Ships sailing parallel, representing Indonesia and Malaysia, amidst nautical charts.

Maritime Boundaries: How Indonesia and Malaysia Negotiated Their Seas

Theo Raines in Global Affairs June 2025 • 4 min read.

"Explore the untold stories of maritime boundary negotiations between Indonesia and Malaysia, shaping Southeast Asian geopolitics."

The seas between nations are not just bodies of water; they are zones of resources, strategy, and sovereignty. For Indonesia and Malaysia, two Southeast Asian giants, defining these maritime boundaries has been a decades-long endeavor filled with diplomatic maneuvering, technical challenges, and the ever-present specter of historical claims.

In 1969, as the world watched the space race and cultural revolutions, delegations from Indonesia and Malaysia gathered in Kuala Lumpur. Their mission: to negotiate the continental shelf boundary between them, an issue that touched upon the Straits of Malacca, the South China Sea, and the Sulawesi Sea. This was not merely about drawing lines on a map; it was about defining future access to resources and solidifying national identities.

This article navigates through the intricacies of these negotiations, drawing upon historical documents and expert insights to reveal the hidden layers of these critical discussions. Discover how individuals like Admiral Adi Sumardiman, armed with maps and legal acumen, played pivotal roles in shaping the maritime landscape we know today.

Charting the Course: The Initial Deliberations of 1969

Ships sailing parallel, representing Indonesia and Malaysia, amidst nautical charts.

In September 1969, representatives from Indonesia and Malaysia convened in Kuala Lumpur to address the complex issue of their shared continental shelf. The negotiations spanned four critical areas: the Straits of Malacca, the eastern and western sectors of the South China Sea, and the waters off East Kalimantan and Sabah in the Sulawesi Sea. The Indonesian delegation included legal and technical experts, with Admiral Adi Sumardiman, a Geodetic Engineer, leading the technical team.

Admiral Sumardiman’s role was critical. Tasked with discussing the precise course of delimitation lines, he had to reconcile geographical data, hydrographical surveys, and historical maps. One of the initial challenges was the differing datums used in Malaysian maps (based on British surveys) and Indonesian maps (based on former Netherlands East Indies surveys). This discrepancy required careful examination to ensure fair and accurate delimitation.

Initial Agreement: Both sides agreed to a median line approach for delimitation purposes.
Technical Teams: The Indonesian Naval Hydrographic Office and the Malaysian National Mapping Department formed teams to designate line coordinates.
Key Participants: Major Oemarijoto and Mr. Soembarjono from Indonesia worked alongside Mr. Hadji Yatim and Kok Swee Tuck from Malaysia.

Two significant problems emerged during the construction of these lines. First, Malaysia questioned Indonesia's use of three outermost islands—Batu Mandi, Tokong Pyramid, and Tokong Burung—as base points, arguing they were not included in Indonesia's Act No. 4 of 1960. After considerable discussion, the Malaysian side conceded. The second issue involved the dividing line in the northern part of Tanjung Datuk, where Malaysia had already entered into a concession agreement with Shell Oil Company, complicating adherence to a median line.

From Disagreement to Delimitation: The Long Road to Maritime Accord

The maritime boundary negotiations between Indonesia and Malaysia were a complex interplay of historical claims, technical data, and geopolitical considerations. Individuals like Admiral Adi Sumardiman played pivotal roles, navigating through cartographic discrepancies and sovereignty disputes. While challenges remain, the collaborative efforts underscore the importance of dialogue and diplomacy in resolving maritime issues, setting a precedent for future negotiations in the region and beyond.

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Everything You Need To Know

What were the primary objectives of Indonesia and Malaysia when they initially met in Kuala Lumpur in 1969 to discuss maritime boundaries?

In 1969, the primary objective of Indonesia and Malaysia was to negotiate the continental shelf boundary between them. This involved defining future access to resources and solidifying national identities in areas such as the Straits of Malacca, the South China Sea, and the Sulawesi Sea. The negotiations aimed to establish clear maritime borders, which were crucial for resource management and asserting sovereignty.

Who was Admiral Adi Sumardiman, and what role did he play in the maritime boundary negotiations between Indonesia and Malaysia?

Admiral Adi Sumardiman was a Geodetic Engineer who led the Indonesian technical team during the maritime boundary negotiations with Malaysia. His role was critical in discussing and reconciling geographical data, hydrographical surveys, and historical maps to determine the precise course of delimitation lines. He addressed discrepancies between the datums used in Malaysian maps (based on British surveys) and Indonesian maps (based on former Netherlands East Indies surveys), ensuring fair and accurate delimitation.

What were the main issues encountered during the initial stages of constructing maritime boundary lines between Indonesia and Malaysia?

Two significant issues emerged during the initial construction of maritime boundary lines. First, Malaysia questioned Indonesia's use of three outermost islands—Batu Mandi, Tokong Pyramid, and Tokong Burung—as base points, arguing they were not included in Indonesia's Act No. 4 of 1960. After discussions, Malaysia conceded. Second, a complication arose in the northern part of Tanjung Datuk, where Malaysia had already entered into a concession agreement with Shell Oil Company, making adherence to a median line difficult.

How did differing cartographic datums between Indonesia and Malaysia complicate the maritime boundary negotiations, and what steps were taken to address this issue?

Differing cartographic datums complicated the negotiations because Malaysian maps were based on British surveys, while Indonesian maps were based on former Netherlands East Indies surveys. This discrepancy required careful examination to ensure fair and accurate delimitation. Admiral Adi Sumardiman and his technical team played a crucial role in reconciling these differences by analyzing and adjusting the geographical data to establish a common reference point for defining the boundary lines. This involved comparing and converting coordinates to ensure both sides agreed on the precise location of the maritime boundaries.

What is the significance of the median line approach in the context of maritime boundary delimitation between Indonesia and Malaysia, and what implications did it have for the two nations?

The median line approach involves drawing a boundary line that is equidistant from the nearest points on the baselines of both countries. In the context of Indonesia and Malaysia, agreeing to a median line approach meant that the maritime boundary would be defined by a line that is equally distant from the coastal points of both nations. This approach is intended to ensure fairness and equity in the division of maritime space, including the continental shelf and its resources. However, the implementation of the median line approach encountered challenges due to factors such as the presence of islands, differing interpretations of baselines, and existing concession agreements. The agreement set a precedent for future negotiations in the region.