A redline drawing is a released drawing marked up in red to show what is wrong with it or what should change. The markup sits over the drawing as an annotation and leaves the drawing underneath exactly as it was issued, so the sheet everyone is working from stays intact while the change is being agreed.
Red is the convention because it separates the comment from the linework at a glance on a printed sheet. The colour carries no meaning beyond that, though many offices extend the palette, using green for a comment that has been actioned or blue for a query rather than an instruction. The word doubles as a verb: to redline a drawing is to mark it up, and a redlined set is one that has been through that process.
When Drawings Get Redlined
CAD redline markups annotate a drawing to indicate an error or suggest an amendment. There are two primary triggers for redline markups.
- Project revisions: When a drawing is in the project/construction phase, it will often undergo several revisions before it is finalised with an as-built status. Redlines provide a convenient method for drawing reviewers to suggest changes to a drawing.
- Operations and maintenance: In real-world scenarios, such as on-site surveys or general maintenance work, errors in drawings can be discovered. CAD redline markups offer a practical solution in such situations, allowing these errors to be highlighted and triggering a revision update on the drawing to rectify the issue, thereby maintaining the integrity of the drawing.
Markup and Revision Are Two Different Jobs
A redline is a request. A revision is the answer to it. Somebody marks the sheet, somebody else opens the native CAD file, makes the change, and issues a new revision that carries it. Those are two pieces of work, usually done by two people, and running them together is where redlines go missing.
The distinction matters because of what each one is evidence of. The markup records what was found, who found it, and when. The revision records what was done about it. Keep both and you can answer why a drawing changed years after the fact. Keep only the revision and you have the change with the reason stripped out of it.
A redline never alters the released sheet. It sits over the top of it until a revision supersedes it. See redline markup for where the line between the two sits.
Redlining Construction Drawings
On site, redlining is how a drawing set keeps up with what is being built. A crew marks the issued sheet wherever the installed work departs from it: a duct routed around an obstruction, a valve set two metres from where it was drawn, a circuit run a different way. The marked set travels with the job, and at the end of it the accumulated redlines are drafted back into the drawings to produce the as-constructed drawings the asset owner will operate from for the life of the asset.
That makes site redlines the highest-value markup in the set and the easiest to lose. They are made in the field on paper, they live in a site office, and they get typed up at the end of a project once the people who made them have moved on. Capturing the markup against the controlled drawing at the moment it is made removes the dependency on a roll of paper surviving the job.
Redline Drafting Services, and When You Need Them
Search for redline services and you will find drafting firms that take a marked-up set, whether that arrives as scanned paper, PDF, or photographs from a phone, and return updated CAD files. The work is real and there is a good reason people buy it. A backlog of site markups is a drafting job, and if you have no drafters, or the ones you have are committed to live projects, sending it out is the sensible answer.
What a service of that kind typically covers:
- Reading the markups and querying the ones that are ambiguous.
- Opening the native DWG or DGN, making the changes, and matching your layer, level, and title block standards.
- Returning the updated files at a new revision, usually with a comparison against the previous one.
What the arrangement cannot do is close the loop quickly. The round trip runs at the speed of the batch: markups accumulate, get packaged, get sent, come back, and get checked, and for the whole of that cycle the current drawing on your system is the one without the changes. The longer the cycle runs, the longer people are building from a sheet that is known to be wrong.
So it is worth being clear about which part you are buying. If you need drafting capacity, a service is a reasonable way to get it, and the work still has to be done by someone. If the problem is that markups are not reaching the drawings at all, more drafting capacity will not fix it, and the markup needs somewhere controlled to live from the moment it is made.
Redline Markups in a Drawing Management System
When drawings are uploaded to Lunr, a few files are generated in the background:
- A thumbnail to support quick previews.
- A PDF rendition to enable viewing in the browser and redline markups.
- An OCR content file to allow users to search for text inside drawings.
Markups are securely stored against the PDF rendition, enabling easy review on the web. Access to redline markups can be controlled using securities, ensuring only authorised users can view and create them. Typically, read-only users will not have access to redline markups, instead, they will only see the latest as-constructed drawing file, maintaining the integrity of the drawing review process.
When drawings are released from document workflow, markups are kept with the current document version, meaning the new as-constructed version does not include the markups from the previous version.