The total area of the Russian-held islands off Hokkaido claimed by Japan has shrunk by about 33 sq. km — thanks to more accurate mapping techniques by the Geospatial Information Authority of Japan (GSI).

The annual survey by the GSI, released Friday, found that the area of the islands — called the Northern Territories in Japan and Southern Kurils in Russia — was 5,003.05 sq. km as of Oct. 1, compared with 5,036.14 sq. km a year ago.

The islands comprise Etorofu, Kunashiri and Shikotan and the Habomai group of islets.

The decrease can be attributed to the introduction of a new topographic map based on satellite images, which enabled the GSI to measure the area more accurately, according to an official of the institution attached to the Land, Infrastructure, Transport and Tourism Ministry.

Until the 2013 survey, the GSI had measured the islands using a 1:50,000 scale map created in 1922, Nobuyuki Watanabe, a section chief in the National Mapping Department, told The Japan Times on Tuesday.

"Using images from the Japanese satellite Daichi, we completed creating a topographic map of the northern areas with a scale of 1:25,000 in February 2014," Watanabe said.

The GSI has utilized a topographic map with a scale of 1:25,000 for surveying areas in other parts of Japan.

The islands were seized by the former Soviet Union at the end of World War II. The institute has been unable to conduct an aerial survey.

The area of Etorofu, the largest of the disputed islands, is now registered as 3,166.64 sq. km, down from 3,182.65 sq. km, while that of Kunashiri was recalculated at 1,489.27 sq. km, down from 1,498.56 sq. km.

Shikotan's area is calculated at 247.65 sq. km, reduced from 250.16 sq. km. The Habomai islets now total 94.84 sq. km, down from 99.94 sq. km.