Thumb basal area factor calculator


If you know about point sampling, then you may also be aware that you can use your thumb instead of a prism.  With your eye over the sample point, hold your thumb at arms length towards each tree.  If you can see the tree at breast height on both sides of your thumb then count it.  If your thumb covers it, it's too far away.  The basal area factor of your thumb can be calculated if you measure the width at its thickest part in millimetres (without squashing it), and measure the distance from your eye to your thumb when your arm is stretched out in front of you in cm.  Enter the values below, and press the calculate button.  Your metric thumb factor will appear in the lower field.

A word of warning: Point sampling with a thumb requires that you hold your thumb in the same position and rotate your whole body around the sample point.  This takes concentration.  If you move your arm without moving your torso, then your thumb basal area factor will change and you'll get biased estimates of basal area/ha.  Thumbs are useful if you want a rough estimate of basal area.  For more reliable estimates, invest in a prism.

      Thumb width (mm):           
      Eye to thumb distance (cm): 
      Your thumb factor:          

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