or buy Best of the four for a garden survey Two, or one with a staff bipod Everything, and it is the method that teaches you the arithmetic Needs a second person, and a bumped tripod invalidates every reading since the last backsight

For a single block, hire. Perth yards stock both instruments — buying only pays if you are staging the build over several years, which on a renovation you probably are.

Set the datum first

A datum is one point on your site that you assign an arbitrary height to and never move. Every other level is expressed relative to it.

Pick something permanent. The best datum on a renovation is a feature of the house itself: the finished floor level at a nominated doorway, or the top of a specific step. It will not shift and you can find it again in December. If you need a datum out in the garden, drive a 1.2 m star picket at least 600 mm in and concrete the collar — a peg in Perth sand moves.

Assign it RL 100.000. Not zero. Surveyors use 100 so every level on the site is a positive number and you never confuse “minus 0.24” with “0.24 below”. A point 240 mm lower is RL 99.760; 150 mm higher is RL 100.150.

Photograph it and describe it in words. “Datum = top of threshold, rear sliding door, assigned RL 100.000” is a sentence you can act on in two years. A paint mark alone is not.

When you need real AHD. An assumed datum is correct practice for a garden survey. You need actual Australian Height Datum values if a development application asks for them, if you are tying into a council drain, or if a shallow watertable makes soakwell invert levels critical. Landgate publishes the AHD heights of WA’s benchmark network free; if no mark is within reach, that is what surveyors are for.

Taking the readings

Work on a grid — 5 m spacing is enough for open lawn, tighter where the ground is obviously moving. Then take these regardless of where the grid falls:

  • Every external door threshold, and the finished floor level inside it
  • All four boundary corners, plus any change of direction
  • Top and bottom of every existing retaining wall or step
  • The invert (the bottom of the hole) of every soakwell, grate, gully and downpipe outlet you can find
  • Top of kerb at the crossover, and the verge
  • The neighbours’ ground level on the far side of each fence
  • The base of every tree you intend to keep

The last two matter more than people expect. You cannot legally dump your water next door, and you cannot bury a mature tree’s root flare in fill and expect it to live.

Booking the readings

Use the height of collimation method. Set the instrument up, take a backsight onto the staff held on your datum, and add it:

Datum RL                   100.000
Backsight on datum           1.450
Height of collimation      101.450

Foresight at rear fence      1.780
RL at rear fence            99.670

Every subsequent reading is height of collimation minus staff reading.

Close the run. When you have finished, put the staff back on the datum and read it again without moving the instrument. If it does not match your opening backsight within about 5 mm, something moved and the whole run is suspect — redo it. Professionals close every level run, and it is the single habit that separates a survey you can build from a page of plausible numbers.

If you must move the instrument mid-survey, take a foresight on a solid temporary point before you move, a backsight on that same point after, and recalculate the height of collimation.

Converting to falls

Fall is the difference between two reduced levels. What varies is how you express it.

  • Fall in millimetres: 100.000 − 99.760 = 240 mm
  • Percentage: fall ÷ run × 100 → 240 ÷ 18 000 × 100 = 1.33%
  • Ratio: run ÷ fall → 18 000 ÷ 240 = 1:75

All three describe the same slope. Contractors quote ratios, engineers quote percentages, and the NCC quotes millimetres over the first metre. The RedBack slope and fall calculator converts instantly and flags your figure against the relevant Australian Standard.

Targets worth knowing before you design:

Situation Target Source
Finished ground away from the house — impermeable surface or access path Minimum 25 mm over the first 1 m NCC 2022 Housing Provisions 3.3.3(a)
Finished ground away from the house — all other cases Minimum 50 mm over the first 1 m NCC 2022 Housing Provisions 3.3.3(a)
Slab floor above finished ground, sandy well-drained site Minimum 100 mm (150 mm generally) NCC 2022 Housing Provisions 3.3.3(b)
Paved surface drainage 1:100 is the standard minimum; build to 1:80 AS/NZS 3500.3, as flagged by the RedBack calculator
Accessible ramp 1:14 maximum AS 1428.1, as flagged by the RedBack calculator

Build to better than the minimum. A 1:100 patio is 1:100 on the day it is laid; after settlement, construction tolerance and a decade of sand movement, it ponds.

The trap nobody warns you about

Falls compound over distance. A modest 1:80 across an 8 m patio is 100 mm of drop. Run that same gradient the full 18 m to the back fence and you have lost 225 mm — which becomes a step, buries the bottom of the fence, or dumps into the neighbour’s yard. Draw a section through the longest run on your block before you commit to a gradient. That is the calculation the levels and grading plan formalises.

The renovation-specific check

Measure the clearance from your finished floor level down to the existing ground and paving hard against the house. On older Perth homes gardened for decades, that gap has usually been eaten away by accumulated mulch, soil and re-paving. If you are down to 50 mm against a permeable garden bed, that is a problem to solve in the design, not something to build on top of.

