If you hike the numerous public trails throughout California, you may have come across a survey monument, especially if you like geocaching.

The metallic geodetic monuments mark key survey points — for example, mountain tops.

Survey monuments are mostly 3-inch cast metal disks affixed to the tops of pipes that have been pounded into the ground or fastened on large rocks or concrete pillars. A legend is usually stamped on their faces.

The monuments give surveyors a point of reference to make field measurements on or near the surface of the Earth to determine topographical locations. The positions are crucial to determining property lines or constructing roads or bridges.

In the United States, there are approximately 240,000 survey monuments that have been installed over hundreds of years. Many in California are more than a century old.

Some of these monuments called “benchmarks” indicate elevations above or below sea level.

The mean sea level, or MSL, reference datum for these benchmarks in North America was derived from 19 years of tide gauge observation at 26 locations on the Pacific Coast, the Gulf of Mexico and Atlantic shorelines in 1926 and was updated in 1988.

Over the ocean, satellites launched from Vandenberg Air Force Base — Jason 1 in 2001, Jason 2 in 2008 and Jason 3 in 2016 — have been measuring sea level.

“These satellites give us a global view of our changing oceans with such exquisite accuracy that even the yearly rise and fall of global sea level is visible, caused by the transfer of water to and from the continents in the form of rain and river runoff,” said climate scientist Josh Willis of NASA’s Jet Propulsion Laboratory in Pasadena.

The satellites measure sea level using an extremely precise radar altimeter that transmits radio waves to the ocean’s surface and measures the time it takes for them to bounce back.

This technique has proven to be particularly precise and accurate. In fact, Jason 3 has a goal to measure sea-level variations of 1 inch.

Over the land, in the quest to further refine and determine elevations above sea level, surveyors turned to LIDAR, which stands for light detection and ranging equipment.

LIDAR is deployed on aircraft and, combined with land-based laser gear, can precisely measure the heights of our lands and verify the elevation of our survey benchmark monuments throughout the county to less than an inch.

Due to the high expense of the programs, LIDAR measurements have been mostly limited to the United States, Canada, Europe, Australia and Japan.

Outside those areas, governments have typically relied on NASA’s Shuttle Radar Topography Mission, or SRTM, to measure elevation.

Unfortunately, SRTM measurements were found to overestimate land elevations, and here’s why:

According to a report published by Climate Central titled “Flooded Future: Global vulnerability to sea-level rise worse than previously understood,” the SRTM data measured the tops of features that protrude from the ground — like buildings and trees — as well as the ground itself.

As a result, SRTM data generally overestimated elevation, particularly in densely forested and built-up areas.

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Globally, the average overestimate appears to be roughly 6 feet, or about two meters. Those values match or exceed most of the highest sea-level rise projections for the entire century.

In other words, the coastal areas of Asia will experience a great deal more flooding due to rising sea levels than previously imagined.

In fact, in countries like Bangladesh, China, India and Vietnam, an additional 157 million people could be affected by 2050 due to sea-level rise.

Globally, it could mean another 221 million people may be displaced than thought initially, or a total of about 300 million — nearly the entire population of the United States.

Willis told me about 40 percent of the recent sea-level increase is due to thermal expansion of the ocean, 30 percent from land glaciers and 30 percent from the melting of the ice sheets in Greenland and Antarctica.

In the future, not only will the rate of sea-level rise increase, but the main contributor will be the melting of the ice sheets.

Today, about 5 million tons of carbon dioxide is dumped into the atmosphere every hour from the burning of fossil fuels.

To learn what you can do to reduce your carbon footprint, visit Pacific Gas and Electric Co.’s website at www.pge.com.

Other excellent sites are Dr. Ray Weymann’s Central Coast Climate Science Education webpage at www.centralcoastclimatescience.org/ and Climate Central at https://climatecentral.org.

John Lindsey is Pacific Gas and Electric Co.’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.


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