PostGIS and Spatial Queries Steve Signell, Instructor Robert Poirier, TA School of Science Rensselaer Polytechnic Institute Monday, March 3, 2014 GIS in the Sciences ERTH 4750 (38031)

Spatial Databases PostGIS is a spatial database. Oracle Spatial and SQL Server 2008 are also spatial databases. But what does that mean; what is it that makes an ordinary database a spatial database? The short answer, is... Spatial databases store and manipulate spatial objects like any other object in the database.

Databases & SQL: Review The four “verbs” of SQL (Structured Query Language) SELECT, returns rows in response to a query INSERT, adds new rows to a table UPDATE, alters existing rows in a table DELETE, removes rows from a table

Databases & SQL: Review SELECT QUERIES Required: SELECT (field(s)) FROM (tables) Optional: JOIN (combines two FROM items) WHERE (conditions) GROUP BY (fields– used for AGGREGATE functions) ORDER BY (field(s)) LIMIT (# rows returned)

JOIN Combines two FROM items (tables) using a common identifier. Usage:

WHERE… WHERE…. ALSO: BETWEEN x AND y Mathematical functions (+ - * /, etc.) See math.htmlhttp:// math.html OperatorDescription <less than >greater than <=less than or equal to >= greater than or equal to =equal <> or !=not equal

Aggregate Functions AGGREGATE functions: compute a single result from a set of input values aggregate.html avg(), sum(), min(), count() Also statistics: corr(), regr_slope(), stdev() Always have a ‘GROUP BY’ in the SQL statement

Spatial Databases 1. Spatial data types refer to shapes such as point, line, and polygon; 2. Multi-dimensional spatial indexing is used for efficient processing of spatial operations; 3. Spatial functions, posed in SQL, are for querying of spatial properties and relationships.SQL

Spatial Indexing Spatial indexing & Bounding Boxes Answering the question “is A inside B?” is very computationally intensive for polygons but very fast in the case of rectangles. Even the most complex polygons and linestrings can be represented by a simple bounding box.

Spatial Functions Conversion: Functions that convert between geometries and external data formats. Management: Functions that manage information about spatial tables and PostGIS administration. Retrieval: Functions that retrieve properties and measurements of a Geometry. Comparison: Functions that compare two geometries with respect to their spatial relation. Generation: Functions that generate new geometries from others.

Conversion Functions Well-known text (WKT)WKT ST_GeomFromText(text, srid) returns geometry ST_AsText(geometry) returns text ST_AsEWKT(geometry) returns text Well-known binary (WKB)WKB ST_GeomFromWKB(bytea) returns geometry ST_AsBinary(geometry) returns bytea ST_AsEWKB(geometry) returns bytea Geographic Mark-up Language (GML)GML ST_GeomFromGML(text) returns geometry ST_AsGML(geometry) returns text Keyhole Mark-up Language (KML)KML ST_GeomFromKML(text) returns geometry ST_AsKML(geometry) returns text GeoJSON ST_AsGeoJSON(geometry) returns text Scalable Vector Graphics (SVG)SVG ST_AsSVG(geometry) returns text

Management Functions AddGeometryColumnAddGeometryColumn - Adds a geometry column to an existing table of attributes. DropGeometryColumnDropGeometryColumn - Removes a geometry column from a spatial table. DropGeometryTableDropGeometryTable - Drops a table and all its references in geometry_columns. PostGIS_VersionPostGIS_Version - Returns PostGIS version number and compile-time options. Populate_Geometry_ColumnsPopulate_Geometry_Columns - Ensures geometry columns have appropriate spatial constraints and exist in the geometry_columns table. Probe_Geometry_ColumnsProbe_Geometry_Columns - Scans all tables with PostGIS geometry constraints and adds them to the geometry_columns table if they are not there. UpdateGeometrySRIDUpdateGeometrySRID - Updates the SRID of all features in a geometry column, geometry_columns metadata and srid table constraint

Management Functions AddGeometryColumnAddGeometryColumn - Adds a geometry column to an existing table of attributes. DropGeometryColumnDropGeometryColumn - Removes a geometry column from a spatial table. DropGeometryTableDropGeometryTable - Drops a table and all its references in geometry_columns. PostGIS_VersionPostGIS_Version - Returns PostGIS version number and compile-time options. Populate_Geometry_ColumnsPopulate_Geometry_Columns - Ensures geometry columns have appropriate spatial constraints and exist in the geometry_columns table. Probe_Geometry_ColumnsProbe_Geometry_Columns - Scans all tables with PostGIS geometry constraints and adds them to the geometry_columns table if they are not there. UpdateGeometrySRIDUpdateGeometrySRID - Updates the SRID of all features in a geometry column, geometry_columns metadata and srid table constraint

Retrieval Functions ST_AreaST_Area: Returns the area of the surface if it is a polygon or multi-polygon. For “geometry” type area is in SRID units. For “geography” area is in square meters. ST_LengthST_Length: Returns the 2d length of the geometry if it is a linestring or multilinestring. geometry are in units of spatial reference and geography are in meters (default spheroid) ST_NPointsST_NPoints: Returns the number of points (vertexes) in a geometry. ST_NumGeometriesST_NumGeometries: If geometry is a GEOMETRYCOLLECTION (or MULTI*) returns the number of geometries, otherwise return NULL. ST_PerimeterST_Perimeter: Returns the length measurement of the boundary of an ST_Surface or ST_MultiSurface value. (Polygon, Multipolygon) ST_StartPointST_StartPoint: Returns the first point of a LINESTRING geometry as a POINT. ST_XST_X: Returns the X coordinate of the point, or NULL if not available. Input must be a point. ST_YST_Y: Returns the Y coordinate of the point, or NULL if not available. Input must be a point.

Comparison Functions Questions like “Which are the closet bike racks to a park?” or “Where are the intersections of subway lines and streets?” can only be answered by comparing geometries representing the bike racks, streets, and subway lines. Other Questions: “What neighborhood and borough is Atlantic Commons in?” “What streets does Atlantic Commons join with?” “Approximately how many people live on (within 50 meters of) Atlantic Commons?”

Comparison Functions

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Part II: Demos Lean-tos Campsites Trail Registers Aquatic Invasives

Questions? 24 Homework for Thursday 1 page proposal for group Project Sections 1-9, Boundless PostGIS Tutorial