Then go outside in the rain

Levels tell you where water should go. A June downpour tells you where it does, and Perth’s rain arrives in short heavy bursts, so the chance comes and goes in twenty minutes. Put it in the calendar. Photograph the ponding; note where water crosses the boundary in either direction, where it sheets off paving, where the downpipes actually discharge, and whether the soakwells are keeping up or surcharging. Mark all of it on the same base plan as the spot levels — water on site takes it further.

What this feeds

Your levels go onto the base plan from measuring up, and together they become the constraints and opportunities plan at the end of this phase. Later, this survey is what the drainage plan is calculated from and what you set out against in earthworks and setting out.

The next lesson identifies what the ground under those levels is made of — which decides how much of it you can move, and at what cost.

Do this on your own block

Field journal entry

Levels survey

Set a permanent datum point on your block and record spot levels on a grid across the site, plus at every door threshold, boundary corner, and existing drain. Mark them on your base plan.

Sign in to write this straight into your project workbook — or just keep it in a notebook.

Checklist

  • Establish and physically mark a permanent datum
  • Record spot levels on a grid and at all critical points
  • Calculate the fall across the site in mm and as a percentage
  • Identify where water currently goes in heavy rain

Watch

How to setup, use and take readings on a Surveying or Dumpy Level

Applied Maths🌏 Works anywhere

Covers the whole instrument in one video — setting up the tripod, levelling the instrument, adjusting the eyepiece and reading the staff. If you have hired a dumpy level and have never touched one, this is the fifteen minutes that stops you wasting the hire day.

Change this for Perth

Instrument technique, not landscaping — no site context, no Perth content. Pair it with the datum and booking method in the lesson body.

How to transfer heights from a control point using a dumpy level - SURVEYING TRAINING

SOM Survey Instruments🌏 Works anywhere

This is the part that matters for your survey: starting at a known control point, taking a backsight, and carrying that height to every other point you care about. Watch this one second, after you can set the instrument up, because it teaches the logic of a datum rather than the mechanics of a tripod.

Change this for Perth

Produced by a survey instrument supplier, so expect their equipment on screen. The method is standard levelling practice and independent of brand.

How to Use a Water Level for Deck Construction

Bailey Line Road🌏 Works anywhere

The cheapest levelling method demonstrated on a real job rather than as a physics curiosity — filling the hose, purging the bubbles, and using it to bring posts to a common height. Useful for exactly the case where a laser struggles: transferring a level around a corner, behind a shed, or through a gap in an existing garden you cannot see across.

Change this for Perth

North American channel, so expect imperial measurements and North American timber sizes. Water finds its own level regardless of units — read the technique, ignore the numbers.

Where to go deeper

Every link below was fetched and checked before it shipped. Nothing here is a guess.

  • Freearticle15 minWorks anywhere

    Gradients and Falls

    Pavingexpert

    Why this oneThe clearest free explanation of the three ways a fall gets written down — decimal, percentage and 1-in-x ratio — and how to convert between them without getting it backwards. It states the arithmetic plainly (gradient = rise ÷ run, and ratio to percentage is 100 ÷ the denominator) and works a real example: a 4.2 m patio at 1:60 needs 4200 ÷ 60 = 70 mm of fall. It also separates endfall from crossfall, which is the distinction most people miss when they set out a patio.

    Change this for Perth

    British site, so the recommended fall ranges are quoted against British Standards rather than AS/NZS. The maths is identical everywhere; check your target falls against the NCC and AS/NZS 3500.3 figures rather than the BS numbers on this page.

    Link checked 2 Aug 2026

  • Freearticle20 minWorks anywhere

    Setting Out — Automatic and Laser Levels

    Pavingexpert

    Why this oneExplains how automatic (dumpy) and rotating laser levels actually work, and then does the thing almost no other free page does — walks a full worked example of a driveway using a relative datum set at 100.000 m, calculating the required staff reading at each point. That worked example is the whole skill. The site sells nothing and says so, which is why the advice reads like a tradesman talking rather than a catalogue.

    Change this for Perth

    British, so it assumes British kit and drainage conventions. The instrument technique, the datum convention and the arithmetic transfer to Perth unchanged.

    Link checked 2 Aug 2026

  • Freegovernment10 minAustralian

    NCC 2022 Housing Provisions — Part 3.3 Drainage

    Australian Building Codes Board

    Why this oneThe primary source for the two numbers that constrain every level on your block: how steeply finished ground must fall away from the house, and how far the floor level must sit above finished ground. Clause 3.3.3 gives 25 mm over the first metre for reasonably impermeable surfaces and access paths, 50 mm over the first metre otherwise, and floor heights of 150 mm above finished ground generally, 100 mm on sandy well-drained sites, 50 mm above impermeable paving that drains away. Free to read online, no login.

    Change this for Perth

    The 100 mm concession for sandy, well-drained areas covers most of the Swan Coastal Plain, but it is the building surveyor's call, not yours. On a renovation the relevant question is what clearance you actually have left today, not what the code required when the house was built.

