Prime Minister Hun Sen and Vietnamese Prime Minister Nguyen Xuan Phuc agreed yesterday to write to the French government to request expert assistance in copying colonial border maps mandated by the Constitution into higher resolution for use in demarcation.

The countries have been working to complete demarcation of the border for more than two decades, with the most recent meeting in August breaking down over Cambodia’s request to seek France’s assistance and for Vietnam to stop building on undemarcated areas.

Only the colonial maps of Cambodia’s border with Vietnam, which use the antiquated Bonne method of map projection at a 1/100,000 scale, can legally be used for erecting border posts, but Hun Sen acknowledged last year that more detailed maps were often employed.

The CNRP has questioned the constitutionality of the practice and expressed concern that those maps – drawn at a 1/50,000 scale using the UTM method, which is compatible with GPS, unlike the Bonne method – might, in fact, be ceding sovereign territory to Vietnam.

Hun Sen has previously proposed making 1/50,000-scale maps from the colonial maps with the help of neutral French cartographers, and after yesterday’s development meeting in Siem Reap, said he had made a deal with Vietnam’s premier to seek such assistance.

“Both prime ministers agreed with each other to make a joint letter to France to ask for experts in making a map to change it from 1/100,000 to 1/50,000,” Kao Kim Houn, a minister attached to the prime minister, told reporters after the meeting between the premiers.

Kim Houn said Foreign Affairs Ministry Secretary of State Long Visalo, who has long worked on demarcation issues, would be placed in charge of developing the maps, which would be used only for the small parts of the border that still remain unmarked.

“He was assigned to work together with a Vietnamese counterpart,” Kim Houn said of Visalo. “For the unclear points, we will ask France for help.”

Kim Houn said the premier had also reiterated Cambodia’s demands for Vietnam to stop constructing buildings in undemarcated “white zones” of the border, but did not say what the Vietnamese premier said in reply.

The border issue is one of the most politically sensitive in Cambodia, with an opposition lawmaker and senator presently in prison for comments made during a 2015 CNRP campaign against Vietnamese border violations.

Hun Sen has pledged to arrest anyone who continues to say the government is using the wrong maps.

CNRP lawmaker Um Sam An was arrested in April for claiming on his Facebook page that the government was using the wrong maps for demarcation, and opposition Senator Hong Sok Hour was arrested in August last year for a lecture in which he presented as real a fake border treaty dissolving the Cambodia-Vietnam border.

However, citing such arrests, the opposition party has since late last year not pushed the issue further, saying they do not want to anger the government.

CNRP spokesman Yim Sovann said yesterday that the opposition welcomed Hun Sen’s proposal to invite French experts to help, and said that it could help to build trust in the new 1/50,000 maps that are produced for use in demarcation of the border.

“If there is a referee in attendance in making the maps, which is something unclear and easy to make errors in, I welcome that, and I hope that France can help, because it would be positive for both countries to end the political border issue,” Sovann said.

A Foreign Ministry press release also said that Foreign Minister Prak Sokhon would next week visit Paris to meet with his French counterpart, Jean-Marc Ayrault, to discuss “cooperation between the two countries, particularly in the fields of politics, economics and culture”.