How accurate is the map?

Never dig from this map

Start here, because it is the only part of this page that can hurt somebody.

A line you or somebody else drew is a note about roughly where a pipe or a cable was believed to run. It is not a located service. It does not know what has been added, moved or abandoned since it was drawn, it carries every error described further down this page, and it was probably walked at the surface above something buried at an unknown depth.

And the reverse is worse: empty map is not empty ground. Nothing on the screen where you are about to dig means nobody has drawn it yet, not that nothing is there.

Before any excavation, get the services located properly. It is free to start at beforeudig.co.nz, and anything near live electricity or gas wants the asset owner or a qualified locator physically on site. People are killed doing this. Use Plotstead to brief them on where to look, and never as the thing you dug on.

The pin: your phone's GPS

When you stand at the bore and drop a pin, your phone is working out where it is from satellites, and it is doing a good job of a genuinely hard problem. In the open, with a clear view of the sky, a modern phone lands within about three to five metres. Newer handsets that pick up two frequencies at once can do better, sometimes a metre or two.

Two things make that worse, and they are the two situations you are most often in on a property.

Sky view. Under trees, beside a shed, in a gully, up against a bank or a tall hedge, the phone can see fewer satellites and also picks up signals that have bounced off things on the way in. Ten metres is easy in that situation and it can be considerably worse. A pin dropped inside a building is close to a guess.

The accuracy circle lies slightly, in a specific way. When a phone tells you it is accurate to five metres, that is a radius it is 68 per cent confident about. Which is another way of saying that about a third of the time, the truth is outside the circle it just drew you. It is one standard deviation, not a promise. Height is worse again, usually two or three times the horizontal error, which is why Plotstead does not ask you to trust an elevation for anything.

For context on the underlying system: the United States government commits to broadcasting the GPS signal with a range error of 7.8 metres or better, 95 per cent of the time. That is a figure about the signal in space, not about your position, and your actual accuracy is that plus satellite geometry, plus the atmosphere, plus whatever is between you and the sky.

Practical version: walk the line with the phone held up and away from your body, take your time at the corners, and treat what you get as a good description rather than a measurement.

The photo underneath: LINZ aerial imagery

The imagery comes from LINZ, and rural New Zealand only gets re-flown every few years. Your paddock can be showing photos from a while back while a town half an hour away is bang up to date. Tap the layers button on the map and it will name the survey and how old it is, so at least you know what you are looking at.

If the photo of your place is too old to work from, Plotstead Plus adds a recent satellite layer you can switch to. It is not as sharp as a low-flown aerial photo, and it is usually a great deal newer, which is the trade worth having when the aerial is from a decade ago.

Aerial photos also lean. Anything with height, a shed, a shelter belt, a power pole, leans away from the centre of the shot, so the roof you can see is not sitting directly over the footprint you are trying to mark. Photos taken near the edge of a flight path get stretched more than ones taken straight down the middle. This is normal and it is why a pin can look wrong against a building while being perfectly correct on the ground.

Your pins sit where you put them whatever the photo underneath is doing.

The boundary line: LINZ parcel data

Close, but not survey accurate. The boundary line and the aerial photo can both sit a bit off where things really are. Photos taken near the edge of a flight path get stretched more than ones taken straight down the middle, and a boundary can land a metre or two from where the pegs actually are.

It is worth understanding what that line actually is. It comes from the cadastre, which is the legal record of how land is divided rather than a survey of where the fences went. In a recently subdivided street it is very good. On an old rural title it can be based on measurements taken a long time ago, and it can be out by more than a metre or two.

For most of what you will do here that is neither here nor there. Dropping a pin on the bore, walking a line down a trench, working out roughly how big the back paddock is: all fine.

Where it matters is right on the line. Before you put up a fence, a shed or a right of way anywhere near a boundary, check the title and the survey plan, and get a surveyor to look at it if there is any doubt at all. The positional accuracy of what is shown, boundaries and aerial photos included, may be approximate, and source documents are what you use to determine positions accurately.

One more, for completeness

New Zealand is on a moving plate and drifts a few centimetres a year. The national coordinate system holds still while the ground does not, so there is a small, consistent offset built into the whole country between where satellites think you are and where the map says you are. It is well under the other errors on this page and you will never notice it. It is mentioned only because it is a real reason two systems can disagree by the same amount everywhere, which otherwise looks like a bug.

So what is it good for

Everything it was built for, which is remembering. A few metres does not matter at all when the job is to walk somebody to the right paddock, point at the right pump, or answer the question of where on earth the shutoff is.

• Finding the bore, the valve, the meter or the trap in a place you do not know well
• Briefing a contractor on roughly where to start looking
• Recording that a line runs from about here to about there, before the trench is filled in
• Rough areas and rough distances
• Remembering which of the four gates is the one that gets padlocked

And the jobs it is not for, in every case because the answer needs to be right to the centimetre and this is right to the metre:

• Deciding where to dig, ever. Get services located
• Placing a fence, a building or a right of way near a boundary
• Settling a disagreement with a neighbour about where the line runs
• Anything a council, a surveyor or an insurer needs a measured figure for
• Working out depth or height from anything the app shows you

Questions

There is more on the map, the layers and how capture works in the help, and the terms of use set out where responsibility sits. If something here is wrong or unclear, please tell us on the contact page, because this is the page we would most like to be right.