Procedures for Connecting SET Bench Marks to the NSRS: Establishing GNSS-Derived Ellipsoid/Ortheometric Heights on Surface Evaluation Table Bench Marks

摘要

These guidelines were developed in response to a growing interest in accurately measuring local sea level rise in relation to coastal land elevation. Central to this task is obtaining accurate and precise connections between wetland surface elevation and local water levels. For over a decade, Surface Elevation Tables (SETs) have been used to measure and monitor coastal wetland surface elevation with respect to an in situ vertical bench mark. However, these bench marks, often located in remote coastal settings, have traditionally been isolated from consistent spatial reference. These guidelines were developed to provide the necessary information to bring ellipsoidal heights to SET bench marks via Global Navigation Satellite System (GNSS) technology. Once vertical control has been established in a coastal wetland setting, other coastal observing systems can likewise be connected, such as local water level recorders, permanent vegetation monitoring plots, etc. Furthermore, by connecting SET bench marks to a consistent vertical framework, the vertical velocity of the bench mark can be measured over time, providing information on local rates of deep subsidence. Bringing accurate and precise elevations to remote coastal wetland settings is fraught with difficulties and logistical constraints. Surveying has traditionally avoided the typically unstable, unconsolidated, and very dynamic wetland soils. Survey methods have to be adapted to this environment, also taking into consideration minimizing impacts to the sediment surface which is being measured. This document draws on extensive field trials conducted at SET sites distributed across southeastern Louisiana, in collaboration with the University of New Orleans, the US Geological Survey, and the Louisiana Department of Natural Resources.