Blog · September 30, 2026
What Is a Basis of Bearing?
The basis of bearing is the note that says what north the plat’s bearings refer to. It may be a recorded deed line, geodetic north, grid north, or an assumed north. Without it, the numbers on the courses have no reference, and you cannot compare them to a compass.
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Why every bearing needs a reference
A bearing such as north 12 degrees east is an angle from a north. Which north has to be stated, or the angle floats. The basis-of-bearing note may say bearings are based on the north line of the parent deed recorded in a book and page, or on state plane grid north, or on GNSS observations tied to a named datum. Some older plats say assumed north, which means the surveyor picked a direction, often the direction of one line, and related every other line to it. Assumed bearings can still describe a perfect figure. They just will not match true north, a phone compass, or the next plat that used a different assumption. Comparing two plats without comparing their bases is how people invent a rotation that is only a change of reference.
Where the note hides
Look in the title block, beside the north arrow, or in the surveyor’s notes, not only in the middle of the drawing. A north arrow with no statement may mean the firm relied on a convention you do not have. Do not fill the silence with magnetic north. If the plat says the basis is the monumented line between two corners of a prior plat, those two corners are the reference. Recreating the basis means recovering that line, which is surveying, not rotating a PDF until it looks level on your screen. Scanned plats are often rotated for filing. The basis note, not the edge of the paper, tells you the reference.
What you can and cannot check at a desk
You can read the note and write it on your copy. You can refuse to mix bearings from a deed based on magnetic north in 1890 with bearings from a 2020 grid plat. You cannot, with a phone compass, test whether the plat was drawn correctly. A phone is not a survey, and magnetic north moves. County GIS has its own grid, which may be a state plane zone. A parcel line’s apparent direction on that screen is a GIS display, not the plat’s basis of bearing. Platbookmapper links to the display. It does not rotate plats or state a basis. When two surveys of the same line show bearings that differ by a small angle, check the basis notes before you conclude the line moved.
What to do next
- Find the basis-of-bearing sentence before you copy any course.
- Write down what north it uses, including a deed book and page if it cites one.
- Do not compare two plats’ bearings until their bases are reconciled.
- Do not use a phone compass as a check on the note.
The mix-up to avoid
Treating the top of the sheet as north and ignoring an assumed or rotated basis. The sheet is paper. The note is the reference.
Questions people ask
What is a bearing equation?
It is a statement that a line has one bearing in one system and a different bearing in another, with the angle between them. You will see it where a new survey meets an old deed basis. Apply the equation as written. Do not average the two bearings.
If the basis is assumed, are the lot dimensions wrong?
No. The angles between the lines can be exact while the whole figure is rotated relative to geodetic north. Buildings fit the figure. A compass walk will not.
Find the county map
Open Platbookmapper and pick the county that taxes the land. Use that viewer to get the parcel id, then match it to the recorded plat if you need the easements and lot numbers.
Not a survey. County GIS and tax parcel maps are assessment and planning layers. They are not a substitute for a recorded plat, deed, or licensed survey.