White Level. The White-Level option controls what colors are white. If the slider to is to the far right, only pure white is made transparent. If the slider is moved to the left, other, near-white colors are made transparent. This useful where the sketch map is drawn on yellow paper or the paper has been stained, so it is less than pure white.
Also, the area around the edges of lines may be less transparent than you want because of color bleed from the lines. Adjust the White Level, can increase the transparency and make the edges of the lines more distinct.