Using Masks and Layers—Painting, Filling, and Color Tools - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 201

• Feather edges : This option creates a “soft border” for the selection.
Feather edges is not the default setting, so you'll want to change the
settings before editing if you want feathered edges. If you click the check
box at Feather edges , a slider titled Radius shows up. Move the Radius
slider to set the feathering width. A value between 2 px and 5 px should
be sufficient, depending on the desired effect and image resolution.
• Select transparent areas : Allows the Fuzzy Select tool
to select completely transparent areas like a color. If the
option is not checked, transparent areas cannot be selected.
The possibility to select transparent areas is rather helpful. For instance, to
trace the contours of an image object on an otherwise transparent layer,
it's often easier to select the transparent area and invert the selection
than to select the object itself.
• Sample merged : This option ensures that all visible image areas on all
layers are included when the selection by color is calculated. If this option
is not checked, the tool will only select the desired color on the active,
single layer.
• Threshold : This option determines the range of similar colors that will be
selected. The higher the threshold, the wider the range of colors.
• Select by : This option offers a drop-down menu. Composite is the default
setting; it actually calculates a selection of the color values of the pixel
chosen with the tool by clicking on it. The other menu options let you
choose which component of the image the program should use to
calculate the image. The components you can choose from are the three
fundamental colors (red, green, and blue), hue, saturation, and value.
Proceed as follows:
• Remove small defects from the sky, if you ind any. Select these spots in
the sky (or selection islands with an animated border) by circling them
with the Free Select tool (Lasso) while either holding the Shift key down
or selecting Mode > Add to current selection from the tool's settings dialog
box.
• Remember that you can use the Zoom tool to magnify an image area.
• When your selection is cleared of “islands” (or spots), increase the
selection size by approximately 4 px using the Select > Grow menu item.
Then choose Select > Feather to add feathering to the selection border
(approximately 5 px). The horizon and the contour of the trees will now
be feathered. After deleting the area of the blurred sky, the remaining
landscape contour will look natural, and the scissors effect will be
avoided. Otherwise, you can do this by using the Feather edges option of
the Fuzzy Select tool .