June 15, 2000
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Bench marks - monuments of the past for future use
An engineer for a local road-construction contractor recently called the U.S. Geological Survey's Hawaiian Volcano Observatory to report his company's disturbance of a bench mark. He realized the significance of this incident and properly reported it to the government agency that would be affected.
What is a bench mark? A bench mark is a metallic disk that is cemented into bedrock or any stable surface and is used as a reference monument by surveyors and geodesists. The monument is about 9 cm (3.5 in.) in diameter with a gentle convex surface defining a high point, usually marked by a cross within a triangle. It is usually made of brass or an aluminum alloy and is stamped with various information.
The information generally found on a bench mark includes the government agency (e.g., U.S. Geological Survey, U.S. Coast & Geodetic Survey, National Geodetic Survey) that set the monument, the name of the site (e.g., Lyman 2, 12YY, HVO87-10, G-20), and the year it was established. It occasionally has the elevation also stamped on it. The oldest bench marks on this island date back to the 1890s.
Bench marks are reference points that provide vertical and/or horizontal information to surveyors and geodesists. As part of a vertical information network, the bench mark's elevation is known relative to a datum, usually mean sea level. For horizontal information, the bench mark's precise location or map coordinates is known relative to two points constituting a baseline.
The elevation of a bench mark is obtained by precise spirit leveling from a tide gage to the bench mark. The first leveling survey on the island was done in 1912, when the elevation of bench marks along a line from Hilo to the summit region of Kilauea was measured. More than 820 bench marks on the island of Hawai`i have known elevations and are elements of the vertical network for the island. Most of the bench marks are on Kilauea and Mauna Loa Volcanoes. The summit elevation of Mauna Loa Volcano is measured by a level line extending from Pier 1 in Hilo to the top of Mauna Loa.
The horizontal control network on the island of Hawai`i was first measured in 1897 by triangulation, a surveying method using angles from two points which are a known distance apart in order to locate a third point. The location of the third point can be calculated relative to the baseline points. The newly located point can then be used with one of the baseline points to find the location of another new point. Eventually, a network of triangles with known locations is created. Such a network provided horizontal control when the topographic map of the island was prepared.
Eventually, the measurement of distances between bench marks replaced the measurement of angles. This method is called trilateration, and the instrument used is a laser-ranging or infrared electronic distance meter. Since 1987 locations have been obtained from measurements with the global positioning system (GPS). HVO monitors the location of 228 bench marks that comprise the horizontal network on the island of Hawai`i.
Changes in the elevation or location of bench marks indicate the deformation of the volcano. The volcano can deform by the movement of magma bodies, by increased gravitational stress, by slope failure, or by a combination of factors. If a bench mark is culturally disturbed between measurements, the change observed can not be interpreted. Therefore, when the construction company disturbed the bench mark along the highway, the history of measurements for that site was broken. Fortunately, we have established a dense network of bench marks in areas where we expect surface deformation, so the loss of a few will not cripple our monitoring. However, if you are planning to bulldoze an area, be sure to check a topographic map to see if there are any bench marks where you plan to raze. The "BMs" on the map are the site of bench marks.
Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the coast near the eastern boundary of Hawai`i Volcanoes National Park. The tube system is now well-established, and surface flows from breakouts of the tube system are rarely seen in the coastal flats these days. Lava is entering the ocean mainly at Waha`ula and at a site 900 meters (1000 yards) to the west. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.
An earthquake was reported felt by residents of Puna and Hilo at 2:35 a.m. on Monday, June 12. The magnitude-3.4 earthquake was located 8 km (4.8 mi) southeast of Mt. View at a depth of 14.6 km (8.8 mi).
The URL of this page is http://hvo.wr.usgs.gov/volcanowatch/archive/2000/00_06_15.html
Updated: 19 June 2000