Iwakuni's Legacy

July 5, 2011
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Nearly 340 years ago, an exceptional team of engineers, mathematicians and surveyors in Japan joined forces to produce an architectural marvel--the wooden Kintai Bridge in Iwakuni.Originally built as a gateway to the Iwakuni Castle across the Nishiki River, the bridge is the oldest of its type in the country and is viewed as one of the most important historic properties in Japan.



Nearly 340 years ago, an exceptional team of engineers, mathematicians and surveyors in Japan joined forces to produce an architectural marvel--the wooden Kintai Bridge in Iwakuni.

Originally built as a gateway to the Iwakuni Castle across the Nishiki River, the bridge is the oldest of its type in the country and is viewed as one of the most important historic properties in Japan. With its six abutments, four of which are set in 115-foot (35.1 meter) interspaces, the Kintai Bridge has the longest span of any historic wooden bridge in the world. Its five, sequential wooden arches cover 634 feet (193.3 meters).

A colorized point cloud of the bridge.

The architectural achievement, combined with the age of the structure, was recognized by the Japanese government in 1922 as one of only three historic bridges in the country to be designated a National Treasure.

Floods have taken their toll on the graceful structure over the years. In fact, the bridge has been rebuilt three times, most recently in 2005. However, each reconstruction has carefully mirrored the original craftsmanship, ensuring that the bridge remains a historic masterpiece.

Today, the bridge continues to draw tourists to Iwakuni, a city with a population of more than 140,000. City officials recently organized a steering committee to promote the uniqueness of the bridge with the goal of having it named a World Heritage Site by the United Nations Educational, Scientific and Cultural Organization (UNESCO). As part of their efforts, they wanted to obtain an accurate as-built record that would provide precise positioning and scientific data on the bridge.

The Daiken Sokuryo Engineer Co., a consulting, survey and design services company headquartered in Kitayushu, was chosen to handle the survey work. “The committee wanted to create a 3D computer graphic so the details of the entire bridge could be seen from all directions,” explains Toshihko Kabashima, company director.

A photograph of Kintai Bridge with the Topcon GLS-1000 in the foreground.

The company used a Topcon GLS-1000 laser scanner to capture the 3D as-built data. The scans were done at a 1-centimeter pitch at a distance of 98 feet (30 meters). According to Kabashima, the results were outstanding. “We took scans of the entire bridge from 22 different locations in just two days,” he says. “We started at 5 a.m. each day and ended at 9 a.m. to avoid the heavy pedestrian traffic in the area. It only took a total of eight hours for the entire scan. The speed at which we completed the job could never have been achieved without using the laser scanning technology.” 

Kabashima says the resulting point clouds and photos were exactly what city officials needed. “The committee highly appreciated the point cloud data we presented; the density and cleanness exceeded their expectations.”

The rebuilt structure is expected to be able to withstand future floods. But if something does happen to the beloved bridge, the 3D record will live on.

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