Static Heights in a Sinking World: The USACE Epoch 2009.55 Methodology

In high-precision geodetic surveying, the ground is rarely as stable as it appears. For the U.S. Army Corps of Engineers (USACE), particularly within the New Orleans District (MVN), the phenomenon of regional subsidence creates a "moving target" for vertical control. To combat the confusion caused by benchmarks that sink at different rates, the USACE utilizes a Time-Locked Epoch—specifically the 2009.55 Epoch. This methodology freezes the published elevations of control points to a specific moment in time (mid-2009), providing a consistent baseline for engineering and flood protection projects. Without this temporal lock, comparing a levee survey from 2015 to one in 2026 would be impossible, as the entire landscape (including the benchmarks) has physically shifted downward.

Table of Content

• Purpose: The Necessity of Time-Locked Data
• Reasoning: Why 2009.55?
• Step-by-Step: Implementing the 2009.55 Methodology
• Use Case: Levee Performance Monitoring
• Best Results: Avoiding "Datum Drift"
• FAQ
• Disclaimer

Purpose

The 2009.55 Epoch serves as a "Geodetic Snapshot." Its primary purposes include:

• Structural Comparison: Allowing engineers to determine if a levee has settled relative to its design grade, independent of whether the underlying tectonic plate has subsided.
• Consistency Across Contracts: Ensuring that multiple survey contractors working on the same hurricane protection system are "keyed-in" to the exact same vertical values.
• Standardization: Providing a unified reference for the North American Vertical Datum of 1988 (NAVD88) in dynamic coastal environments.

Reasoning: Why 2009.55?

The choice of the 2009.55 epoch is not arbitrary. Following Hurricane Katrina, a massive effort was undertaken to redefine the vertical control network in Southern Louisiana. The 2009.55 adjustment represented the most comprehensive update to the National Spatial Reference System (NSRS) at that time, incorporating new GPS observations and leveling data that accounted for the rapid subsidence observed between 2004 and 2009.

By "locking" to 2009.55, the USACE effectively treats the benchmarks as if they have stopped sinking. When a surveyor occupies a monument in 2026, they do not use the current (subsided) elevation; they force the equipment to accept the published 2009.55 value. This "arbitrary" setting ensures that all measured data remains relative to the original post-Katrina design specifications.

Step-by-Step: Implementing the 2009.55 Methodology

1. Identify Published 2009.55 Control

Consult the USACE MVN Survey Section or the SONRIS database to find benchmarks with authorized 2009.55 orthometric heights. Do not use standard NGS datasheets if they have been updated to a newer epoch (e.g., 2010.00) unless a transformation is applied.

2. Calibrate the Base Station (Key-In)

When setting up a GNSS base station or performing static observations, manually "key-in" the published 2009.55 elevation for the monument.

Example: 
Monument: BH1234
Actual 2026 GPS Elevation: 2.15m
Published 2009.55 Elevation: 2.30m
Action: Set base to 2.30m

3. Perform the Survey

Conduct the topographic or hydrographic survey as usual. All collected points will now be "shifted" upward to match the 2009.55 vertical framework.

4. Metadata Documentation

Crucially, label all deliverables with the specific epoch. In GEE or ArcGIS Pro, add an attribute field for V_DATUM and EPOCH to ensure future analysts know the data is time-locked.

Use Case: Levee Performance Monitoring

An engineering firm is tasked with checking the "armoring" on a 5-mile stretch of levee in Plaquemines Parish.

• The Challenge: The benchmarks in the area have subsided 0.15ft since the levee was built. If the surveyor uses "current" elevations, the levee will appear 0.15ft lower than it actually is relative to the structures.
• The Solution: By using the 2009.55 Epoch, the surveyor "ignores" the regional sinking.
• The Result: The final report accurately shows that the levee crown is still at the required 15.0ft elevation relative to the original benchmarks, confirming the structure has not settled internally.

Best Results

FAQ

Why can't I just use a computer program to convert to 2009.55?

Subsidence in Louisiana is non-linear and varies by city block. Software like CORPSCON or NCAT can handle horizontal datum shifts well, but vertical "epoch" shifts in subsiding areas require re-occupying physical benchmarks to determine the current localized offset.

What happens if the benchmark I'm using is destroyed?

You must find the nearest "Order 1" or "Order 2" 2009.55 monument and bring the elevation forward using differential leveling or high-precision static GPS, maintaining the 2009.55 value from the source mark.

Is 2009.55 used outside of Louisiana?

While the concept of "epochs" is used globally, the specific 2009.55 designation is primarily a standard of the USACE New Orleans District. Other districts may use different epochs (e.g., 2004.65) depending on local tectonic activity.

Disclaimer

Surveying in subsiding regions is technically complex. The use of the 2009.55 epoch is a specific policy requirement for USACE MVN projects and may not be appropriate for general property surveys or FEMA flood insurance rate maps (FIRMs), which may use different standards. Always refer to the latest EM 1110-1-1005 and project-specific Scope of Work (SOW). March 2026.

Tags: USACE_Survey, Geodetic_Epoch, NAVD88, Subsidence_Mapping