GNSS receivers track port movements with CORS corrections

The largest seaportS on America’s West Coast are the Ports of Long Beach and Los Angeles, located next to each other in San Pedro, California. (Photo: Art Wager/E+/Getty Images)

The Port of Long Beach, California, is moving up and down because it sits on fault blocks that move like pistons due to subsidence caused by oil extraction. To accurately keep track of these movements, the port’s surveyors use GNSS receivers that receive corrections from continuously operating reference stations (CORS) operated by the port and by the City of Long Beach.

CORS corrections compensate for errors inherent in GPS — clock drift, orbit errors, signal errors and atmospheric errors.

Monitoring Subsidence. A monitoring receiver is placed on each fault block’s anticline, said Kim Holtz, director of survey for the Port of Long Beach. Her agency has 15 stations, along the coast, and a couple in the Port of Los Angeles. They were installed originally in the 1990s, using Trimble 5700s. “We are constantly monitoring to make sure that the fault blocks are not moving too much and that they are not moving horizontally other than all together, as the plates move to the north,” Holtz said.

Also, the Long Beach Energy Resources Department has 14 Trimble R9 base stations. While Energy Resources uses the equipment to get precise elevation differences and measure subsidence for movement of more than 0.025 feet, the port uses them mainly for horizontal measurements for construction.

The port’s hydrographic survey boat, the pilot boats, and the dig alert crew that marks utilities for construction operations also use the receivers to tie into the CORS network. “The stations are about eight or nine years old and Energy Resources is getting ready to replace all of them with Trimble Alloy GNSS reference receivers, over a three-year period,” Holtz said.

Digital Level Run. The port normally performs a digital level run from a tidal wave base station in San Pedro, which dates to the 1920s. “We run a level run from that and, at the same time, Energy Resources does a GPS subsidence survey, where they get elevation,” Holtz explained. “Last year, we combined the two surveys, to compare the data and see whether we could use some of their GPS data for our level run. It was very promising. We are going to do it again in November.

“Then, if it works, we will cut our level run, which normally takes two months, down to about a week or two. We will just come off of the main benchmarks on which Energy Resources puts a GPS elevation.”

To keep the elevations tight, more than 10 years ago Long Beach created its own geoid. “It is a hybrid of GEOID12B, and we’ve updated it a couple of times,” Holtz said.