A scanning tunneling microscope (STM) operating in air was used to image an array of Vickers indentations made on hardened steel, having diagonal lengths ranging from 1 to 70 mum. The morphology of the indentations shows that the boundary of the indentation is not sharply defined. Therefore, accurate STM-based microhardness measurements need image analysis methods for measuring either the indentation area or a parameter well correlated with the geometry of the indentation. Three methods have been investigated. Two methods use different methodologies to estimate the level of the undisturbed surface of the tested material before the indentation is made, and the indentation area is subsequently calculated. The third one analyzes locally the slope of the indentation boundary area. The area is obtained from the profile of the indentation, as in the traditional optical measurements. The repeatability of the indentation area measurements obtained by using the different methods, has been evaluated by imaging the indentations at different sampling areas. The influence of the sample orientation and tip scan rate on the measurement repeatability has also been studied.