On IAP Charts, the height of obstructions is always depicted in what measurement?

The height of obstructions depicted on Instrument Approach Procedure (IAP) charts is consistently represented in relation to Mean Sea Level (MSL). This standard measurement ensures a uniform reference point that air traffic controllers and pilots can rely upon for safety and navigation.

Using MSL as a reference for obstructions helps pilots during approaches and landings by providing a clear understanding of how high an obstacle is compared to a widely recognized and stable datum—mean sea level. This is crucial because it allows for consistency across all charts and navigational aids, reducing confusion and potential errors when flying in various terrains and regions.

In contrast, other measurements like feet above ground level would vary depending on the elevation of the terrain underneath the obstruction. Similarly, above sea level does not specify a uniform reference, as the term "sea level" could vary slightly based on local geographic conditions. Local elevation would also not provide a standardized frame of reference and would change based on where the measurement is taken. By utilizing MSL, IAP charts maintain clarity and safety for all pilots navigating the airspace.