> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/templates/openbrimfea/coordinate-systems-fea.md). # Coordinate Systems \[FEA] Coordinate Systems holds the reference frames the model is built and reported against. **Alignments** define the roadway geometry - horizontal path, vertical profile and cross-slopes - so objects can be positioned by station and offset rather than by absolute coordinates, and so the model follows the roadway when it changes. **Coor. Sys.** defines a local frame with its own origin and rotation. Its main structural purpose is skewed and inclined boundary conditions: a bearing on a skewed abutment is fixed along the bearing line and free across it, which cannot be expressed in global axes at all. Assigning a coordinate system to a node reinterprets its coordinates **and** its restraint directions, and reports its displacements and reactions in that frame - so a result that appears to disagree with a global hand-check is often correct and simply expressed locally. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.openbrim.org/templates/openbrimfea/coordinate-systems-fea.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.