Final answer:
Without additional information about which contour line is labeled, the elevation of the index contour cannot be determined solely from a 5ft contour interval.
Step-by-step explanation:
If the contour interval is 5ft, the index contour cannot be determined with only that information. The index contour is typically a contour line that is thicker or more prominent on a map and is used as a reference point to count contours up or down from to determine elevation. The index contour typically represents a multiple of the contour interval such as 25ft, 50ft, 100ft, etc., depending on the map. So, without further information about which contour line is labeled or highlighted as the index contour, we cannot determine the elevation of the index contour just from knowing the contour interval is 5ft.