Thai-Cambodian border row: does 1:50,000 or 1:200,000 mapping give the edge?

When people think of the Thai–Cambodian border dispute, they often view it as a deeply complex political or historical matter. But at the root of this century-long conflict lies something deceptively technical: the difference between two map scales—1:50,000 and 1:200,000—used by the two countries to define their shared boundary.

During the Sixth Thai–Cambodian Joint Boundary Commission (JBC) meeting held in Phnom Penh on June 15, 2025, Lam Chea, the Cambodian Minister in charge of Border Affairs, declared unequivocally that Cambodia would only recognise the 1:200,000-scale map, citing its basis in the Franco–Siamese Treaties of 1904 and 1907. He firmly rejected Thailand’s 1:50,000-scale maps, which were unilaterally produced.

The core difference: 1:50,000 vs 1:200,000 maps

To understand why this difference is so contentious, one must first understand what map scales mean.

Thailand uses 1:50,000-scale maps, meaning 1 centimetre on the map represents 500 metres on the ground. This scale provides highly detailed topographical information, including hills, streams, and minor roads, making it ideal for precise boundary demarcation.

Cambodia, on the other hand, insists on using the 1:200,000-scale map, where 1 centimetre equals 2 kilometres in reality. This French-produced map is four times less detailed than Thailand’s but covers a much broader area. It is better suited for large-scale regional planning rather than fine-grained boundary work.

Projection systems: More than just scale

Beyond scale, a more critical technical difference lies in the projection systems used to convert the Earth’s curved surface into a flat map.

Thailand’s 1:50,000 maps use the Mercator Projection, a cylindrical mapping system that transforms the Earth’s curved surface into a flat map. This system, developed with technical assistance from the United States, is particularly effective in representing accurate directions and distances.

When navigating from one point to another on a Mercator map, distances and bearings can be measured with high precision. However, the system has a key limitation: it distorts the size of geographic areas, especially those far from the equator. The farther a location is from the poles, the greater the distortion becomes.

In contrast, Cambodia’s 1:200,000 maps rely on the Sinusoidal Projection, often compared to peeling and flattening an orange or an onion. Developed by French surveyors, this projection maintains area accuracy, ensuring that every square on the map corresponds evenly to real-world land area.

This makes the sinusoidal projection useful for land-use planning and resource evaluation, but less reliable for measuring distances, especially diagonal ones or in areas far from the map’s central meridian.

Why can't the two maps align

The greatest challenge in using both the 1:50,000 and 1:200,000 scale maps is that they cannot be perfectly overlaid. This misalignment stems from fundamental differences in projection systems and inherent map properties.

When surveyors attempt to overlay Thailand’s Mercator projection maps onto Cambodia’s Sinusoidal projection maps, they find that the boundary lines do not match, even when referencing the exact same physical terrain. This discrepancy is not due to surveying errors, but to the fact that each map transforms the Earth’s curved surface into a flat representation in different mathematical ways.

In certain areas, Thailand’s maps may show the boundary passing through one location, while Cambodia’s maps indicate it lies hundreds of metres—or even several kilometres—away. This shift is a direct consequence of the incompatible mapping systems, not deliberate manipulation.

A historical backdrop to differing cartographic choices

To understand why Thailand and Cambodia rely on different sets of maps, we must return to the early 20th century, when colonial France and Siam were negotiating their territorial boundaries. At that time, cartographic technology was still developing, and each country followed mapping conventions influenced by different global powers.

The turning point came on October 3, 1893, during the Paknam Incident, when France used naval force to pressure Siam into signing a treaty ceding large territories. 

The 1904 Franco–Siamese Convention established the use of watershed lines as the basis for the boundary and created a joint boundary commission to survey and map the border.

The 1907 Convention forced Siam to cede Battambang, Siem Reap, and Sisophon to France in exchange for the return of Dan Sai and Trat. These agreements significantly shaped today's borders.

Although a joint commission was formed to carry out boundary demarcation and data collection, it disbanded before the 1:200,000-scale map was finalised and printed in August 1908.

