Blog · September 30, 2026
What Is a Delta Angle on a Curve?
Delta is the central angle of a circular curve, the angle at the radius point between the start of the curve and the end. It is not the bearing of the road and not the deflection you see standing on the curb. With the radius, delta fixes the curve.
Platbookmapper is an Amazon Associate. If you buy through an Amazon link on this page, we earn a commission. The link is a product, not the county map.
The angle at the center
Picture a circle and a pie slice. The two straight edges of the slice are radii. The angle between them is delta. The crust is the arc, which is the curved property line or the curved centerline. A large delta is a sharp turn through many degrees. A small delta is a gentle kink. Delta is usually given in degrees, minutes, and seconds. It is not a distance. People add it to a foot dimension and produce nonsense. On a plat it may be labeled with a triangle, the word delta, or the letter D, and the legend must confirm D, because D is also used for other things in some offices. Chord length, radius, and arc length are the companions. Delta and radius together determine the arc length and the chord. If the table’s arc length does not match that relationship, you have a copying problem to note, not a new geometry to invent.
Direction of the curve
Delta has no left or right of its own. The plat also has to say whether the curve bends left or right as you travel, or it gives a chord bearing that implies the bend, or it shows a radius bearing to the center. Without a direction, you can draw the curve two ways and both will have the same delta. Lot lines that follow a curving street inherit this. A lot on the outside of a curve and a lot on the inside can share a delta in a table and still have different radii, because they sit at different distances from the center. Do not reuse the street centerline’s radius as the lot front’s radius. The front is offset by half the right of way, or by whatever offset the plat states.
Desk check, not a field stake
You can test whether delta, radius, and arc length are consistent on the page. Arc length equals radius times delta in radians, which is a calculator check, not a survey. If they disagree by a rounding tenth, the plat may have rounded. If they disagree by a lot, look for a transposed digit before you build. A laser measure along the curb measures a polyline you chose, not delta. A phone GPS track of a curve is not a survey. County GIS often replaces the curve with a few straight segments. Measuring those segments will not recover delta. Platbookmapper does not compute curves. It links to the county viewer so you can find the parcel, then you return to the recorded plat for the curve data.
What to do next
- Find delta in the curve table or on the arc.
- Keep it as an angle, and pair it with the radius of that same curve.
- Read the direction the curve turns.
- Check arc length against radius and delta only as a consistency check on the paper.
The mix-up to avoid
Using the street centerline delta and radius to lay out the lot front. The front is a different curve, concentric if the plat designed it that way, with its own radius.
Questions people ask
Is delta the same as the deflection angle?
Not quite. In route work, deflection is often the angle between a back tangent and a forward tangent, and for a simple curve it equals delta. The words get used loosely. Read the label on the sheet rather than importing a highway definition the plat did not use.
What if delta is missing but radius and arc length are present?
You can compute delta for your sketch from those two, and you should label it as computed, not as printed. The plat’s silence may be an omission. A surveyor confirms it if the curve has to be staked.
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.