GIS Lecture 8 Notes PDF

Overview of Vector Geoprocessing Definition of Geoprocessing ​ Geoprocessing involves transforming spatial objects into new or modified ones. ​ Unlike spatial queries or spatial joins (which do not modify geometry), geoprocessing modifies both attributes and geometry. ​ Common modif...

Overview of Vector Geoprocessing Definition of Geoprocessing ​ Geoprocessing involves transforming spatial objects into new or modified ones. ​ Unlike spatial queries or spatial joins (which do not modify geometry), geoprocessing modifies both attributes and geometry. ​ Common modifications: ○​ Creation or deletion of features. ○​ Changes in size or shape. ○​ Physical cutting of features. Key Questions for Geoprocessing Operations 1.​ Does the operation change the geometry? 2.​ Does the operation change the attributes? 3.​ What is retained versus discarded in the process? Types of Geoprocessing Operations 1.​ Overlays 2.​ Other Geoprocessing Operations Overlay Operations Overlay operations involve placing one or more spatial layers on top of another to determine spatial relationships and generate new information. They require at least two layers. 1. Clip ​ Purpose: Cuts a portion of the first input layer using a second input layer as a "cookie cutter." ​ Result: Retains geometry from the input layer within the clip boundary. ​ Attributes: ○​ The output table matches the input layer attributes. ○​ No attributes from the clip feature are included. ○​ Example: If the input area is physically reduced, the attribute table (e.g., area field) will not update automatically. 2. Erase  ​ Purpose: Removes the part of the first input layer that overlaps with the second input layer. ​ Result: Retains geometry outside the erase layer boundary. ​ Attributes: ○​ Matches the input layer attributes. ○​ No attributes from the erase feature are included. 3. Union ​ Purpose: Combines all areas of overlap and non-overlap from two or more input layers. ​ Result: Produces polygons that reflect all overlaps and non-overlapping features. ​ Attributes: ○​ Output contains attributes from all input layers. ○​ Overlapping features are duplicated, which can cause double-counting of area or features. 4. Intersect ​ Purpose: Retains only areas common to all input layers. ​ Result: ○​ Output geometries can be polygons, lines, or points depending on input. ○​ Only overlapping areas are included. ​ Attributes: ○​ Retains attributes from all input layers. 5. Identity ​ Purpose: Combines input layer geometry with attributes from an identity layer, but keeps all features from the input layer. ​ Result: ○​ Geometry: Retains input layer geometry. ○​ Attributes: Adds attributes from the identity layer where overlap occurs; assigns null values where no overlap exists. Other Geoprocessing Operations These operations modify features and attributes but do not involve overlapping layers. 1. Merge ​ Purpose: Combines features from two or more layers into a single layer. ​ Result: Produces a unified layer containing all input features. ​ Attributes: ○​ Attributes can be summarized or preserved, depending on user specifications.  ○​ Example: Combining state-level data for a region (e.g., Midwest). 2. Dissolve ​ Purpose: Aggregates features based on specified attributes, forming larger features from smaller ones. ​ Result: ○​ Reduces the number of features by combining polygons that share a common attribute. ○​ Example: Combining counties into water management districts. ​ Attributes: ○​ Summarized based on the aggregation process. 3. Buffer ​ Purpose: Creates a zone of specified width around a spatial feature. ​ Result: Produces a polygon layer, regardless of whether the input is a point, line, or polygon. ​ Attributes: ○​ Can dissolve overlapping buffers to prevent double-counting of areas. Comparing Operations Overlay Operations vs. Other Operations ​ Overlay operations involve spatial relationships between layers and often produce attributes from multiple sources. ​ Other operations focus on modifying single layers or combining layers without creating spatial relationships between them. Critical Questions For each geoprocessing operation, consider: 1.​ Geometry: ○​ Is it modified? How? ○​ How many features are created or remain? 2.​ Attributes: ○​ Are they retained or modified? ○​ Which attributes are included in the output? Practical Considerations  1.​ Area Calculation: ○​ Geometry modifications (e.g., clip, erase) require manual updating of area-related fields in the attribute table. ○​ Overlaps in buffers or unions can lead to overestimation if not dissolved or handled carefully. 2.​ Choosing the Right Tool: ○​ Geoprocessing operations are goal-specific. ○​ Example: ​ Use clip to define boundaries. ​ Use union to combine overlapping attributes. ​ Use buffer for distance-based analyses. Summary ​ Geoprocessing is a fundamental aspect of spatial analysis in GIS. ​ Overlay operations like clip, erase, union, intersect, and identity are used to analyze spatial relationships. ​ Other operations like merge, dissolve, and buffer modify or combine features and attributes for different purposes. ​ A deep understanding of how each operation affects geometry and attributes ensures accurate and meaningful analysis results.