When France later handed over this map to Siam, it was essentially an informal delivery of a document, not an officially endorsed boundary agreement. This has since cast legal doubt on the legitimacy of the 1:200,000 map as a binding reference, particularly in the eyes of Thailand.

Map development in the post-war era and great power influence

Following World War II, the United States began to play an increasingly influential role in Southeast Asia. As a close ally of the US, Thailand received technical assistance in modernising its cartographic systems. This cooperation led to the adoption of the Mercator Projection and the development of 1:50,000-scale maps, which remain Thailand’s standard for detailed topographic mapping.

Thailand’s 1:50,000 maps are highly detailed and significantly more accurate than the older French colonial maps. Thanks to more advanced surveying technologies, the maps offer high-resolution representations of terrain features, allowing for greater precision in boundary demarcation. From a technical standpoint, Thailand’s maps are generally considered superior in quality and reliability.

By contrast, Cambodia, as a former French colony, continues to rely on cartographic systems inherited from the colonial era. Its insistence on using the 1:200,000-scale map reflects not only a technical preference but also deep political and historical dimensions.

Border impact: When two map systems collide

Using two incompatible mapping systems—1:50,000 and 1:200,000—to define the same international boundary can lead to massive discrepancies, sometimes spanning several square kilometres. In certain areas, the Thai and Cambodian maps place the border kilometres apart, resulting in overlapping claims and the creation of disputed zones.

These zones are not just lines on a map—they are real areas with forests, water sources, natural resources, and, most importantly, local populations. The inability to definitively demarcate borders in such areas creates obstacles to governance, public service delivery, and citizens’ rights along the frontier.

One of the most prominent examples is the Preah Vihear Temple case. In 1962, the International Court of Justice (ICJ) ruled that the temple belonged to Cambodia, basing its decision in part on the 1:200,000-scale map. While the ICJ explicitly noted that it was not endorsing the map’s accuracy, the map was used as supporting evidence in reaching its conclusion, ultimately transferring sovereignty over the temple to Cambodia.

Efforts to resolve a long-standing issue

Since 1997, Thailand and Cambodia have sought to resolve their border disputes through the JBC. Over the past 25 years, progress has been made, with 603 kilometres of the 798-kilometre boundary successfully demarcated. However, the remaining 195 kilometres have yet to be settled.

The 2000 Memorandum of Understanding (MOU) attempted to establish a cooperative framework, relying on three sets of reference documents: the 1904 Convention, the 1907 Treaty, and maps prepared by the Franco-Siamese Border Commission. Yet, diverging interpretations of these documents continue to pose challenges.

At the most recent JBC meeting on June 15, 2025, the fundamental obstacle remained unresolved: conflicting references to 1:50,000 and 1:200,000 scale maps. This technical disagreement has become a flashpoint, particularly after Cambodia decided to bring the disputes over four areas— Emerald Triangle, Ta Moan Thom temple, Ta Moan Toch temple, and Ta Krabei temple—to the International Court of Justice (ICJ).

Pathways toward resolution

Resolving the discrepancy between 1:50,000 and 1:200,000 scale maps requires a combination of scientific precision and diplomatic finesse. The use of modern technology—such as GPS, high-resolution satellite imagery, and Geographic Information Systems (GIS)—may offer a way to create new maps that are mutually acceptable.

Other possibilities include identifying midpoints between the two mapping systems’ reference lines, negotiating land swaps to ensure fairness, or engaging international mediators to facilitate agreement. Crucially, the two countries must foster a shared understanding that the issue is not merely technical. It profoundly affects the lives of people living along the border.

Ultimately, the difference between the 1:50,000 and 1:200,000 scale maps may appear to be a matter of numbers and cartographic methodology. But behind these numbers lie deeper concerns: history, sovereignty, and the future of Thai-Cambodian relations. A fair and lasting resolution will require not only scientific knowledge, but also wisdom, patience, and goodwill from all involved.