Correctly Measuring Girth   Colby Rucker
  Aug 11, 2002 20:32 PDT 
Big trees on steep slopes often require a variety of measurements to allow comparisons with other trees.

All height measurements start from the same place - "where the acorn sprouted." If you stand at mid-slope and sight down the centerline of the trunk, that line meets the soil at the central basal contour. You can't determine the contour by averaging the elevation on the high and low sides of the base; roots enter the soil abruptly on the high side, but run all over the hillside on the lower. The central basal contour is a constant elevation; it extends level from your side of the trunk, through the spot where the tree first sprouted, and out the other side. Like all contour lines, it may curve, but it's at a constant elevation. If you measure upwards with the lean of the tree 4.5 feet above that contour, that point is breast height on either side of the tree.

Circumference is measured at right angles to the lean of the trunk. If you temporarily mark breast height on both sides of a large tree with a scrap of paper and a thumb tack you can be sure the tape is even. You can carry some thumb tacks pressed in a wine cork. Of course, if erosion has left the tree standing on its taproot, you'll have to estimate the elevation where the tree first started. If floodplain deposition has raised the grade, you'll have to accept the existing grade level. We don't dig 'em out.

On a very large tree, a tape set at breast height may still be below grade on the high side. In that case, raise the tape to a higher elevation and attach that elevation to your circumference record. Actually, a variety of measurements is very useful, including several taken with the slope, starting at grade. This will allow others to model the architecture of the base in the future.

Yes, getting the tape even on the downhill side can be difficult. I had to use a telescoping aluminum pole to raise the tape on a 22' 8" tuliptree on a slope above a precipitous drop at the head of a ravine.

We've found a simple way to measure trunk diameters at any height with laser rangefinders. This, together with taped measurements of the base, allows us to calculate the volume of the entire trunk. For the western giants, volume is the best means of comparison. Bob Van Pelt is the expert on this, and I highly recommend his book.

Hope some of this is helpful.