Notes and Suggestions for Botanical Garden Surveyors - Part 10

By Walt Dunlap, Mapping Specialist, The New York Botanical Garden
© 2001 The New York Botanical Garden
As a mapping specialist at The New York Botanical Garden and a professional land surveyor for the last 20 years, I am offering consulting services on land surveying to any garden that is mapping or planning to map their collections or garden.

Magnetic Bearings

Historically, the easiest way to describe a direction from one point to another, whether for mapping or for travel, has been by compass bearing. Mariners developed an extremely precise division of the 360° of a circle to better describe their headings so that they might reckon their route back home again, as well as be able to share the information of their sailing courses with others. Much of this information survives in beautiful ocean-going maps from long ago. Many of these maps have very finely drawn compass "roses" which helped a fellow navigator to orient his compass to the territory depicted by the map. Indeed apprentice mariners who were expected to stand at the helm were required to learn all the points (sequential divisions such as north-northeast, northeast, east-northeast) of the compass and to recite them flawlessly. This was known as "boxing the compass", a sort of oral exam.

Land surveyors have relied on the compass to provide the same directional clues for anyone wishing to trace their footsteps literally or mathematically. The system has been used continuously in the United States since colonial days to describe land parcels in deeds. Most users limit themselves to the northeast, southeast, southwest and northwest quadrants for descriptions, such as N 40° 56’ E or S 55° 30’ W, although a few very old deeds in nautical-tradition states sometimes recite terms such as NNE, WSW, etc.

Unfortunately if you took a deed from the colonial days and started at the exact same starting point of the description, you might wind up at the start again but your route would be deflected by almost 9° westerly here in Bronx, New York. Even over a short distance that would be a substantial difference in the position of every angle point.

First of all the actual direction toward "North", as in the astronomic value, is not the same as magnetic north because what attracts the compass needle for us in North America is the great Canadian iron deposit just above the upper peninsula of Michigan. This vast deposit actually draws the compass needle toward it today by as much as 13° westerly of true North (in the Bronx). This is called magnetic declination and it was only just a bit more than 4° westerly in 1776. This value is constantly changing as the earth’s crust shifts and churns and it is different for all areas in this country along roughly north-south lines that radiate from the Canadian iron. The position of 0° magnetic declination (the agonic line) runs roughly through central South Carolina. Compasses to the west experience an easterly pull, just as lands to the east experience a westerly compass deflection.

If you take into account the magnetic declination in addition to the whole host of local influences on a compass needle such as locally magnetized (iron-bearing) rocks and manmade metallic structures or electrical currents, you can well appreciate the inaccuracy of compass readings even if measured with precision. This does not mean that compasses are useless, far from it. It just means that you must be keenly aware of the influences that bear on their effective use.

Just about all a plant mapper has to know is that, within a restricted geographic area, you can use a compass for locating plants or objects. But, you must also be mindful to add enough accuracy checks on your data to allow you to confirm that it meets your mapping standards. A handheld compass that allows a viewfinder to be held to the eye and is calibrated in degrees may be adequate for distances under 50’. Any compass that you use ought to be compared to some of your known control points so that you are sure that the basis of bearing is the same or that it can be equated mathematically.



Updated January 9, 2001.
Send comments and questions to BG-Map Support