list manipulation - Selecting points on either side of a curve

I have the following array of data. Suppose I want only to keep data within the two lines. I can achieve my result using the following code:

data = Table[{i, RandomReal[{-10, 10}]}, {i, 0, 100}];
line1[x_] := 0.1 x - 10;
line2[x_] := -0.1 x + 10;

rf = RegionMember[Polygon[{{0, line1[0]}, {0, line2[0]}, {100,line1[100]}}]];
bool1 = rf[data];
notbool1 = Not[#] & /@ bool1
datain = Pick[data, bool1];

dataout = Pick[data, notbool1];

This has the desired effect.

Now suppose I only have one line, since I can't make a closed region what would be the best way to select points either above or below a line? What if the curve is not a straight line? Any ideas?

Answer

Some knowledge about (planar) analytic geometry facilitates this problem. Suppose now you only have line1, this

Select[data, line1[ #[[1]] ] < #[[2]] &]

selects out the points above line1; the key lies in the inequality. If it is connected by >, then the points below line1 you will get.