Alan Sharp

Alignment Spiral Types and Our Support At Trimble

Blog Post created by Alan Sharp on Nov 27, 2018

We are getting the occasional question about our support of Spiral Types other than the traditional Clothoid Spiral that is used in Road Construction. There are many types of Spiral out there that get used in different parts of the world and primarily for Railway work. Spiral Types include

  • Clothoid
  • BLOSS Curves
  • Cubic Parabolas
  • Korean Cubic Parabolas
  • Half Sine Spirals
  • NSW Cubic Parabola


Business Center - HCE and Trimble Business Center supports all of the above Spiral Types. There are Spiral Types that it doesn't support. This is however only half the challenge.


If you create a Corridor Model that uses these types of Spiral, can you use the data downstream in any of the Trimble Field Systems. Below I will try to address this issue.


Trimble GCS900 / Trimble Earthworks / Trimble PCS900 Machine Control Systems

These systems all employ either SVD / SVL or DSZ file formats for the design models. The SVL or DSZ file is what carries the alignment geometry. Those file types actually use a chorded approximation of the alignment, chorded to a density that is unnoticeable on the machines. In the file format the chords all have the true stationing at that location stored as a part of the file format, so while in each chord, as you move towards the end of the chord, the station will be off fractionally, it will be unnoticeable on the machine. 


As a result of this approach, the Machine Control Systems all work with all of the above Spiral Types because they are not actually dependent on the alignment geometry or a calculation engine for the computation of Station and Offset from these Spiral Type elements.


Trimble SCS900

Trimble SCS900 utilizes the Terramodel PRO file for its corridor models. The PRO File format only supports the Clothoid Spiral. There is no support in SCS900 or the Field Data Exporter for Alignments or Road Surface Models for any alignment other than one that utilizes a Clothoid Spiral.


Trimble Siteworks

Trimble Siteworks is officially the same as Trimble SCS900. However Siteworks also has initial support for TBC VCL files as a Design Model. Currently you would have to select an Alignment and a Normal Surface Model (not a corridor surface model) and any linework required, and then run the normal Exporter, Select VCL File and output the data to the correct Design Folder for the correct device(s). When you open this type of file in Siteworks, the alignment will support all of the above spirals with the exception of the Half Sine Spiral currently. This is a VCL File Version Issue - Siteworks has not yet been updated to the BC-HCE 4.12 or TBC v5 VCL Version, once that is done, it will support all of the spiral types from TBC using the VCL format.


Trimble SitePulse

This has the same data structure as Trimble SiteWorks and also supports VCL format in the same way. It also has the same VCL version issue for Half Sine Spirals.


Trimble Access

Trimble Access Portal states that it supports the following Spiral Types in its Road Model definition (RXL Format)


  • Clothoid Spiral
  • Egg-Shaped Clothoid Spiral
  • Cubic Spiral (Cubic Parabola)
  • Korean Cubic Parabola
  • NSW Cubic Parabola


Note that the Egg-Shaped Spiral is not currently supported in Trimble Business Center v5.0.


People often ask - Can I just change the spiral type, and if I do what difference will it make? The simple answer is No you cannot just change the spiral type and expect it to give you the same numbers exactly. If you change the spiral type several things can happen


  1. The spiral can change position along its length
  2. The spiral may start at the same location but it may end at a different location
  3. The Spiral elements may no longer be Tangential with the inbound or outbound alignment elements


The errors associated with a change of Spiral Type will of course depend on the following

  1. The start and end radii of the spiral
  2. The length of the spiral
  3. The formula used to define the spiral


In some scenarios, it may be possible to change the Spiral Type and still get results that approximate to within an acceptable tolerance of the original data, however you would need to validate every part of an alignment after making such a change - Trimble cannot support that this is either viable, acceptable or good practice. As an industry professional you would be entirely responsible for making such a decision and the consequences of making any such change. Business center provides you with the tools necessary to explore these types of changes to see what effect it may have on the data. 


When checking or validating such a change, be sure to create a set of points from the original data that are fixed in space, that follow the entire original alignment, so that you can compare their position to any adjusted alignment. You have to validate along the entire length of all spiral elements not just at the end points, as some spiral types bulge more or less than other spiral types.


I hope that this helps provide background to this subject area