GIS Lecture 7b PDF

Spatial Queries in GIS Spatial Queries are used to select geographic features based on their location or spatial relationship with other features. Unlike attribute queries (which search by attribute values like population or name), spatial queries work with geometry (the shape and position of objec...

Spatial Queries in GIS Spatial Queries are used to select geographic features based on their location or spatial relationship with other features. Unlike attribute queries (which search by attribute values like population or name), spatial queries work with geometry (the shape and position of objects in space). They help answer questions like: "Which polygons intersect with this point?" or "Which areas are within a certain distance of this feature?" Key Spatial Relationships: 1. Intersect: Selects features that overlap (fully or partially) with another feature. ○ Works with all vector objects (points, lines, polygons). 2. Within a Distance: Creates a buffer around features and selects those that intersect the buffer zone. ○ Works for all vector objects. 3. Within: Selects features where the geometry is inside another feature’s geometry (e.g., selecting points inside a polygon). ○ A polygon cannot be "within" a point (as a point is 0-dimensional). 4. Completely Within: Selects features that are completely inside another feature (without touching boundaries). 5. Contains: Opposite of "within." The source feature's geometry must contain the target feature's geometry (including boundaries). Select by Location in ArcGIS: This tool is used to conduct spatial queries. Target Layer: The layer from which you want to select features. Source Layer: The layer you're comparing to. Spatial Relationship: Defines how the two layers are spatially related (e.g., intersect, within). Common Mistake: When checking how many features were selected, make sure you're looking at the correct layer's attribute table, not the total count for all layers. Spatial Joins in GIS A Spatial Join is a type of join where features from one layer are added to another layer based on their spatial relationship. Unlike attribute joins (which are based on a common field), spatial joins use geographic relationships between features. Key Types of Spatial Joins: 1. Distance-based Join: Joins based on proximity, such as the nearest feature or those within a set distance.  2. Containment-based Join: Joins features based on spatial containment (e.g., a park contained within a county). Types of Spatial Joins: 1. Simple Join: A one-to-one or many-to-one relationship where attributes are copied directly from the source to the target feature. 2. Summarize Join: Used when there are many records in the source layer that need to be summarized (e.g., counting how many parks fall within a county). ○ You can calculate statistics like count, average, sum, etc. Example: Polygon to Polygon Join (e.g., county data to park data): ○ Summarize Join: Counts how many parks intersect each county. ○ Simple Join: Copies attributes of the first county that a park intersects. Geometry Types in Spatial Joins: Point to Point Point to Line Point to Polygon Line to Line Line to Polygon Polygon to Polygon Additional Notes: Spatial queries and joins in GIS can involve complex calculations, but the software (like ArcGIS) helps automate this process. The Plumb Line Algorithm is a common method used in spatial queries, such as checking if a point is within a polygon.