The Effects of Symbol Size and Workload Level on Status Awareness of Unmanned Ground Vehicles
The objective of this study was to determine which size symbols should be used by the U.S. Army for an operator control unit to indicate the status of unmanned ground vehicles (UGVs). Three sizes of symbols were studied. The symbols subtended 20, 40, and 69 minutes of arc corresponding to 0.116, 0.233, and 0.400 inches high when viewed at a distance of 20 inches from a touch screen. Twelve participants were asked to watch the symbols on a map display and touch one of four UGV symbols when it stopped moving. Different numbers (0, 8 and 12) of distracter symbols with the same height as the UGV symbols appeared during the experimental trials. The time to notice that a UGV symbol had stopped (recognition time) and to touch the screen (response time) were measured. Participants were asked for Subjective Workload Assessment Technique (SWAT) ratings for each combination of symbol size and number of distracter symbols. Errors committed while attempting to touch the correct symbol were counted. Participants made very few errors attempting to touch the wrong symbol.
Results for the time and error measures were as expected for changes in symbol size. As symbol size increased, recognition time, response time, and extra touches decreased. Significant differences were seen in these measures between the subtending 20 and 40 minutes of arc and between symbols subtending 20 and 69 minutes of arc. Also, as expected, subjective mental workload increased as symbol size decreased with differences seen between all symbol size levels. No significant differences were observed for workload manipulation (number of distracter symbols) as measured by time and error. However, SWAT scores did show a significant difference as a result of number of distracters. The differences between 0 and 8 distracters and between 0 and 12 distracters were significant. There was no significant interaction between symbol size and number of distracters for any of the measures. Overall results suggest that symbols smaller than those recommended for keypads may be sufficient for interactive map displays. For static platforms with barehanded operators, symbols that subtend 40 minutes of arc may be sufficiently large to ensure adequate touch screen performance under low to moderate workload conditions.