How Accurate Are Online Property Lines?

There is no single number. In some places an online parcel line sits within a few feet of the surveyed boundary. In others it is off by much more. What you see depends on where the data came from, how it was made and how it is displayed.

"How accurate is this?" is really several questions. Each has its own source of error, and the errors stack.

What Counties Say About Their Own Maps

The people who publish parcel data are the best source on its limits, and they tend to be blunt.

• Pitkin County, Colorado builds its parcel lines from deeds, plat descriptions, CAD files and georeferenced plats, then fits them together using aerial photos to close gaps and prevent overlaps. Its disclaimer says positional errors may range from several feet to several hundred feet.
• Sacramento County, California says its assessor maps are made for assessment, show information as recorded, may not match what is measured on the ground and are not surveys. Its FAQ also warns that an aerial photo laid under the lines may not reflect the actual boundaries.
• Snohomish County, Washington says its parcel lines are compiled from records and not adjusted to match fences, and that they cannot be used to prove one owner is encroaching on another.

Different counties, same message. The line is an index of the records, and it is not a measurement of your yard. One more detail from Pitkin's description is worth a second look: when a county fits records together to avoid gaps and overlaps, some lines get nudged from where any single document puts them.

Two Different Errors on One Map

When you open a map on your phone and look at the property line, you are looking at two separate pieces of information:

1. The parcel boundary. A line drawn from public records.
2. Your phone's position. The dot that shows where you are.

Each can be wrong independently of the other. A very precise GPS dot tells you nothing about whether the boundary line is in the right place. A perfect boundary line does not make your phone's dot accurate. GPS.gov makes a similar point from the other side: users are sometimes misled by incorrectly drawn maps, which the government cannot correct.

Where Parcel Boundary Errors Come From

The original records

Many boundaries start as written descriptions in deeds. Some are very old and refer to features like a creek, a fence or "the oak tree". Others use measurements that were rounded or never closed properly. A map drawn from imprecise records will be imprecise too, however carefully it is drawn.

How the parcel geometry was created

Counties build parcel maps in different ways. Some draw parcels from surveys and recorded plats. Others trace them over aerial photographs. Some convert older paper tax maps. The method behind the data makes a big difference in how well the lines match the ground.

Digitization

Turning a paper map into a digital one adds small errors. A pencil line that was a few feet wide on a large-scale map can represent a considerable distance on the ground. Stretching, folding and scanning can add more.

Map alignment

Parcel data is placed on the earth using coordinates. If the original map was not tied to those coordinates carefully, the whole layer can be shifted. A shift often looks like every parcel in the area is a few feet off in the same direction.

Satellite imagery alignment

Most apps show parcel lines over satellite images. The imagery has its own alignment error, especially on hills and next to tall buildings, which can appear to lean. If a parcel line looks like it cuts through a house, the problem may be the image rather than the line.

Neighboring data sets

Where two counties meet, their parcel data can come from different sources and be shifted slightly relative to one another. Lines near a county border can look odd for this reason.

Delays

A lot split or boundary change takes time to reach the map. Pitkin says its data can lag recording by up to sixty days, and Sacramento says its GIS layer can trail assessor changes by up to four weeks.

Where Phone GPS Errors Come From

GPS.gov says smartphones are typically accurate to within a 4.9 m (16 ft) radius under open sky, and that accuracy worsens near buildings, bridges and trees. Signals bounce off surfaces, fewer satellites are visible and the phone's estimate becomes less certain. The accuracy circle around the dot is the phone's own estimate of how far off it might be. How to Find Property Lines With GPS goes into this in more detail.

How Far Off Can a Parcel Map Be?

There is no honest universal answer. In well-mapped urban areas, the offset is often small, a few feet. In rural areas, where parcels are described by older records and drawn at a lower level of detail, it can be larger, and in some places much larger. Even within one county the quality varies from one neighborhood to the next.

The safest assumption is that the line is approximate, and that the closer something is to it, the less you should rely on the map to tell you which side it is on.

A Quick Local Check

You can get a feel for local quality without any equipment. If your lot has a recorded width, say 75 feet on the plat, measure the same edge on the map and compare. A close match is a good sign for the shape of the parcel.

It does not prove the position, though. A layer shifted a few feet in one direction keeps every distance the same, so the width check passes while every line sits in the wrong place. Pair it with a look at where the lines fall against buildings you know are inside the lot.

Are Property Line Apps Accurate?

Apps display parcel data that comes from counties or data providers. They are as good as that data. A good app can make the data easy to use, but it cannot make the underlying records more precise. The right expectation is "useful for research and orientation, approximate for boundaries".

Why Don't Property Lines Match Satellite Images?

Because the images and the lines are two separate data sets with their own errors. A line in a record also does not have to match anything visible. A boundary may run through the middle of a lawn where there is no fence, or fall a few feet away from a fence that was built in the wrong place decades ago. Do not expect the line to follow what you can see.

What If the Map and the Ground Disagree?

Start by checking a second source: a plat, a deed or another map. Look for markers on the ground. If the difference is small and nothing depends on it, it may not matter.

An approximate line is fine for research, comparing properties, getting oriented and walking the land. It stops being fine when a few feet change the outcome, such as a fence, a new building or a dispute. For a closer look at what each tool can and cannot do, see Property Line Map vs Land Survey.

Common Mistakes

• Assuming that a thin line on a map has a precise position.
• Blaming GPS for an error that is actually in the parcel line, or the reverse.
• Comparing the line to a fence and assuming the fence is correct.
• Using a map for a decision that needs a survey.

Frequently Asked Questions

How many feet off can a property line map be?

It varies. One Colorado county says errors can run from several feet to several hundred feet, and other counties give no figure. Assume the line is approximate and give it more room the closer you are to it.

Are county GIS parcel lines legal boundaries?

No. Counties that publish them typically say so in their disclaimers. The legal boundary comes from the recorded documents and is located on the ground by a survey.

Why does my fence not match the parcel line?

Fences are built by owners, often without a survey, and parcel lines are drawn from records. Neither is guaranteed to match the true line, and they often do not match each other.

If you are not sure where to begin, How to Find Your Property Lines walks through all the methods.

Sources

• Parcel Data Disclaimer (Pitkin County, Colorado) How the parcel layer is built, error range and update lag.
• About Assessor Parcel Maps (Sacramento County Assessor) Assessor maps are for assessment, not surveys; aerial overlays and GIS lag.
• Snohomish County Assessor FAQ (Washington) Parcel lines are compiled from records and are not adjusted to fences.
• GPS Accuracy (GPS.gov) Smartphone accuracy, causes of error and incorrectly drawn maps.