What term refers to the reference systems used to measure the position of points on the earth's surface?

The term that refers to the reference systems used to measure the position of points on the earth's surface is "datums." A datum provides a frame of reference for measuring locations on the earth. Essentially, it defines how the coordinate system is aligned with the real-world surface and establishes a baseline for the geographical data collection.

Datums are critical in geospatial analysis as they help ensure that spatial data is accurate and can be reliably compared across different datasets. For example, when mapping or analyzing data, knowing which datum was used allows users to correctly understand and interpret the spatial relationships between different geographic points.

The other terms provided do not serve the same purpose. "Coordinates" refers to the numerical values that represent a specific location within a given datum, but they do not define the reference system themselves. "Models" generally pertain to representations or simulations of geographic phenomena, and "measures" imply the act of quantifying, which may pertain to various attributes but does not specifically refer to the framework for positioning geographic features. Thus, "datums" is the most accurate choice for the reference systems that determine how we measure locations on the earth’s surface.