MISSISSIPPI HEIGHT MODERNIZATION PROJECT JUNE 11, 2009 By Ronnie L. Taylor Chief, Geodetic Advisor Branch NOAA, National Geodetic Survey

The existing vertical reference network has been rendered unreliable by the effects of continuing regional subsidence. This subsidence affects the entire Mississippi Gulf Coast region, and much of the remaining areas of Mississippi as well. By contrast, the region around Pensacola, Florida, has been shown to have been relatively stable vertically during the 20 th century. Geodetic leveling is the only survey method that is sufficiently accurate to restore credible, current, and precise NAVD 88 elevation values to southern Mississippi. Once restored, the future accuracy of the re-established elevation reference network can be maintained by monitoring changes in ellipsoid heights on survey monuments in the region, and then applying those changes to the elevation (orthometric height) values. This future monitoring can be preformed quickly and cost- effectively using Global Positioning System technology.

The purpose of this project is to improve and modernize the vertical geodetic control network, based on the North American Vertical Datum of 1988 (NAVD 88), to support MDOT’s deployment of significant survey technology improvements. The project consists of three parts: Geodetic leveling: The Geodetic leveling portion shall meet the National Geodetic Survey (NGS) standards for first order, class two geodetic leveling as defined in the Standards and Specifications for Geodetic Control Networks, September Global Positioning System (GPS) observations: The GPS observations portion shall consist of static GPS observations that meet the standards for B order and 1 st order surveys as defined in Geometric Geodetic Accuracy Standards and Specifications for GPS Relative Positioning Techniques, August Processing and adjusting a GPS survey network: The results of the vector processing and network adjustment portion will conform to Input Formats and Specifications of the National Geodetic Survey Data Base.

Before you build inspect your foundation. D E F I N I T I O N

The purpose of this project is to improve and modernize the vertical geodetic control network, based on the North American Vertical Datum of 1988 (NAVD 88), to support MDOT’s deployment of significant survey technology improvements. The project consists of three parts: Geodetic leveling: The Geodetic leveling portion shall meet the National Geodetic Survey (NGS) standards for first order, class two geodetic leveling as defined in the Standards and Specifications for Geodetic Control Networks, September 1984.

The purpose of this project is to improve and modernize the vertical geodetic control network, based on the North American Vertical Datum of 1988 (NAVD 88), to support MDOT’s deployment of significant survey technology improvements. The project consists of three parts: Global Positioning System (GPS) observations: The GPS observations portion shall consist of static GPS observations that meet the standards for B order and 1 st order surveys as defined in Geometric Geodetic Accuracy Standards and Specifications for GPS Relative Positioning Techniques, August 1989.

Final GPS Network Survey Observations Complete Processing Data Now

The purpose of this project is to improve and modernize the vertical geodetic control network, based on the North American Vertical Datum of 1988 (NAVD 88), to support MDOT’s deployment of significant survey technology improvements. The project consists of three parts: Processing and adjusting a GPS survey network: The results of the vector processing and network adjustment portion will conform to Input Formats and Specifications of the National Geodetic Survey Data Base.

GPS ON BENCH MARKS Part of this project shall complete static GPS observations on a subset of the leveled benchmarks. The GPS project shall follow the standards and specifications for B order and 1 st order surveys as described in the relevant documents. The spatial extent of this GPS survey is limited to the entire portion of the new leveling conducted within Mississippi and to the lines connecting points within Mississippi to the junction point in Mobile, Alabama (e.g. M 406).

The static observing plan shall include all of the following elements: at least two independent 3-hour ties between adjacent points along the line, whether primary to intermediate, or between intermediates; at least one independent 6-hour tie directly between adjacent primary points along the line; at least three independent 6-hour ties between primary points and a HARN station or other high-order control (e.g. USACE Height Modernization) from the 2007 national re-adjustment (points to be specified by MDOT). The approach to the static GPS ties is illustrated in Figure 3. Repeat observations of individual vectors shall be made at different times of the day, as specified by MDOT.

Figure 3, schematic of static GPS ties configuration.

VERTICAL DATUMS What types of heights are there? Orthometric heights Ellipsoid heights Geoid heights

VERTICAL DATUMS Local/Regional –Assumed –City, County –International Great Lakes Datum 1955 (IGLD55) –International Great Lakes Datum 1985 (IGLD85) National –Sea Level Datum 1929 (SLD29) –National Geodetic Vertical Datum of 1929 (NGVD29) (As of July 2, 1973) –North American Vertical Datum of 1988 (NAVD88) (As of June 24, 1993) –Tidal Datums

NATIONAL SPATIAL REFERENCE SYSTEM (NSRS) The National Spatial Reference System (NSRS) is the name given to all geodetic control contained in the National Geodetic Survey (NGS) Data Base. This includes: A, B, First, Second and Third-Order horizontal and vertical control, Geoid models such as GEOID 99, precise GPS orbits and Continuously Operating Reference Stations (CORS), observed by NGS as well as data submitted by other Federal, State, and local agencies, academic institutions and the private sector

NATIONAL SPATIAL REFERENCE SYSTEM (NSRS) ACCURATE -- CM accuracy on a global scale MULTIPURPOSE -- Supports Geodesy, Geophysics, Land Surveying, Navigation, Mapping, Charting and GIS activities and derived products ACTIVE -- Accessible through Continuously Operating Reference Stations (CORS) INTEGRATED -- Related to International services and standards (e.g. International Earth Rotation and Reference System Service, International GPS Service etc.)