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At an elevation of 8836 feet, Half Dome could be visible out to about 110 nautical miles, that's w/o adjusting for elevation of the viewing point (in the valley). Given reasonable weather conditions, one can see snowy peaks from 99 driving up or down the valley. Half Dome is not obscured from the area of the Rostrum so it becomes a matter of determining if one can see out the Merced river canyon area sufficiently or if the curves or intervening hills obscure it. Aside from driving slowly along some of the area roads and knowing where you are, where Half Dome is (hardly a problem with gps) on a good visibility day, it could be worked out from available info. There are tools available for use with Google Earth which will provide an elevation profile between two points. Google Earth I've got, the "tools," I don't.