Feature-Based Grading Tool: Grade to Distance, Elevation & Relative Height

The Linear Grading command in SPCAD allows you to grade daylight from a selected linear feature, such as a polyline, 3D polyline, line, or arc-tessellated polyline. The tool supports three main grading workflows:

1. Grade to Distance – Offset a fixed horizontal distance from the base feature.
2. Grade to Elevation – Grade until a fixed target elevation is reached.
3. Grade to Relative Elevation – Grade to a constant vertical offset from the feature (e.g., +2.5 m above or −1.2 m below).

Outputs include daylight points, optional daylight linework, and a triangle strip (TIN faces) connecting the base and the daylight.

Overview 

1- Feature-Based Grading Tool: Selecting Features & Range

1. Click Select… and pick a polyline/3D polyline (open or closed). 
   • LWPOLYLINE bulges (arcs) are respected via tessellation. 
   • 3D Polylines use their existing vertices. 
2. (Optional) Apply to the entire length 
   • Checked: grading is computed along the whole feature. 
   • Unchecked: a sub-length jig starts: 
     • Pick a start point on the feature. 
     • Move the cursor along the feature to preview the graded span (blue preview). 
     • Press Enter to accept, or type E to grade the entire length. 

Tip: The jig works on both Polyline and 3D Polyline. When a WPF window is open, SPCAD uses EditorUserInteraction so clicks go to CAD as expected. 

2- Common Grading Options 

• Grading side: Left or Right relative to the feature direction (vertex order). 
  If the daylight looks “inside-out”, reverse the polyline or switch sides. 
• Grading Surface Name: Name used for the new graded TIN metadata and visualization. 
• Draw Day Light Line: Adds a 3D polyline through daylight points (good for QA or styling). 

3- Feature-Based Grading Tool Modes & Parameters

A) Grade to Distance 

Offset a constant horizontal distance and apply a single slope. 

• Distance (H): positive, non-zero. 
• Cut slope (H:V) fields are hidden in this mode; you enter one slope H:V: 
  • SPCAD uses V/H internally (rise over run). 
  • Direction of grade (up/down): Use the sign on V to control fall when needed. 
    • Example: H=1, V=2 → grades up 2:1. 
    • H=1, V=-2 → grades down 2:1 (daylight Z lower than base). 

Behavior: 

• The software computes the daylight by offsetting the feature by Distance on the chosen side.
• The program automatically fillets concave corners on the chosen side (arc-approx) using a radius equal to the offset distance. This step prevents self-intersections.
• The system calculates Daylight Z from the base Z ± (ΔH × slope).

Good to know: 

• Use positive H. Use negative V only when you want explicit “downward” grading. 

• If the offset distance is very small at a very sharp corner, SPCAD will clamp the fillet to fit the segments, keeping the daylight valid. 

B- Grade to Elevation 

Grade from the base to a fixed target elevation using separate cut & fill slopes. 

• Target elevation: absolute elevation to hit. 
• Cut slope (H:V) and Fill slope (H:V): positive values (H>0, V>0). 
  • SPCAD chooses which slope to use at each vertex/segment based on whether the target lies below (cut) or above (fill) the base. 

Behavior: 

• The system logically splits the base where it crosses the target plane to prevent slopes from “flipping” mid-segment.
• The software computes the horizontal distance to the daylight for each vertex: |ΔZ| / (V/H).
• It applies variable-radius fillets to concave corners on the chosen side, using the per-vertex distance. This prevents the “pinch” that appears at bends.
• The program lifts daylight points to Z = Target elevation.
• It builds triangles for each base segment (without using CDT), avoiding crossing or missing faces.

Tips: 

• If the daylight touches the base at a vertex (because base Z = target Z), you’ll still get valid triangulation; the fan method handles degenerate spans. 
• If you expected a fillet but see a sharp corner, check that the radius implied by |ΔZ|/(V/H) is not effectively zero at that vertex. 

C- Grade to Relative Elevation 

Grade to a constant vertical offset from the base (e.g., berm or swale). 

• Relative elevation: positive means raise (fill), negative means lower (cut). 
• Cut slope (H:V) and Fill slope (H:V): positive values (H>0, V>0). 
• SPCAD chooses fill slope when Relative ≥ 0, cut slope when Relative < 0. 
• The horizontal distance is constant for the whole run: |Relative| / (V/H) for the chosen slope. 

Behavior: 

• A constant-offset daylight is built in 2D, then filleted at concave corners (radius = that distance). 
• Daylight Z is baseZ + Relative everywhere. 
• Per-segment triangulation ensures no crossing faces around bends. 

4- Output 

• Daylight (optional): a 3D polyline on the current layer (or the feature’s layer). 
• Triangles: transient-drawn (and/or persisted by your workflow) with metadata: 
  • Unique TIN Guid, Name, Min/Max extents, counts, etc. 
  • Color scheme per your Elevation ramp/metadata configuration. 

5- Practical Tips 

• Choosing Slope Signs 
  • For Grade to Distance only: use a negative V when you want the grade to go down from the base; H stays positive. 
  • For Grade to Elevation / Relative: enter positive H:V for both Cut and Fill; the tool picks the correct one automatically based on whether the target/relative is above or below the base. 
• Grading Side 
  • “Left/Right” is relative to the vertex order. If results appear on the wrong side, reverse the polyline or switch the side in the UI. 
• Closed vs Open 
  • Closed features are supported. Stitching and wrap-around are handled so there are no missing triangles at the seam. 
• Corners & Fillets 
  • Fillets apply only at concave corners on the grading side. 
  • If an offset is too big for a very short span, the fillet radius is reduced automatically to keep tangency and avoid geometry failure. 
• Performance 
  • Very dense polylines or many short bulged segments can be heavy. If preview feels slow, simplify the base or reduce bulge tessellation tolerance before grading. 
• QA Checks 
  • Turn on Draw Day Light Line and compare with the base; there should be no intersections. 
  • Inspect corners: you should see small arc-like runs at concave bends (not spikes). 

6- Troubleshooting 

• Daylight intersects the base 
  • Check grading side vs feature direction. 
  • For Grade to Distance, verify the V sign (use negative V for downward). 
  • Ensure Distance is not zero and slopes are reasonable. 
  • For Grade to Elevation/Relative, make sure H and V are positive, and the expected slope (cut vs fill) is logically selected by the target/relative. 
• Missing/odd triangles near corners 
  • This usually indicates the daylight mapping jumped across more than one base segment. SPCAD’s per-segment triangulation and index monotonicity fix this; if you imported legacy code, keep the “per-segment fan” triangulator enabled. 
• No fillet where expected 
  • The corner may be convex on the chosen side (fillet not needed). 
  • The effective radius could be effectively zero at that vertex (e.g., ΔZ≈0 or very small distance). 
• Jig doesn’t accept input while the dialog is open 
  • Ensure the command was started from the dialog’s Apply to entire length checkbox handler; SPCAD uses EditorUserInteraction so clicks go to AutoCAD. If you opened the dialog outside a document context, start a new command in an active drawing. 

7- Reference: Field Meanings 

• Distance – Horizontal offset (drawing units). 
• Target elevation – Absolute Z (same units as drawing). 
• Relative elevation – ΔZ added to the base (+ up, − down). 
• Cut slope (H:V) – Use positive H and positive V. Steeper slope ⇒ bigger V for the same H. 
• Fill slope (H:V) – Same entry rules; used automatically when grading “up”.