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Length of line features, such as forest border, is among the ecologically interesting attributes estimated from forest inventories. In hilly terrain, observed line lengths must be corrected for slope. Contrary to the correction for standard area-related attributes (like volume per hectare), an overall correction of plot size is not sufficient, but the actual inclination of each individual line segment must be used for slope correction. This topic is discussed, and a mean correction factor is calculated as a function of terrain inclination assuming a uniform angular distribution of lines on the slope. Furthermore, the question is discussed whether the standard slope correction procedure for fixed-area circular field plots may possibly introduce a systematic error into the estimation of line length and also of standard area-related attributes. It is concluded that no relevant error is to be expected, neither with respect to point estimates nor to interval estimates. Data from the second Swiss National Forest Inventory serves for illustration.