Preface

Further review of NOAA Technical Report 50: Rates of Vertical Displacement at Benchmarks in the Lower Mississippi Valley and the Northern Gulf Coast supported clarification of the following:

1. The rates and computed elevations in the study area covered by this report were derived through the analysis of leveling projects in the National Geodetic Survey (NGS) database observed between 1920 and 1995. As such, it is important to note that the vertical displacement rates in NOAA Technical Report 50 may not reflect the current rate of subsidence. Present-day surveys and accurate GPS measurements, including ties to Continuously Operating Reference Stations (CORS), must be used to validate the rates before attempting to determine elevations.
   
2. The original leveling observations used to compute subsidence rates in NOAA Technical Report 50 were observed according to NGS procedures and specifications for the specified order of accuracy and class of survey. It is important to remember that the leveling network is comprised of a multitude of level lines adjusted to minimize errors while tying junctions together.
   
3. The National Ocean Service tide gauge data used in the analysis measures relative sea level rise at a specific location and also depicts relative subsidence within an area compared to a fixed water level datum. It should be pointed out that factors such as number of years of tide gauge records, short-term secular variation in water levels, and differences in decadal water level trends which are not related to the subsidence rates might be areas of additional study that would prove useful to this study.

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NOAA Technical Report 50: Rates of Vertical Displacement at Benchmarks in the Lower Mississippi Valley and the Northern Gulf Coast PDF

NOTE: APPENDIX 3 – TABLE OF RATES, ELEVATIONS, AND POSITIONS

Rates and computed elevations for benchmarks in the subsidence network listed in APPENDIX 3 – Table of Rates, Elevations, and Positions are neither final nor publishable for vertical control. The subsidence rates have been analyzed and validated for the base years spanned by the historic leveling projects. While elevations derived using these rates are likely better than the heights currently in the National Spatial Reference System, there is no guarantee that the subsidence rates are constant over time. Therefore, vertical velocities must be validated independently when used to extrapolate elevations into the future.

(Note: The following paragraph is a summary of how the Technical Report fits into the Vertical Time Dependent Positioning plan.)

NOAA Technical Report 50: Rates of Vertical Displacement at Benchmarks in the Lower Mississippi Valley and the Northern Gulf Coast is the first step in the development and implementation of Vertical Time-Dependent Positioning (VTDP) plan. A joint effort between the Louisiana Spatial Reference Center (LSRC) at Louisiana State University (LSU) and NOAA’s National Geodetic Survey (NGS), the VTDP plan will estimate and update NAVD 88 heights on a number of benchmarks. These benchmarks, along with supporting programs and documentation, will provide the vertical control for survey work in subsidence areas. Derived from the modeling of empirical data from historic leveling projects, the geoid and GPS observations, the VTDP plan is an iterative approach to modeling and defining vertical control in the dynamic state of Louisiana.