Nice smooooth roling hils in edit terrain

I am in the process of making a house with its basement in a hill. I selected the piece of land that I wanted for the house and flattened the hill, and lowered it to the desired position, and that worked fine.

But but but. In the process the surrounding land got a lot of small triangular tops and holes. And I am not able to smooth them out

I have tried to use the smooth tool, but it does not do a nice enough job. I also tried to flatten it, but since this is a hill, I do not want to flatten it further. I tried to flatten just small parts, then each of them becomes flat, but terraced. I then thought I might smooth the terraces, but then it just become triangular again.

How do I make a nice smooth piece of rolling hill???

I think you are talking about the edges of the parcel, right?
If so, then sometimes a simple smooth won't work. Rather, you need to use the lower/raise tool to make it roughly match the edge, and then use smooth.

No, I was not talking about the edges of my parcel, but the problem is on the edges of my dug out basement. Not so much in the basement itself, but on the ground around the house.

The triangular spikes and wholes are not very tall. at most 1/2 meter, but they are not smooth like a lawn, more like - well the underlying triangular grid. All spike like.

But terracing and then smoothing is your suggestion? I'll try that out then.

Areas around holes like that usually end up spiky like that, so making underground stuff in SL can be kind of complicated. My suggestion is to smooth it and use prim grass to cover the smoother holes, or maybe cover the spikes with something else. Maybe a garden around the house?

Those spikes are interpolation glitches, and they are a pain in the butt to remove, even for a pro.

Sometimes the smoothing tools will help. Sometimes you have to raise or lower a *nearby* point just a bit to allow the other area to smooth out.

The problem is caused by how coarse the SL terrain grid is. A whole sim is only 256 by 256 data points, or points one meter apart, and the default vertical resolution is one meter increments on each point. SL interpolates between these points to give you a relatively smooth terrain. But it was a system designed for rolling hills - not for deep and steep-sided excavations. Make a square hole in a flat surface, and you get spikes.

Thanks all, both to the comments in here and those who dropped by inworld. The solution is as you say, not too steep hills. I had drilled a square hole from 10 meter above water all the way to the sea-bed, a drop of approx 30 meter. I was trying carefully smooth the edges as someone suggested, and it did not really work out.
At some point I had just reselected the edit terrain tool, and had just selected the smooth small smooth tool, when my cat stepped on the keyboard and there pressed "smooth selected area" - which was my entire parcel - et voila, it was nice an smooth, all spikes gone, and my deep basement basically intact, but with the walls just a bit sloping. So the cat got a bonus yesterday.
No professional can beat pure lock as our saying goes.

Let's hear it for "Feline Intervention"! Give that kitty some Tuna. *grins*

Rumor is some spikes also get to the point where they are impossible to remove without the intervention of a linden, happened to my buddie a few times...lol