Cheugugi: Standardizing Rainfall Observation in 1442

Joseon struggled to measure rainfall accurately, which was necessary for responding to drought. It developed the Cheugugi in 1441 and refined its measurement and reporting standards in 1442. This system is remembered for establishing a basis for comparing observations across regions.

This article follows the records in the National Institute of Korean History's Annals of King Sejong, checked in November 2026.

Why the Cheugugi Was Developed and the Limits of Earlier Observations

The instrument was developed because rainfall was difficult to determine from the depth of rainwater that had seeped into the ground alone. Joseon was already receiving reports on rainfall in the provinces. But the ability to submit reports and the accuracy of the measurements were separate matters.

The way rainwater seeped into the soil varied depending on how dry or wet it was. The depth of wet soil therefore could not be equated directly with rainfall. A 1441 memorial from the Ministry of Taxation pointed out this limitation in measurement.

Determining how much rain had fallen for agriculture was fundamental to responding to drought. The aim was not to improve the natural phenomenon of rainfall itself. It was to improve the method used to observe and measure it.

This shift is also explained in Uriyeoksanet's “Making the Cheugugi” commentary.

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The 1442 regulations made local magistrates personally responsible for measuring rainfall.AI-generated image

Timeline from Development in 1441 to Refinement of the System in 1442

In 1441, instrument-based observation was pursued, and in 1442, specifications and operating standards were detailed. Combining the two years into a single event obscures the process by which the system was refined. All dates from the annals below use the lunar calendar.

The key historical source is the Ministry of Taxation memorial dated the 8th day of the 5th lunar month, 1442. A memorial is a report submitted by an official to the king. This entry also records that the king accepted the ministry's proposal.

How the Cheugugi Worked and the Role of Each Part

The measurement principle was to use a ruler to read the depth of rainwater collected in a cylindrical vessel. The volume of water entering the vessel was not itself reported. The observed value was a length indicating how high the collected water stood.

This principle is easy to understand when the area receiving the rainwater equals the cross-sectional area in which the water collects. Dividing the volume of collected water by the receiving area gives the water depth. In a straight-sided cylinder, therefore, the water depth represents the amount of rain that fell at that location.

Cheok, chon, and bun are the units of length used in the records. Converting them into modern millimeters requires a conversion standard for the jucheok. This article retains the specifications and recording units used in the annals.

Observation Standards for the Capital and Provinces in 1442

The 1442 rules allowed materials to vary by region while requiring consistent measurement standards. The instrument specified for use in the capital was made of iron. Local districts were allowed to use porcelain or earthenware vessels made to the same specifications.

Seoun-gwan was the government office responsible for astronomy, weather, and related matters at the time. Guesthouses were facilities for receiving visitors at local government offices. The rules explicitly made the responsible officials accountable for examining rainfall conditions themselves.

The observation duties described in the memorial can be organized in the following sequence.

1. Prepare an instrument and ruler that meet the specified standards.
2. Install the instrument on a platform at the designated location.
3. After rainfall, have the responsible official personally examine rainfall conditions.
4. Measure the water depth with the jucheok and record the dates and times when rain began and cleared.
5. Report the measurements and preserve them in a ledger.

The standardization of specifications and observation procedures stands out more than uniformity of materials. Construction methods were left open so that local districts could also obtain instruments meeting the standard. At the same time, responsibility for records was assigned, making the system part of administrative work.

Achievements of Institutionalization and the Surviving Artifact

The achievement of institutionalization was the creation of a framework for measuring and recording rainfall according to consistent standards. If instruments alone had been distributed, observation times and recording methods could have varied. The 1442 rules addressed these matters together.

However, these rules alone do not show how much drought damage was reduced. The memorial also does not state how much agricultural production increased. The achievement that can be confirmed is the development of a detailed observation and reporting system.

The system established under Sejong did not continue without interruption thereafter. The National Meteorological Museum explains that the system was interrupted by war and revived under Yeongjo. This history can be traced in the National Meteorological Museum's Cheugugi guide.

The only known surviving Cheugugi from the Joseon period is the Rain Gauge of Chungcheong Provincial Office, Gongju. This artifact was made in 1837. It is not an original instrument made under Sejong.

According to Korea Heritage Service records, the artifact was returned from Japan in 1971. It is currently managed by the National Meteorological Museum. Its designation as a National Treasure on February 27, 2020, is also documented. The source is the artifact record in the National Heritage Digital Service.

Records and Interpretation

The rules recorded in the annals should be distinguished from later historical assessments. The rules show what people were instructed to do at the time. They do not prove every aspect of actual operation and results.

What the Records State Directly

The 1442 memorial specified instrument dimensions and observation duties in detail. It also explicitly required the use of the jucheok and personal observation by the responsible officials. The dates and times when rain began and cleared were to be recorded alongside the water depth.

The king's acceptance of the proposal is also recorded. However, this entry does not say when every district completed installation. How many local observation ledgers survive must also be checked separately.

Significance Assigned in Historical Commentary

Uriyeoksanet's commentary emphasizes the nationwide implementation of a system for measuring rainfall quantitatively. This assessment highlights the combination of an invented instrument with administrative recordkeeping. It interprets the system as an attempt to record natural phenomena in comparable numerical terms.

The Korea Heritage Service presents the Cheugugi as the world's first scientific rainfall-measuring instrument. This is that institution's assessment of its significance in the history of science. This article focuses on the development and refinement of the system that can be confirmed through the annals.

What Can Be Confirmed About the Inventor and Detailed Circumstances

Uriyeoksanet's commentary explains that the crown prince, who later became Munjong, was involved in the invention. However, the 1442 memorial itself is not an account of an individual's development process. This entry alone cannot establish that a particular person was the sole inventor.

No basis is therefore established here for describing it as Jang Yeong-sil's sole invention. The memorial also does not describe the specific experiments involved in reducing the instrument's size. Unverified details should not be filled in with dialogue or anecdotes about historical figures.

Distinctions Needed When Reading Observation Records as Data

The date when an observation system began and the extent of the material available today are different matters. The 1442 rules provide evidence that records were required. They do not mean that observations from every region have been preserved without gaps from that year onward.

This is an interpretation of the rules in the annals from a modern data perspective. An isolated water-depth value makes it difficult to identify the location and time of observation. To compare records, the conditions attached to the figures must also be preserved.

In particular, the absence of a record is different from a conclusion that rainfall was 0. Filling gaps in the material with arbitrary figures can distort past weather. This is why the existence of a system, the performance of observations, and the preservation of records must be considered separately.

What This Means for Understanding Rainfall Observation Today

Even today, the meaning of an observation depends on measurement standards and recording conditions. Figures with the same name still require care in comparison if their units or observation periods differ. The 1442 rules offer a historical example for understanding these issues.

The significance of this example lies not only in the form of the invention. It also lies in the decision to specify who would measure and what would be recorded. Both instruments and recording procedures must be in place for observations to serve as evidence in the future.