Mark is 10 days into his cross-Mars road-trip and has just reached the entrance to Mawrth Vallis. It's been smooth sailing so far, but, as Venkat explains on The Mark Watney Report, he'll reach the edge of the dust storm "twelve days from now" (22.20).
Finally, Mark emerges from Mawrth Vallis into a large craterous region called Arabia Terra. Luckily, he brought along some makeshift "sixteenth-century" navigational tools that should give him a fighting chance.
By now, Mark is "already fifty kilometers into the storm," though he still has no idea (22.69). He has a lot on his plate, though, so it's understandable—he's currently trying to drive directly south of a giant crater dubbed Marth.
However, Mark has realized that his batteries are holding a tiny bit less charge. Regardless, he's putting the pedal to metal and praying that he's aimed towards the south end of the giant crater.
Well, guess what—he miscalculated his route and ended up hitting the crater dead in the face. This thing is massive—about 100 kilometers wide—so he's unsure which direction provides the most efficient route.
After making some calculations, Mark realizes that he "hit Marth Crater dead-on," which is just about the worst thing that could've happened (22.94). Although this might seem insignificant, determining the closest route will literally be a matter of life and death.
As he's doing that, however, he finally realizes that he's in the middle of a dust storm.