Use the Multipoly tool |CC2MPOLY2| to combine them into one shape.

Two smooth shapes with different properties.

As long as they aren't touching, all shapes will be preserved as they are, but multipoly will make them behave as if they are one shape and impose universal properties on them. These two solid shapes took on the green colour picked in the palette and active in the Status Bar at the time the multipoly was created.

If they partially overlap you will end up with a hole where the overlap is.

If one lies within the other, the smaller inner shape will become a hole in the larger shape.

You can change the properties of a multipoly, but you can't edit the shape or trace it. To do either of those things you have to explode them again using the Explode button, which can be inconvenient. Unless there is a good reason for using a multipoly (for example, if you need to be certain that all the land of a global landmass will be affected if you change properties by picking once on any part of it), I recommend leaving the polys as they are - separate.

They really are below the landmass.

The sheets are in the order in which they are rendered, which means the RIVERS sheet is rendered before the LAKES sheet, and both are rendered before the LAND sheet. Move the LAND sheet up to just underneath the NAVIGATION LINES sheet and you should be ok.

Two different views...

Since the same thing is happening on FB where I asked the same question, then at a guess I've got it about average, which is what I wanted. Some of the buildings might have more or less snow, just like in the photos we've seen above.

Something a bit like this, then... Or is this too melted to go with the thatch?

Oh shoot! I forgot to cut the chimney extent out of the map file, so its a lot brighter than it should be, but you get the general idea.

On the progress side (or lack of it today) I've had quite a struggle getting the gradient shading of the map file correct so that the hollows around the dormer look filled with snow, and everything the right size and shape to be the same as the original but with a layer of snow on it. Not sure how all this is going to work when I start combining them, but we'll see in the morning now.