JOINT PROJECT AGREEMENT CONCERNING THE NATIONAL SPATIAL REFERENCE SYSTEM IN CALIFORNIA

Yehuda Bock (SIO)

Link to NOAA Strategic Plan: NOAA's Mission Goal 4: Support the Nation’s Commerce with Information for Safe, Efficient, and Environmentally Sound Transportation

RESEARCH OBJECTIVES AND SPECIFIC PLANS TO ACHIEVE THEM

NOAA’s National Geodetic Survey (NGS) and the California Spatial Reference Center (CSRC) have joined in partnership for the purpose of researching precise spatial referencing and height modernization for the public good. Although focused on California, our goal is to contribute directly to the development by the NGS of public guidelines and procedures for other states and entities interested in implementing height modernization and spatial reference systems. The specific objectives of the project are to research and implement the scientific and infrastructure basis for the California Spatial Reference System (CSRS). There are several outstanding research questions related to spatial referencing that are being addressed:

(1)            What is the proper observation mix to maintain a modern height network within a spatial referencing environment, and how should these measurements be optimally combined? Observation types include continuous GPS (CGPS), field GPS surveys at passive monuments, spirit leveling, and gravity surveys.

(2)            What is the proper mix of geoid models and local corrector surfaces, in converting from GPS-determined geodetic heights to orthometric heights?

(3)            Can we apply and enhance modern IT methods to provide timely access to height modernization and spatial referencing information?

(4)            How does one develop and implement a precise GIS for the purposes of height modernization and spatial referencing?

(5)            How can real-time (RT) CGPS networks, such as those being created in California, be best used to directly support height modernization and spatial referencing?

The CSRC has been established to achieve the above research objectives. The R&D and operational arm of the center is located at Scripps, and leverages the resources of the Scripps Orbit and Permanent Array Center (SOPAC). The CSRC also consists of an Executive Committee, Coordinating Council, and a user community, organized as a UCSD Support Group. Along with our sponsors at NGS, the Exec. Committee provides advice on the research direction, the relevant civil applications, and the allocation of resources. The overall success and management of the project is the responsibility of the PI.

RESEARCH ACCOMPLISHMENTS

One of the primary problems in maintaining an accurate and consistent reference frame (geodetic datum) in California is the presence of significant tectonic motion and related medium to large earthquakes, which cause significant crustal deformation. NOAA/NGS provides reference epochs to which station coordinates throughout the nation are referred. CSRC developed 2004.0 coordinates for all continuous GPS sites in California (the previous epoch was 1991.35), and later epoch 2007.0 which has been incorporated by NGS in their national readjustment of NAD83 (2007.0). The problem in California is being able to refer true-of-date survey coordinates to the reference epoch. This requires a crustal deformation model. This year we developed a crustal motion model for California (Figure 1) using the DEFNODE software (http://www.rpi.edu/~mccafr/defnode/). DEFNODE models elastic lithospheric block rotations and strains, and locking or coseismic slip on block-bounding faults. Block motions are specified by spherical Earth angular velocities (Euler rotation poles) and interseismic backslip is applied along faults that separate blocks, based on the routines of Okada (1985; 1992). Coordinates of nodes along the fault plane specify the fault model. The parameters are estimated by simulated annealing or grid search. The input to the model is the set of velocity vectors computed by SOPAC for all continuous GPS stations in California. The final product is an on-the-fly, Web-based tool that allows conversion of coordinates between epoch dates. The tool is fully integrated into the SOPAC/CSRC data portal and accessible through Web Services. CSRC plans to maintain and update these tools/models on a regular basis as new data are collected and analyzed, as well as react quickly to irregular but anticipated events such as large earthquakes.

We analyzed and archived observations of the Northern San Joaquin Valley Project 2006 and the Southern California Height Modernization Project 2006, as part of the CSRC’s Master Plan to maintain the California Spatial Reference System (CSRS). The Southern California project researched the use of real-time networks for conducting height modernization surveys. Some advantages of this approach are the use of uncoordinated individual survey crews, rather than a time coordinated survey by teams of surveyors, and the ability to generate results in near-real-time, rather than require a time-consuming network adjustment. We used the second generation Pocket GPS Manager (PGM), a modern server/client application for planning, executing, analyzing, visualizing and archiving geodetic control and height modernization, to manage these projects. PGM was also used to manage a large NGS height modernization project in Louisiana, in response to hurricane Katrina.

We have incorporated into the SOPAC/CSRC archive this past year data and metadata from the growing California Real Time Network (CRTN) (http://sopac.ucsd.edu/projects/realtime/). This year we expanded CRTN into the Imperial Valley (Figure 2) with funding from NASA and in collaboration with UNAVCO. We also continued to integrate all new continuous GPS stations of Earthscope’s Plate Boundary Observatory (PBO). The continuous GPS stations make up the backbone of the CSRS. The CRTN stations allow us to monitor crustal motions and react to significant seismic events in virtually real-time.

 

Fig. 1 Crustal deformation model developed by the CSRC for California. The velocity grid is given with respect to a North America fixed reference frame and shows the transition from North America to Pacific plate motion, a total of almost 50 mm/yr. The sharpest transition is in the Imperial Valley in southern California. The blue lines depict the block model.

 

Fig. 2 Map of continuous GPS stations in the Imperial Valley (blue circles) that have been upgraded to real-time (less than 1 s latency) high-rate (1 Hz) operations. White dots are stations planning to be upgraded. Other continuous GPS stations are denoted by white triangles. The velocity vectors are with respect to a fixed North America reference frame, and illustrate the sharp transition between North America and Pacific plates. The graphic was generated with the Scripps Online Map Interface (SOMI) application, embedded in the CRTN Web Page.