Understanding Lines, Linestrings, and Alignments

Linear data in your project can be a CAD line, linestring, or alignment, depending on whether you import, create, or edit it. Use these definitions to understand the basic differences between the types of lines. For additional information on each, continue reading.

 
Line

In general, a line is an object consisting of two or more points and the connections (straight or curved) drawn between each consecutive point.

Note: The term "line" (as in the Line menu) is used generically throughout the software and documentation to denote any linear data, including CAD lines, linestrings, alignments, breaklines, and boundaries.
CAD line

CAD lines encompass most linear data that is imported from CAD files (.dxf, .dwg, .dgn, or .svl), including polylines, multi-lines, circles, arcs, ellipses, and splines.

When you edit any of these CAD line types, they are converted into linestrings.

Note: Only CAD lines that have been fixed (cleaned up) by the command are converted into linestrings. To convert multiple lines into linestrings at once, use the Elevate Lines command as a work around. See Elevate Lines by Text, Points, and Lines.
Linestring

A linestring is a highly-versatile, often multi-segmented line defined by points and the connections between the points. Linestrings are used to represent linear/curvilinear objects, can be named, and can be either 2D or 3D.

When you edit a CAD line, it is converted into a linestring.

In addition, when you process a line feature, it is converted to a linestring. For more information, see View and Edit a Linestring's Feature Properties.
Alignment

An alignment is a line used to denote the path of a roadway, railway, pipe, or other utility or right-of way. Alignments can support spiral and arc segments and parabolic or circular vertical curves. Alignment properties include stationing and optionally, station equations.

Alignments include a horizontal component and may include one or more vertical components:

Horizontal Alignment (HAL) - A line used to denote the path of a roadway or similar alignment on a horizontal plane (as seen in Plan View / from above).

Vertical Alignment (VAL) - A line used to denote the path of a roadway or similar alignment (as seen in Profile View / from the side).
Breakline

A breakline is a linestring used in a surface to indicate where a change in topography occurs. Breaklines are used to create or edit surfaces so that the surfaces accurately reflect existing or design topography.
Boundary

A boundary is a closed polyline. Boundaries are sometimes used to 'clip' a surface, forming subset of the original surface.

Lines

When you import lines in CAD files, the software creates numerous types of line objects, which are each designed to closely match the identity, character, and properties of the source data.

A few common examples include objects identified in the software as:

  • CAD Line
  • CAD Arc
  • CAD Polyline
  • CAD 3D Polyline

The goal is to reflect the source line’s character as closely as possible, as opposed to “translating” that data into a dissimilar data structure. As a result, these imported lines also retain the various limitations that tend to be inherent in the source CAD data. This reduces the value of the imported data in many cases and for many uses. The software currently supports many different types of lines derived from imported data.

Although you can import CAD lines, when you create similar lines within the software they are linestrings, which are much more versatile and editable. When you edit any line imported from CAD data, this software will automatically convert it into a linestring.

Linestrings

Linestrings are generic line objects that are composed of:

  • Consecutive 3D segments (see notes below)
  • Vertical control points (see notes below)

These entities (segments and points) are defined by:

  • Location - Where this entity begins or its position in 3D space, defined by parameters and possibly other entities
  • Type - How this entity connects to next entity in series both horizontally and vertically

The advantages of linestrings:

  • They provide a single way to create and edit a simple, yet versatile, line object capable of representing numerous common and generic site features.
  • They minimize the number of line objects with which you must be familiar.
  • They overcome the most common faults inherent in imported CAD line data.
  • All lines, other than alignments, can be edited as linestrings.
    • Imported CAD lines are converted into linestrings when they are edited.
  • Linestrings support important additional object properties than can be assigned to imported lines.
  • Linestrings support much more sophisticated geometric construction features than many imported lines can represent.
  • Linestrings enable key field data preparation operations.
  • Linestrings use the concept of 'distance along', which is analogous to stationing for alignments.

