> ## Documentation Index
> Fetch the complete documentation index at: https://withforerunner.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Approximate BFE tool

> Estimate Base Flood Elevations between FIS cross sections for riverine properties

In riverine communities without automated Base Flood Elevations (BFEs) on their FIRMs, determining a property's BFE typically requires manually reading the Flood Insurance Study (FIS) profile. The Approximate BFE tool calculates an estimated BFE directly in Forerunner by interpolating between two FIS cross sections.

<Note>
  Approximate BFE values are estimates. They are not the official BFE for the property. Always refer to the associated Flood Insurance Study (FIS) to verify any value used for regulatory or compliance purposes.
</Note>

## When to use the tool

Use the Approximate BFE tool when:

* The property is in a riverine Special Flood Hazard Area without a published BFE on the FIRM
* The relevant FIS includes a profile baseline and cross sections with BFE values
* You need a working estimate to support floodplain review, permitting, or resident communication

The tool will not produce a result if the surrounding cross sections do not have BFE values, since there is not enough information to interpolate.

<Note>
  When BFEs are already published for a zone, the estimation tool does not apply. Forerunner relies strictly on FEMA's mapped values and boundaries for zones with published BFE data.
</Note>

## How the tool calculates an approximate BFE

Forerunner uses the property's location and the FIS profile data to estimate a BFE:

<Steps>
  <Step title="Locate the property on the profile baseline">
    The tool projects the property's latitude and longitude onto the nearest point on the profile baseline between two selected cross sections.
  </Step>

  <Step title="Measure the property's relative position">
    It calculates how far along the baseline the property sits between the upstream and downstream cross sections.
  </Step>

  <Step title="Interpolate between the two cross-section BFEs">
    It linearly interpolates between the BFE values at the two cross sections, weighted by the property's relative distance along the baseline.
  </Step>
</Steps>

The result is an estimated BFE for the property's location along the reach.

## Using the tool

<Steps>
  <Step title="Open a property without a published BFE">
    Navigate to a property in a riverine SFHA where no BFE is published on the FIRM.
  </Step>

  <Step title="Start the Approximate BFE tool">
    Click **Add** in the BFE row on the property and select **Use approximate BFE tool**. This opens a map interface for the calculation.
  </Step>

  <Step title="Select two cross sections">
    Pick two cross sections with a continuous profile baseline running through the property—one upstream and one downstream of the property.
  </Step>

  <Step title="Review and save the estimated value">
    Forerunner calculates the interpolated BFE based on the distance between the two cross sections and their elevation values, and saves the result to the property with an **approximate** flag to distinguish it from published FEMA values.
  </Step>
</Steps>

## Updating an approximate BFE manually

If you have calculated a more precise BFE using the FIS profile directly, a raster dataset, or another engineering method, you can override the tool's value and store your own calculated BFE on the property.

## Limitations

* **Estimates, not official values.** Approximate BFEs are not a substitute for the official BFE. For regulatory determinations, always reference the FIS.
* **Requires cross sections with BFE values.** If the bracketing cross sections do not have BFE values on the FIS profile, the tool cannot interpolate.
* **Linear interpolation between cross sections.** The tool assumes a straight-line change in BFE between two cross sections along the profile baseline. Where the FIS profile is highly non-linear between two cross sections, the interpolated value will diverge from the value you would read directly off the FIS profile.

<Tip>
  For the most accurate result, select the closest cross sections that bracket the property, rather than cross sections farther up or down the reach.
</Tip>