Merging geometry on a hidden layer

[Involute]

I have a lot of layers in my drawing, so I turn visibility of various ones on and off depending on what I'm working on. I draw a square on one layer and get a warning that my "recent operation has caused visible geometry to merge with existing geometry on a hidden layer."

1. If the layer is hidden, why is my geometry merging with it?

2. How can I prevent this from happening?

3. How can I identify which hidden layer is being merged with?

Thanks.

[TIG]

First rule:
Sketchup Layers are NOT like CAD Layers.
In CAD the Layers 'separate' Objects, but in Sketchup Layers do NOT separate them, irrespective of the Layers' visibility.
Sketchup's Geometry [faces/edges etc] "sticks together" and "interacts", irrespective of the Layer it has assigned to it.
In Sketchup Layers simply control the "visibility" [and consequently the "selectablity"] of objects.
Sketchup Geometry that is on different Layers will "interact" even if some of it is on Layers that are not visible - causing unexpected results and issues [erase an edge on a visible layer and the face on the hidden layer that needed that edge to exist will vanish too - unseen!].
A recipe for eventual madness

Second rule:
Always drawn Geometry [faces/edges etc] on the 'default-layer' [=='Layer0'].
Always have your 'active-layer' set to be 'Layer0' to avoid a mess up.
Then 'Group'* related Geometry, and give that Group a sensible Name AND a specific Layer that you can then turn on/off to control its visibility - for anything on Layer0 inside the Group will vanish too when the Group's Layer is 'off'.
Groups do not "interact" with external Objects/Geometry - so unexpected geometry changes are avoided.
*A 'Group' is a collection of Objects [Geometry and 'nested' Groups] that you will use once - e.g. your site's topographic mesh: if you have something that you are likely to use on several occasions in the SKP [or even exported for use in new SKPs] - like a Door, Window, Column etc then make it a 'Component' - which is similar to a 'Group', but it can have multiple Instances.
Components also do not "interact" with other Objects - except for those with special types of Component behavior set: 'gluing' which glues instances to preset 'planes' - e.g. an artwork that only glues onto a vertical wall: 'cutting' which lets a 'glued' object cut a holes in the face it is placed onto [if in the same context] - useful for windows/doors/rooflights/etc.
The SKP's Components appear in the Component-Browser, and therefore additional Instances can be added to the SKP as needed later. You can easily assign specific Layers to selected Groups/Instances using the Layer-toolbar or the Entity-Info dialog.
You can also bring in external SKPs as new Components in the model, or save the model's Components as new external SKPs.
Making the contents of a Component on Layer0 means that you can do things like - the 'Door' Component Instances on the 'first-floor' Layer can be switched on while other Instances of the same 'Door' on the 'second-floor' can be switched off.
The advantage of using Components is that when you Edit one Instance and save it the changes are reflected across all Instances [you can of course use 'Make Unique' to ensure that particular Instances are made separated from their siblings and then changes do not affect all others].
A Groups or Component is easily edited by double-clicking on it, then when you are inside its entities then geometry in that context interacts as it does when made in the model itself.
In fact each Group or Component can be thought of being like a "mini-model", nested inside the main SKP etc...

[Involute]

Thanks for the thoughtful reply, TIG. I've been using SketchUp for years, though somewhat casually. Nevertheless, I thought I understood the minutiae of how layers worked, but I guess I didn't. Thanks for straightening me out.

[Involute]

On the other hand, TIG, what's the difference between creating some geometry on Layer 0 and later assigning it to Layer X, and just creating it on Layer X to begin with? If all your geometry ends up on different layers, some of which are visible and some of which aren't, how is this approach going to avoid the problem I'm having? I can see how ensuring everything is grouped appropriately makes a difference, but that's a separate issue.

[TIG]

You CAN create all of you geometry on ANY layer.
BUT you are likely to get confused and mess up at some point.
What benefit do you get having some lines on some layers and some faces on others etc, if some of those layers are 'off' you will make unseen changes to their geometry whilst editing things on a visible layer ! As I said erase a seemingly faceless edge on a visible layer might also remove a connected face on a hidden layer, unexpectedly - only to be realized when that layer is made visible and stuff is found to have 'gone' - too late to fix without extra work. If you really must have the face on another layer keep all of the geometry on Layer0 and group the face and assign the layer to the group. Erasing the now unconnected edge on the other layer no longer affects the face as grouping just a face will replicate the required edges for the face inside the group automatically.
If you make all of your raw-geometry on Layer0, and then group OR componentize it, and then assign a layer to that 'container' there are no issues of some unseen geometry on an off-layer being connected to other visible geometry on an on-layer - live is easy !
Having a layer other than Layer0 set as 'active' is a recipe for errors and confusion.

There may be times that assigning a layer to non-grouped entities is useful - e.g. TEXT or DIMS; BUT raw Edges/Faces need a different approach, this is because they 'stick together' irrespective of their layers or those layers' visibility settings. Layers don't separate geometry, grouping/componentizing geometry does separate them - their only 'interaction' then being with other geometry within the same 'context'...

[Gaieus]

I do not think there is anything I could add to TIG's explanation but if you are a visual type, watch this video:

[Dave R]

That video should almost be compulsory for SketchUp users. This sort of thing comes up all the time.