How to use knife tool. Mine just draws a purple line

Extracted a model of a pier from google maps. I need to trim the excess water from around the edges. The vertices overlap the water and the pier. I want to keep the pier. I am able to use the knife tool to cut new lines across the overlapping vertices and remove some of the water. In other areas the knife tool appears as normal but won’t cut any new lined. Just draws a purple line. No green squares, using z key. I think it may be the mesh as knife works in other models. Thanks in advance for your time and help. ![knifeTool|579x500] Holding shift or z doesn’t help. I think there is something about parts of the model that is preventing this from working. It worked for a bit then just stopped as I moved around the model. I’ve saved and reopened.
(upload://8vqQHrlaKw8h4VwXKOpLWrjp9sg.jpeg)

It’s the mesh. There looks to be some weird topology there (overlapping edges, stray vertices). Other similar tools like Intersect, Boolean, etc. are likely to also have issues with this. Try running some basic cleanup first by selecting everything and doing a Mesh -> Cleanup -> Degenerate Dissolve and/or Limited Dissolve with low angle setting.

Thanks so much for your help. I did the clean up and it simplified the model considerably but still the knife is not allowing me to continue to trim the fat off this model. Is there another tool or method to trim off parts you don’t want or no longer need? The knife icon dissapears when I grab the capture but you can see the purple line.
Thanks again.
knifeError2

Well, you could try the Intersect (Knife) tool: add a plane (in edit mode, not object mode). Position/scale it as if you’re cutting the mesh with it, select it (only it) and do a Face -> Intersect (Knife), and a bit of cleanup after (e.g. delete the remains of the plane). Not sure how well that’d work there though.

I just thought of something. If this is a GM import, how large in the mesh in world space? Did you adjust your view clipping to see it? Knife works off of view matrix, if you have a huge clipping range it always has issues.

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Thank you Stan, your help and experience is much appreciated. I will try that and let you know how it turns out. It is three docks/piers along a ship port. I cut them at the shoreline so it is only three piers. Small model and I can see all three of them in the blender screen when trying to cut. I only posted a small portion of the screen in the screen captures. Thanks again!

I am able to use the bisect tool portion of the knife with no problems. I bisect a line through the mesh, select>select loops>select lops inner region, then X key to delete vertices and the portion I selected is gone. I don’t know why the knife stopped working but I am able to work around it with the bisect tool.

Thank you Stan. Your help is much appreciated.

You’re welcome :slight_smile:

If you could upload a (small) portion of that mesh I could take a look to see if I could find what, exactly, stumps the knife tool.