Theoretically, increase of steepness in natural or artificial slopes; raise the potential of instability. Zonation mapping, based on the statistical analysis of different slope classes, in northwest Iran (Khalkhal-Koloor region), showed that the percentage of unstable areas in the steepest slopes are less than slopes of medium or low steepness. This phenomena, causes a systematic error in the results of zonation mapping, which is normally based on the percentage of unstable areas in different classes of data layers. Following a slope rupture, the average gradient of slope decreases and slipped materials move to a flatter area compared to the initial slope. This final gradient is usually used to rank the slope parameter. On the other hand the map area of working units is less than their actual area, especially in steeper slopes. This also causes systematic error in ranking slope gradient and other parameters. Assessment of slope instability concentrations in eight directions in studied area, shows that landslides facing north and north-east are more frequent and those facing west and southwest are less frequent than the average slope instability concentration in the entire region. Present paper discusses some methods to overcome the mentioned pitfalls.
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