The Earth is a solid known as an oblate spheroid, somewhat squashed down at the poles and bulging at the equator. How do we know this? Because of the work of geodesists who measure and study the shape of the Earth.
Geodesy is the science of measuring and monitoring the size and shape of the Earth and the location of points on its surface. NOAA's National Geodetic Survey (NGS) is responsible for the development and maintenance of a national geodetic survey system that is used for navigation, transportation and communication systems, and mapping and charting. Even the location of satellites and the Global Positioning System are based on the geodetic framework established by NOAA, its ancestor agencies, and sister agencies.