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Forest Surveying Methods - Distances and Angles

Using a Compass and Chain to Reconstruct a Forest Boundary

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Forester or builder marking trees with ora...
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With the advent of public use of geographic positioning systems and the availability of aerial photographs (Google Earth) for free over the Internet, forest surveyors now have extraordinary tools available to do make accurate surveys of forests. Still, along with these new tools, foresters also depend on time tested techniques to reconstruct forest boundaries. Remember that professional surveyors have traditionally established nearly all original land lines but landowners and foresters have a need to retrace and reestablish lines which either disappear or become difficult to find as time passes.

A Fundamental Unit of Horizontal Measurement - The Chain

The fundamental unit of horizontal land measurement used by foresters and forest owners is the surveyors' or Gunter's chain with a length of 66 feet. This metal "tape" chain is often scribed into 100 equal parts which are called "links", but I digress.

The important thing about using the "chain" is that it is the preferred unit of measure on all public U.S. Government Land Survey maps (mostly west of the Mississippi River) - which include millions of mapped acres charted in sections, townships and ranges. Foresters prefer using the same system and units of measure that were originally used to survey most forest boundaries on public land.

A simple calculation from chained dimensions to acres is the reason the chain was used in the initial public land survey and the reason it is still so popular today. Areas expressed in square chains can be easily converted to acres by dividing by 10 - ten square chains equals one acre! Even more attractive is that if a tract of land is a mile square or 80 chains on each side you have 640 acres or a "section" of land. That section can be quartered again and again to 160 acres and 40 acres.

One problem using the chain universally is that it was not used when land was measured and mapped in the original 13 American colonies. Metes and bounds (basically physical descriptions of trees, fences and waterways) were used by colonial surveyors and adopted by owners before the public lands system was adopted. These have now been replaced by bearings and distances off permanent corners and monuments.

Measuring Horizontal Distance

There are two preferred ways foresters measure horizontal distance - either by pacing or by chaining. Pacing is a rudimentary technique that roughly estimates a distance while chaining more accurately determines distance. They both have a place when determining horizontal distance on forested tracts.

Pacing is used when a quick search for survey monuments/waypoints/points of interest might be useful but when you don't have the help or time to carry and drop a chain. Pacing is more accurate on moderate terrain where a natural step can be taken but can be used in most situations with practice and the use of topographic maps or aerial photo maps.

Foresters of average height and stride have a natural pace (two steps) of 12 to 13. To determine your natural two step pace: pace the 66 foot distance enough times to determine your personal average two step pace.

Chaining is a more exact measurement using two people with a 66 foot steel tape and a compass. Pins are used to accurately determine the count of chain length "drops" and the rear chainman uses the compass to determine the correct bearing. In rough or sloping terrain, a chain has to be held high off the ground to "level" position to increase accuracy.

Using a Compass to Determine Bearings and Angles

Compasses come in many variations but most are either hand-held or mounted on a staff or tripod. A known starting point and a bearing is necessary for beginning any land survey and finding points or corners. Knowing local sources of magnetic interference on your compass and setting the correct magnetic declination is important.

The compass most used (like a Silva Ranger 15) for forest surveying has a magnetized needle mounted on a pivot point and enclosed in a waterproof housing that has been graduated in degrees. The housing is attached to a sighting base with a mirrored sight. A hinged mirror lid allows you to look at the needle at the same moment you site your destination point.

The graduated degrees displayed on a compass are horizontal angles called bearings or azimuths and expressed in degrees (°). There are 360 degree marks (azimuths) inscribed on a survey compass face as well as bearing quadrants (NE, SE, SW, or NW) broken into 90 degree bearings. So, azimuths are expressed as one of 360 degrees while bearings are expressed as a degree within a specific quadrant. Example: azimuth of 240° = bearing of S60°W and so on.

One thing to remember is that your compass needle always points to magnetic north, not true north (the north pole). Magnetic north can change as much as +-20° in North America and can significantly effect compass accuracy if not corrected (especially in the North East and far West). This change from true north is called magnetic declination and the best survey compasses have an adjustment feature. These corrections can be found on isogonic charts provided by this U.S. Geological Survey download.

On reestablishing or retracing property lines, all angles should be recorded as the true bearing and not the declination corrected bearing. You need to set the declination value where the north end of the compass needle reads true north when the line of sight points in that direction. Most compasses have a graduated degree circle that can be turned counterclockwise for east declination and clockwise for west declination. Changing magnetic bearings to true bearings is slightly more complicated as declinations must be added in two quadrants and subtracted in the other two.

If there is no way to set your compass declination directly, you can mentally make an allowance in the field or record magnetic bearings and correct later in the office.

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