What is necessary to ensure proper alignment when overlaying two datasets in ArcGIS?

To ensure proper alignment when overlaying two datasets in ArcGIS, it is essential for the datasets to have coordinate system compatibility. This means that both datasets must be using the same spatial reference or be transformed into a common coordinate system. If the coordinate systems are not compatible, the features from one dataset may not align correctly with those from another dataset, leading to misrepresentation or confusion in spatial analysis.

Coordinate systems determine how feature locations are represented on the Earth's surface. Different datasets might have different projections or units, and if these are not aligned, the overlay will produce inaccurate results. Hence, transformation or reprojecting of datasets into a common coordinate system is a crucial step to achieve proper alignment.

While having the same attribute structure, equal scale, or matching geometries may enhance the comparison or visualization of the datasets, they do not directly address the fundamental need for alignment based on spatial reference. Without compatible coordinate systems, no amount of similarity in attributes, scale, or geometry will allow the datasets to overlay properly. Thus, ensuring that the coordinate systems are compatible is the key factor in achieving effective overlay in ArcGIS.