In GIS, what does a planar measurement refer to?

A planar measurement in GIS specifically refers to measurements that are made in a two-dimensional space. This typically involves calculating distances, areas, and angles without considering the vertical dimension, or altitude. In GIS applications, planar measurements are commonly applied when analyzing maps, creating layouts, or working with data that is projected onto a flat surface.

By focusing solely on two dimensions (length and width), planar measurements simplify the processes of spatial analysis and geographic modeling. This is crucial when dealing with data that is represented in a 2D format, such as most digital maps.

The other options present definitions that do not align with the concept of planar measurement. For example, measurements in 3D space involve altitude and depth, which is not part of planar measurement. Similarly, altitude itself is a vertical measure and does not fit within the parameters of planar analysis. Lastly, measurements using three reference points typically relate to triangulation methods which again introduce a third dimension. Therefore, the focus on two-dimensional space solidifies why this choice is the accurate definition of planar measurement in GIS.