DeChant Curve (A7) or the Double Arc
What?
Generate a smooth approximation of a street corner, sidewalk, island, or intersection where tracing with
a curve or single arc isn't enough. Snap to measured points using the DeChant Curve, press Enter and
watch your curve regenerate into two arcs that will create a near perfect representation of the corner.
Why?
The DeChant Arc was developed in part to assist investigators in creating best-fit arcs; defining road
geometry such as, but not limited to curbing, painted fog lines and median radii. The DeChant Arc works
excellent as it allows the Arc to be created from minimal points, saving time not only during the field
work, but also in the diagramming phase within The Crash and Crime Zone. Originally developed to
assist in image-based close-range photogrammetric surveys, it also works great with total stations, and
encoder-based lasers and point cloud data.
TIP – Fewer Selected Points are Required to Create a Near Perfect Curve
How?
1) Click on the DeChant Arc tool.
2) Pick points along the curve you’re tracing. This process is the same as drawing a standard curve.
3) When you’ve selected the last point, press Enter. The curve outline will convert to the DeChant Curve
or two arcs.
TIP – See the Geometry Used to Construct the DeChant Curve
If you want to see the geometry used to generate the DeChant Curve (two arcs) hold down the Shift key
and then press Enter when you’re finishing the curve. Refresh your screen.
By looking at the construction of the DeChant curve you can see how the two arcs are formed.
A) A chord is drawn between the start and end points of the curve.
B) The first chord is bisected with the bisected line terminating at the curve generated when first
construction the DeChant Curve.
C&D) Next, two chords are created, one from the start point to the bisected point and the other from
the end point to the bisected point.
E&F) Each of the new chords is bisected, terminating at the initial curve.
G) Finally, two arcs are drawn between the five key points. (Start, E, B, F, End)
NOTE – DeChant Arcs are 3D Entities
If you snap to points with 3D data the two arcs forming the DeChant Curve will be drawn on the 3D
plane defined by the selected points.
Fix it!
If part of the DeChant Curve isn’t as good as you want it you can clean it up. Because the DeChant Curve
is made up of two arcs you can delete either of the arcs you don’t want and create another DeChant
Curve in place of the deleted arc. The new section will then consist of two more arcs in place of the one
you deleted.
You Can’t Find it Anywhere Else
The DeChant Curve is trademarked by The CAD Zone, Inc. and can only be found in CAD Zone drawing
programs. This technique was developed by The CAD Zone in conjunction with Lee DeChant of DeChant
Consulting Services, co-creator of the iWitness photogrammetric software system.