From celestial objects to every feature on Earth, geodesy provides a reference frame and is the foundation for surveying, mapping, and other geoscience activities. In Nepal, geodesy was officially introduced after 1924 to prepare the topographic map series. Although the previous geodetic project occurred with foreign assistance, Nepal is using national resources to conduct milestone projects such as the re-measurement of Mount Everest height in 2020 and the ongoing LiDAR survey of western Terai. Taking the 2015 Gorkha earthquake as a reference, this paper reviews the past and present geodetic activities in Nepal. It presents the history of conventional Nepal datum as a horizontal datum and Indian mean sea level-based vertical datum, and modern satellite geodesy works on the Himalayas. Considering recent earthquakes, continuous crustal motion, international and global compliance, and increasing demand for precise positional accuracy from the users and stakeholders, this paper discusses future directions to build, establish, maintain, and operate modern terrestrial, height, and gravity reference systems and frames. This paper consolidates many reports and experiences from Nepal and will serve as useful documentation for newcomers whose interests align in geodesy and Nepal.

Orthometric height is the generally adopted type of height worldwide and in geomatics community. Precise levelling has been the method of obtaining orthometric height in past for most of the country, so as the Nepal. However, due to wide usage of Global Navigation Satellite System (GNSS), the alternative approach of combining GNSS derived ellipsoidal height with geoid undulation to get GNSS derived orthometric height, has been used extensively. In Nepal, this technique was officially adopted in 2020 for Everest height measurement and understood as the efficient way to comply with levelling height. In this study, GNSS surveying was conducted on 15 stations located at the lowland region of Nepal and orthometric heights were obtained from GNSS and geoid method. When compared GNSS derived orthometric height with precise levelling height, the difference remained within threshold of 5cm for majority of observation stations. However, these differences are not sufficient to support the standards set for the third order levelling by Survey Department (SD). The accuracy of GNSS derived orthometric height can be significantly affected by various environment and existing resources such as existing accuracy of geoid, nature of precise levelling height. Considering the revisit upon these conditions, we expect GNSS-levelling as a strong alternative to time consuming, tedious, and costly precise levelling which is most suitable method of obtaining orthometric height in lowland topography at a precision less than 4 cm.

Gravity plays very crucial role in geodetic issues as gravity field determines the shape of the earth. This paper reviews – beginning from defining the gravity, various applications of gravity in geodesy and in other fields, nature of gravity field of our terrain, instruments of gravity measurement, and techniques of gravity survey. This paper first gives information about gravity networks in Nepal, presents an equation to compute orthometric correction (OC) from height obtained from levelling. Gravimetric geoid-determination method is explained here with various gravity reduction methods and topographic-isostatic gravity reduction method is recommended for N- separation computation as this method results gravity anomalies near to zero.

The height measurement of the highest peak of the world “Sagarmatha” was conducted by Nepal for the first time. The methodology for the measurement was finalized from the workshop held in Kathmandu with the constructive comments from national and international experts. Trignometrical levelling, precise levelling, GNSS survey and gravity survey was conducted. Previous air borne gravity data and present surface gravity data was used to determine the precise regional geoid for this program. Thus orthometric height was determined as 8848.86 m from the ellipsoid height observed at the top of Sagarmatha and precise geoid determined. The height was determined on the base of International Height Reference System (IHRS) and final height was announced jointly from Nepal and China on 8th of December 2020 from Kathmandu and Beijing through virtual media.