How is the working height of an aerial ladder measured?

The working height of an aerial ladder is determined by measuring the distance from the ground to the highest ladder rung that can safely support personnel or equipment in operation. This measurement provides a practical understanding of how high the ladder can reach relative to the surface below, ensuring that firefighters and rescue personnel can effectively access structures or perform rescues at greater heights.

When considering the other options, measuring from the height of the vehicle chassis does not provide an accurate representation of the ladder's reach, as it doesn’t account for the actual extension of the ladder. Similarly, measuring from the base to the lowest rung ignores the functional aspect of the ladder's height in terms of operations and access points. The maximum extension of the ladder could be misleading if not aligned with the usable height, as factors such as angle of deployment can affect the effective working height. Therefore, focusing on the height to the highest rung is essential for proper operational planning and safety in aerial operations.