Relascope Scales and BAFs
Converting metric to English units for BAFs is a snap. You just multiply the metric BAF by 4.356 to convert square meters per hectare to square feet per acre. You will probably recognize 43,560 as the number of square feet in an acre.
If you use prisms, when it comes time to “go metric” remember this conversion, rather than buying all new sets of “metric” prisms. You can also scribe the Metric BAF on each prism with an engraving tool – but Relascopes are an even better tool, so what scale should you use?
The Metric CP is the newest of the scales, and in some ways superior to the older Metric Scale. It specifically incorporates a metric 1,2,3 and 4 BAF (that’s in square meters per hectare), but you will probably only use the 3 or 4.
What is very useful is that it has several bars of the same width. Each of these is called a “Relascope Unit”, or “RU”. It is always the bar that gives you a metric BAF of “1”, or any bars of that same width. Specifically the Metric CP scale has 5 (with the 4 smaller quarter bars making another full bar to form 6). The other Relascope bars often are often a black/white pair that forms 2 Relascope units (as you see between 5 and 7 in the illustration for this article). To get the BAF, just square the number of Relascope Units. 3 standard bars = 9 BAF metric (9*4.356 = BAF 39.2 in English units).
Many of us prefer the Wide Scale Relascope, because it has lots of these bars of the same width, and can easily generate some whopper BAFs (like 144 Metric – over 600 English) when you need them. The Metric CP is probably the next best choice for a scale. It is also a good match when you have mixed Relascopes in an organization, since so many of the same BAF’s are common to both the Metric CP and the Wide Scale. Equipment firms like Forestry Suppliers can replace the scale in your Relascope for quite a reasonable price.
With the quarter bars, you can get finer gradations. For instance, 3.25 (3 full and 1 quarter bar) give you (3.252 = 10.5625) metric or 46.01 English.
You can download an EXCEL file that will compute this for you, as well as the Plot Radius Factors that go with it. [Note: a bug was found in the spreadsheet. A corrected version will be available soon. ]
We have already printed a very detailed list of BAF’s for the American Relascope (also included in this spreadsheet), but you should consider the other scales of the Relascope. It’s not hard to convert, and the main thing is for the field crew to be able to easily and reliably use the bars when sighting trees. That is far more important than having factors that are very even.
We have previously talked about using a temporary overhead marker to color the scales, so you do not need to count them each time. There are small illustrations of all these scales at the Relascope website (see the links on the John Bell Website. Be patient - it takes a while to load all the illustrations and headers). The Relascope people can also send you Relascope manuals if yours have been lost.
All these scales are also shown in Walter Bitterlich’s book “The Relascope Idea”. This book is hard to get, and the best source is currently the Relascope Website. They will soon be out of print and unavailable, so you might want to order one for your company library before they disappear entirely.
Originally published January 2005
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