Which term best describes the concept of a uniform change across a surface?

The concept of a uniform change across a surface is best described by the term "gradient." A gradient represents the rate of change of a quantity over a specified distance and is commonly understood in the context of geography and geospatial analysis where it conveys how something, such as elevation or temperature, varies across a landscape.

In geographic terms, when discussing a surface—be it a topographic map or a geospatial model—a gradient indicates a smooth and linear transition from one value to another, reflecting an even distribution of change. This is crucial for myriad applications, including environmental modeling, urban planning, and resource management, where understanding how one variable changes over space assists in making informed decisions.

In contrast, the other terms signify different concepts. Orthometric height pertains to the height of a point above the geoid, which is not necessarily uniform across the surface. Tidal fluctuation relates to variations in sea level due to tidal forces, which can be highly variable and location-dependent. The geoid is a model representing mean sea level and illustrates a surface of equal gravitational potential but does not inherently describe a uniform change across all surfaces. Therefore, gradient distinctly captures the essence of uniform change, making it the most suitable choice in this context.