protected float getHeight(int x, int z, float xm, float zm) {

        QuadrantChild match = findMatchingChild(x,z);
        if (match != null) {
            if (match.child instanceof TerrainQuad) {
                return ((TerrainQuad) match.child).getHeight(match.col, match.row, xm, zm);
            } else if (match.child instanceof TerrainPatch) {
                return ((TerrainPatch) match.child).getHeight(match.col, match.row, xm, zm);
            }
        }
        return Float.NaN;
    }
gets an interpolated value at the specified point
jme3-terrain/src/main/java/com/jme3/terrain/geomipmap/TerrainQuad.java