Eagle - changing board dimensions?

Community PCB Design
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I’ve currently used a ProtoShield (DEV-13820) to make a custom shield for a project I’m working on. The wiring ends up being, well, kind of messy. Certainly, it’s functional, but I figured I might us it as a jumping off point to design and make a custom PCB.

So, I’ve taken the Eagle files for the ProtoShield as a basis to start from. I’m having trouble redefining the board dimensions, though. I don’t need the BlueSmirf area, or the breakoff area at the bottom, so I’d like to remove them. But, anything I try to change them on layer 20 (Dimension) is unsuccessful - Eagle just beeps at every mouse click using the Move, Group, Delete tools, etc.

I have the same result on layers 51 (tDocu), and for one area of silkscreen on 21 (tPlace) that corresponds to the ICSP area.

I thought something might be locked, etc, but I haven’t been able to unlock those areas. This is probably just some stupid thing I’m missing, but I’ve spent a couple hours trying to work this out, now, and I’m just beating my head on the wall, so… ha ha

Current schematic and board files attached for reference… Thanks for any help!

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Dimension layer - Arduino Layout: this is not editable unless you modify the library because the outline is a “part” and not individually editable lines. This is the same for the other two layers. They’re part of a part’s package. They weren’t individually drawn in the project’s board files. The only way to edit them is by editing their libraries.

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Ah… Interesting. That makes sense - so, it looks like the original was designed off of the Arduino R3 Shield part, but was then modified by editing the library (in a temporary library, from the look of it). So, I can’t actually edit it directly.

A better route would be to start fresh, and make use of the Arduino R3 Shield part directly, I’m guessing - which should be easy.

Thanks for the clue, Brandon! Not sure I would’ve figured that out on my own!

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No worries, that answer comes from 100+ hours of my own frustrations and failures of learning Eagle! Good luck.