Compared to the number of field inventory programs that monitor change in vegetation with visual cover estimations, very few studies have been conducted to show how accurate thistype of data is. In addition, no previous studies have determined whether efficient calibration of field observers can improve such data. This study concerns the design and evaluation of a computer program consisting of images of vegetation on which the true cover of vegetation has been digitally calculated. The calibration consists of estimation with immediate feedback of the true cover. The results show that even a short time of calibration greatly improves the estimations and can also drastically reduce the influence of different backgrounds, aggregation patterns and personnel experience.