What is the required vertical accuracy for passing NMAS?

The requirement for vertical accuracy to pass the National Map Accuracy Standards (NMAS) is established to ensure that the elevation data represented on a map is reliable. The standard specifies that vertical accuracy should be at least half of the contour interval, with a 90% confidence level for accuracy.

This means that when measuring the vertical accuracy of the dataset, 90% of the points should fall within the specified vertical tolerance, which is calculated as half of the contour interval. For example, if the contour interval is 20 feet, the vertical accuracy must be within ±10 feet for 90% of the sampled data points. This rigorous standard is designed to maintain map quality and reliability for various applications, including land use planning, engineering, and environmental assessment.

Other options do not meet the NMAS requirements. For instance, suggesting a smaller fraction of the contour interval, like a third or a quarter, would not ensure the level of precision necessary for most applications. Equally, stating that the full contour interval must be the standard does not reflect the allowance made in NMAS for some variability within the accuracy assessment.