Okay, let's break down what you're asking and the potential steps to convert from a NAVD88 elevation to an Above Finished Floor (AFF) elevation.

Understanding the Terms NAVD88 (North American Vertical Datum of 1988)

This is a standardized reference point for measuring elevations (heights above sea level) in North America. It's like a common "zero" point for all surveyed heights.

Elevation

Simply the height of a point above a vertical datum (like NAVD88). So, a NAVD88 elevation of 13 means that point is 13 units (usually feet or meters) above the NAVD88 datum.

AFF (Above Finished Floor)

This is a relative elevation. It's the height of a point above the finished surface of the floor within a building. The finished floor elevation is the new zero.

Why a Simple Conversion Is Impossible

You can't directly convert a NAVD88 elevation to an AFF elevation without knowing the NAVD88 elevation of the finished floor. AFF is relative to the floor, not to sea level.

Steps to Convert NAVD88 to AFF1. Determine the NAVD88 Elevation of the Finished Floor

This is the most critical step. You need to establish what the NAVD88 elevation is at the point where the finished floor surface sits within your building. There are a few ways to do this:

Surveying

The most accurate method is to hire a surveyor. They can use surveying equipment (total station, GPS, level) to determine the NAVD88 elevation of your finished floor.

Benchmark

Look for nearby benchmarks (established, surveyed points with known NAVD88 elevations). A surveyor can tie your finished floor elevation to a nearby benchmark.

As-Built Drawings

If available, as-built drawings might contain information about the finished floor elevation and its relationship to a known benchmark or datum. However, rely on these with caution, as accuracy can vary.

Construction Documents

If the building is new or recently renovated, construction documents may state a "finished floor elevation" that is referenced to NAVD88 or another datum. Check architectural or structural drawings, especially the "site plan" or "elevations" sheets.2.

Calculate the AFF Elevation

`AFF Elevation = NAVD88 Elevation of the Point - NAVD88 Elevation of Finished Floor`

Example

NAVD88 Elevation of the point: 14 feet NAVD88 Elevation of the Finished Floor: 10 feet AFF Elevation: 14 feet - 10 feet = 4 feet AFF

Important Considerations Units

Make sure all your elevations (NAVD88 and AFF) are in the same units (feet or meters).

Accuracy

The accuracy of your AFF elevation depends entirely on the accuracy of the NAVD88 elevation of the finished floor. A quick, approximate method is fine for some applications, but for critical dimensions, a professional survey is essential.

Floor Variations

Be aware that even within a building, the finished floor elevation might vary slightly from room to room or area to area.

Corrosion

I see you mentioned corrosion. If you are looking at corrosion on something on or near the floor, then understanding the elevation and where it is coming from is important. However, the NAVD88 number won't directly measure it.

In summary, to find the AFF elevation, you need to find the NAVD88 elevation of the Finished Floor and then subtract that value from the point. Flag for review