Coordinate and Measurement Conventions¶

This page documents the sign conventions, coordinate systems, and units used throughout TunnelCAD.

Coordinate Systems¶

World Coordinates¶

TunnelCAD uses a standard surveying coordinate system:

Northing = Y axis
Easting = X axis
Elevation = Z axis

The Alignment canvas tab displays data in world coordinates.

Section Local Coordinates¶

Cross-sections are displayed in a local coordinate system relative to the alignment axis at that station:

X = horizontal offset from the alignment axis
Y = elevation

Offset Convention¶

Offsets are measured perpendicular to the alignment, looking in the direction of increasing chainage:

Positive (+) = right of the alignment axis
Negative (-) = left of the alignment axis

This convention applies to all offset values in section data, template definitions, and setout calculations.

Angular Conventions¶

ALM arc/spiral sign: Positive (+) = right turn (clockwise). Negative (-) = left turn (counter-clockwise).
Bearing display: Bearings shown in the interface use the surveying convention, converted from the internal math angle via: surveying bearing = Pi/2 - math angle.

Gradient¶

Gradient values are stored and displayed in percent (%). The value read from the alignment engine is already in percent – it must not be multiplied by 100 again.

Deviation Radial (DR)¶

The deviation radial is the difference between the measured section and the design template at a given angular position:

Positive DR = overbreak (the measured point is outside the template envelope)
Negative DR = underbreak (the measured point is inside the template envelope)

DR text labels use a default height of 0.15 m in model units.

Section Point Ordering¶

Section points are ordered clockwise starting from the bottom (negative Y axis direction) after radial sorting around the section center.

Template Bulge¶

Template line segments may include an arc defined by a bulge value:

Positive bulge = arc bulging to the left
Negative bulge = arc bulging to the right

Note

When constructing templates programmatically, the bulge value must be set after adding the end point of the segment, because the bulge array requires at least two points to exist.

Elevation in Setout Calculations¶

Two related but distinct elevation values appear in setout offset data:

Elevation: The elevation of the alignment offset axis at the section station. This value is constant for a given station regardless of lateral position.
PointHeight: The actual elevation at a specific lateral offset, which varies according to superelevation (cross-fall) applied at that station.