Notes on a linestring's 3D segments:

  • Segments can be either 2D or 3D; they are not constrained to a horizontal plane.
  • Each segment is defined by:
    • A connection type (straight line segment, arc, 3-point arc, or deflection from last segment)
    • An end point (the first segment in the linestring is defined by a start point as well)
  • Segment end points can be defined by:
    • A fixed 2D or 3D coordinate (the Coordinate option in the Edit Linestring dialog's Type list)
    • A dependent relationship with a named 2D or 3D point object (the Point ID option in the Edit Linestring dialog's Type list)
  • For fixed coordinate end points of segments:
    • You can establish a VPI on the line by specifying the elevation of that end point. That VPI will remain coincidental with that end point as the end point's location is subsequently edited. However, in that case as well, a VPI otherwise (even coincidentally) established at the same distance along value will override that elevation assignment.
  • For segment end points dependent on named point objects:
    • If the point is 2D, you can establish a VPI on the line at that location by setting the elevation of that point (if otherwise practical), in which case the VPI is established at the related segment end point as opposed to being established at a fixed distance along the line. You cannot otherwise associate an elevation with that segment end point (as you can when defining the end point's coordinate as an integral part of the linestring's definition). However, you can independently establish a VPI at that distance along the line at which that end point currently occurs, which will remain at that distance along measurement as the line's geometry is edited.
    • If a 3D point, then the elevation of that point establishes a vertical point of intersection (VPI) on the line at the location of that end point (unless overridden by a VPI defined in the Vertical tab

Notes on Vertical Points of Intersection (VPIs)

  • The vertical component (elevation) of a linestring can be established in one of three ways:
    • By assigning elevations to the fixed start and end points of a linestring's segments (discussed above). A VPI established in this manner cannot have a vertical curve. A VPI of this nature remains located at that particular horizontal segment end point, even if the horizontal alignment geometry is edited, thereby potentially placing that end point at a different distance along the linestring's length. The resulting VPI can be overridden by a VPI that might be assigned, as a part of the linestring definition, at the same distance along measurement at which the internally defined end point currently lies.
    • By assigning elevations to dependent points that a linestring's segments use as start and end points. The elevation of the line at each point is defined by the point object's elevations. A VPI established in this manner cannot have a vertical curve. Vertical control points that you add at the same distance along the linestring override VPIs established using either of the two ways described above.
    • By adding vertical control points (discussed below) along the linestring.

Vertical control points:

  • Each vertical control point is defined by:
    • A curve type (parabolic or no curve)
    • A distance along the linestring
    • An elevation
  • A VPI established in this manner has no influence on the line's horizontal alignment.

Vertical control points that you add at the same distance along the linestring override VPIs established by fixed coordinates or named points. However, such a VPI will remain at that distance along the entire line as its horizontal geometry is edited, rather than remaining at that particular segment end point.

Alignments

An alignment defines a linear feature, such as a road centerline. Alignments consist of horizontal geometry (as seen in the plan view) and optional vertical geometry (as seen in the profile view). Alignments have associated stationing, and can also include station equations, if appropriate.

You can import alignments from LandXML or GENIO string files, or create them by specifying their horizontal and vertical components, or by converting them from vertices, lines, and polylines in your project.

You can create horizontal alignments using lines/tangents, arcs, and clothoid spirals. You can create vertical alignments with lines (constant slope), arcs, and parabolas. Each vertical alignment is associated with a horizontal alignment, and there can be multiple vertical alignments for each horizontal alignment. The values you can enter depend on the alignment settings for the project.

Usually, the goal in creating an alignment is to create a digital file of the design that you can upload to a field device, e.g. for staking.

Alignments and linestrings compliment each other to provide two unique ways to represent linear forms in your project. Each has properties appropriate to its intended use.

Alignment stationing is typically specified and established by the design engineer; within this software, stationing values should only be imported or transcribed from plans.

Alignment to Linestring Comparison

Alignments

Linestrings

Alignments support stationing and station equations that are established in the original design.

Linestrings address the accumulative distance along the line.

Alignments support spiral horizontal curve segments.

Linestrings are limited to circular arcs.

Alignments support symmetrical and asymmetrical parabolic vertical curves.

A linestring’s parabolic vertical curve must be symmetrical.

Alignments can serve as the basis for profile and cross-section views.

Linestrings cannot currently serve in that manner.

The effect of an alignment is always rendered sharply.

Linestrings have a surface sharpness property (soft or sharp).

Related topics

Understanding Alignments

Understanding Breaklines