An underlying assumption in point sampling to determine cover is that the sample unit is theoretically reduced to an infinitely small, dimensionless point. In practice, however, most instruments employed as points possess some dimension, with increasingly large points leading to a greater loss of accuracy by overestimating cover. A wide variety of improvisions have been used as points during field sampling, including pins or sharpened rods, bayonet tips, intersections on a grid-frame, etc. Crosswires of telescopes and sighting scopes probably provide the finest type of point, but their use is often cumbersome. Viewing the ground vertically through the point will also ensure that the point is as small as possible.
Bias becomes critical if an absolute measure of cover is required, but may be acceptable if the same sized point is used when monitoring differences in cover among sites or over time.
References and Further Reading
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Bonham, C.D. Measurements of terrestrial vegetation. John Wiley Sons, New York, NY. p.111-115.
Burzlaff, D.F. 1966. The focal point method of vegetation inventory. Journal of Range Management. 19:222-223. (pdf)
Cook, C.W., and T.W. Box. 1961. A comparison of the loop and point methods of analyzing vegetation. Journal of Range Management 14:22-27. (pdf)
Greig-Smith, P. 1983. Quantitative plant ecology. Academic Press, New York, NY. pp 41-45.
Goodall, D.W. 1952. Some considerations in the use of point quadrats for the analysis of vegetation. Australian Journal of Scientific Research 5:1-41
Poissonet, P.S., Daget, P.M., Poissonet, J.A., and G.A. Long. 1972. Rapid point survey by bayonet blade. Journal of Range Management 25:313. (pdf)
Warren-Wilson, J. 1963. Errors resulting from thickness of point quadrats. Australian Journal of Botany 11:178-188.
Winkworth, R.E., and D.W. Goodall. 1962. A cross-wire sighting tube for point-quadrat analysis. Ecology 43:342-343.