- Describe the difference between intersect and union, including the purposes for which they are used.
- Which different types of outputs are possible (points, lines, polygons) when performing intersect and union?
The Correct Answer and Explanation is :
1. Difference Between Intersect and Union:
- Intersect: The intersect operation is used to find the common areas between two spatial datasets. In other words, it identifies the region where two geometries overlap. For example, if you intersect two polygons, the resulting geometry will be the area where both polygons share space. Intersect is particularly useful for analyzing overlapping regions or features. This operation is often applied in geospatial analysis to find areas that meet certain criteria from two datasets, such as identifying shared territories between two land use areas.
- Union: The union operation, on the other hand, combines two geometries into one, essentially merging the datasets. It includes all areas from both the input geometries, creating a new geometry that encompasses the entire space covered by both. When union is applied to two polygons, the resulting geometry will be the total area covered by both polygons, without considering where they overlap. The union operation is useful for combining multiple datasets into a comprehensive area and is commonly used when attempting to merge spatial features, like land parcels or zones, for a broader analysis.
2. Possible Outputs (Points, Lines, Polygons) When Performing Intersect and Union:
- Intersect: The outputs of an intersect operation depend on the types of geometries being intersected:
- Point: If the input geometries are lines or polygons, the intersection may result in a point if the two geometries touch at a single location (like where two roads meet).
- Line: If a polygon intersects with another polygon or a line, the output can be a line, particularly when the polygons share a boundary or the line cuts through them.
- Polygon: When two polygons intersect, the output is often a smaller polygon that represents the shared area.
- Union: Similarly, the union operation can also result in different outputs based on the input types:
- Point: If both input datasets are point layers, the union will combine the points into a single layer of points.
- Line: If you union lines, the output will be a continuous line geometry that includes the combined stretches of both input lines.
- Polygon: When unioning two polygon layers, the output is typically a larger polygon that contains all the areas from both polygons, including any overlap.
In summary, both intersect and union can produce points, lines, or polygons as outputs, depending on the types of input geometries and the nature of the operation being performed. Intersect tends to retain the spatial integrity of the overlapping region, while union combines all the areas, forming a new, unified geometry.