MB&G and the Glass Wedge Prism

When former MB&G Principal Lucien (“Lu”) B. Alexander introduced the wedge prism method of timber cruising in the Northwest in 1955, it immediately drew the attention of foresters to the possibilities of speeding up and reducing the costs of cruising timber.

The technique was based on papers published by Walter Bitterlich, an Austrian forest scientist, in 1947 and 1948. The concept developed by Bitterlich forms the basis of what is now known as point sampling, or variable radius sampling – a highly-efficient sampling technique in common use today. Central to the idea was the use of an “angle gauge” to determine which trees are to be measured.

In 1952, two American foresters, Grosenbaugh and Spurr, published accounts of Bitterlich’s method, bringing the idea to the U.S. forestry community. Various types of angle gauges were proposed. In 1953, MB&G developed a prism mirror device for establishing reference angles. In 1955, David Bruce, son of Donald Bruce and then a research forester at the U.S. Forest Service Southern Experiment Station, published an article proposing the use of a small wedge-shaped glass prism as an angle gauge. It was a modification of MB&G’s device. The idea grew from there and became the predominant approach.

Also, in 1955, Lu Alexander made a presentation at a meeting of the Society of American Foresters in Eugene, in which he described the technique. Lu and Dr. John Bell published “Application of the Variable Plot Method of Sampling Forest Stands” in 1957. It provided detailed field procedures for using wedge prisms for determining basal area and volume.

In 1961, Mason, Bruce & Girard published Prism Cruising in the Western United States and Volume Tables for Use Therewith by Donald Bruce. It was one of the publications he was most proud of.

Although more sophisticated optical and electronic tools been developed over the years since, wedge prisms are still in use today because they are inexpensive, lightweight, and accurate.