    Link checked 2 Aug 2026

  • Freecalculator2 minAustralian

    Slope and Fall Calculator

    RedBack Lasers

    Why this oneType a fall in any one of four formats — ratio, percentage, millimetres per metre or degrees — and it converts to the other three live, with a diagram. The second calculator takes a run or a fall and a target gradient and gives you the missing figure. Most usefully it flags your value against AS/NZS 3500.3 (1:100 minimum for surface drainage), AS 1428.1 (1:14 maximum for accessible ramps) and AS 3740, so you find out immediately whether the number you just invented is legal.

    Change this for Perth

    This is a laser level retailer's tool, so the surrounding site is selling instruments. The calculator itself is free, needs no sign-up and cites the standards it checks against — treat the product pages as advertising and the calculator as a utility.

    Link checked 2 Aug 2026

  • Freegovernment15 minPerth / WA

    Survey mark information

    Landgate

    Why this oneIf you ever need real Australian Height Datum values rather than an assumed datum, this is where WA's benchmark network lives. Landgate publishes the geodetic mark reports — AHD71 heights, positional uncertainty, physical condition and access diagrams — free, through Map Viewer Plus, the Locate open data platform, data.wa.gov.au and a downloadable Google Earth KML of every mark in the state. Worth knowing exists before you pay a surveyor to tell you a number that is already published.

    Change this for Perth

    Finding a benchmark near your block is one thing; levelling accurately from it across several hundred metres of suburb is another, and beyond what a garden survey needs. Use this when a council wants AHD levels and there is a mark in your street. Otherwise an assumed datum on your own block is correct practice.

    Link checked 2 Aug 2026

  • Freegovernment10 minPerth / WA

    Requirements for Onsite Stormwater Drainage (v3.0, October 2021)

    Town of East Fremantle

    Why this oneA short, specific Perth council specification that shows exactly what a levels survey is eventually for. It requires the submitted site plan to show existing ground levels or contours in AHD and proposed levels of paved or concrete areas in AHD; sets the minimum soakwell at 900 mm diameter by 600 mm deep; requires soakwells no closer than 1 m or their own depth, whichever is greater, from a footing or boundary; and gives the sizing formula as impervious area in square metres multiplied by 0.0125 m to get the required capacity in cubic metres.

    Change this for Perth

    This is one council's specification, not a statewide rule, and East Fremantle is a small older municipality with tight lots. The AHD requirement and the 0.0125 m depth factor are common across Perth councils but the numbers vary — read your own council's version before you design anything.

    Link checked 2 Aug 2026

  • Freegovernment5 minPerth / WA

    Stormwater Runoff — Information Sheet

    City of Wanneroo

    Why this oneTwo pages, written for homeowners, and it states the obligation that drives this whole lesson in one sentence: property owners have a statutory obligation under common law precedents and the Local Government Act 1995 to confine water runoff within their property boundaries. It also gives the practical numbers — roughly one downpipe per 15 square metres of roof, downpipes no more than 12 m apart, soakwell setback at least the width of the soakwell from buildings and boundaries — and a table converting soakwell size to the roof or paved area it can serve.

    Change this for Perth

    Dated July 2019 and written for Wanneroo's sandy northern suburbs. Confirm the current sheet and the sizing table with your own council, particularly if you are on shallow limestone or over a high watertable where infiltration rates are worse than the table assumes.

    Link checked 2 Aug 2026

  • Freegovernment10 minPerth / WA

    Retaining walls and site works

    City of Joondalup

    Why this oneThe page to read the moment your levels survey suggests you want to cut or fill. It sets out when retaining and site works need a development application rather than complying with the R-Codes, gives the height-to-setback relationship behind the street setback (a 0.5 m wall can sit on the boundary, a 1 m wall needs 1 m of setback, a 3 m wall needs 3 m), states that a building permit is not required for a wall not exceeding 0.5 m that requires no additional building work and does not adversely affect neighbours, and makes clear that whoever alters the natural slope owns the retaining.

    Change this for Perth

    Joondalup's policy, not a statewide one. The 0.5 m building permit threshold and the general principle are consistent across Perth, but setback tables and local planning policies differ by council — confirm the current thresholds with yours before you design a level change.

    Link checked 2 Aug 2026

  • Paid · Not published online — ring the branch for a day ratesupplierPerth / WA

    Laser level and dumpy level hire, Perth

    Classic Hire

    Why this oneA Perth hire company that actually stocks the three instruments this lesson discusses — dumpy level, laser level and pipe laser — across eight metro branches (Hamilton Hill, Morley, Maddington, Cockburn Central, Kelmscott, Wangara, Kewdale and Mandurah). Included because hiring for a weekend is almost always the right answer for a one-off garden survey, and because it saves you searching for who has one.

    Change this for Perth

    Rates are not published on the site — the page says prices are subject to change and include 10% GST, and directs you to call. Price-check against Kennards and Coates, both of which also hire levels in Perth, before you book.

    Link checked 2 Aug 2026

The rules, with sources

WA regulation changes. Each point below names its primary source and the date it was last checked — always confirm with your own council before you build.