Before the shoreline survey

Mara delivered the instructions with brisk certainty, dividing the beach into sections and assuring us that the boundary markers would be easy to follow. Once we reached the sand, however, she studied the faded survey sheet in silence. Two posts had disappeared since the previous visit, and she admitted that we would have to judge where one section ended and the next began.

From the passage, select one word or phrase that presents Mara as confident at the start. Explain how the later details qualify this impression.