Vector Geoprocessing 2007 Austin Troy Introduction to GIS

Vector Geoprocessing @ 2007 Austin Troy

Introduction to GIS Geoprocessing • Geoprocessing is the processing of geographic information. • Perform spatial analysis and modeling via tools that transform datasets • Includes methods to automate GIS workflows @ 2007 Austin Troy

Introduction to GIS Geoprocessing • Three general classes of tools – Breaking features into smaller features (e. g. Clip, Intersect, Union) – Aggregating features into larger features (e. g. Dissolve, Merge) – Creating new polygon features through buffering (e. g. Buffer) @ 2007 Austin Troy

Introduction to GIS Geoprocessing in Arc. GIS • Perform geoprocessing in Arc. GIS – Run a tool using its dialog box. – Run tools at a command line. – Build and run a model – Create and run a script @ 2007 Austin Troy

Introduction to GIS Geoprocessing in Arc. GIS • Several ways to do this…. – Run a tool using its dialog box. – Run tools at a command line. – Build and run a model – Create and run a script @ 2007 Austin Troy

Introduction to GIS Finding Geoprocessing Tools @ 2007 Austin Troy

Introduction to GIS Vector Overlay Functions @ 2007 Austin Troy Slide courtesy of Leslie Morrissey

Introduction to GIS Union Combines features of two or several themes Keeps all line work (extent contains both inputs) Breaks down features, and creates new polygons Keeps all attributes Polygon only (no points/lines) @ 2007 Austin Troy Image source: ESRI Arc Info electronic help

Introduction to GIS Tools: Union Polygons only A list of Polygon datasets @ 2007 Austin Troy

Introduction to GIS Intersect Yields areas that are common to both layers Preserves line work within common extent Usually creates many new, smaller polygons Preserves all attributes from both Polygon overlaid with… Polygon Line (output: line) Point (output: point) @ 2007 Austin Troy

Introduction to GIS Tools: Intersect Two layers @ 2007 Austin Troy

Introduction to GIS Union vs. Intersection Union is the complete combination of two overlapping sets of features and intersect is the intersection of inputs (only the overlapping parts) “ 1 AND 2” Intersect: Layer 1 + Layer 2 Union: “ 1 OR 2” Layer 1 + Layer 2 @ 2007 Austin Troy

Introduction to GIS Union vs. Intersection: Example Here’s an example. Say we have deer wintering areas in one layer and conserved lands in another. @ 2007 Austin Troy

Introduction to GIS Union vs. Intersection: Example Union gives us land that is EITHER conserved OR that is a deer wintering areas @ 2007 Austin Troy

Introduction to GIS Union vs. Intersection: Example Intersect gives us land that is BOTH, and preserves all polygon boundaries within that common extent @ 2007 Austin Troy

Introduction to GIS Identity performs a special type of intersection Keeps all “input” features and attributes but only overlapping “identity” features and their attributes First (“input”) layer determines geographic extent Polygon with polygon, point, or line Kept, as with union Common (intersecting) areas Not kept, as with intersect @ 2007 Austin Troy

Introduction to GIS Overlay trouble warning…. Output layer combines spatial polygons and attribute tables Overlay Parcels Impervious/Pervious Yikes! Zillions of polygons! @ 2007 Austin Troy Illustrative material courtesy of Leslie Morrissey

Introduction to GIS Overlay trouble warnings…. Can result in polygon slivers Yikes! Output file type (shapefile vs. feature class) Shapefile output: no recalculation of area, perimeter, or length fields Double yikes! Output as GDB feature class for accurate area, perimeter, and length calculations! Input layers must have matching projection/datum (spatial reference) No automatic recalculation of numeric attributes for polygons that depend on a spatial unit! More error! @ 2007 Austin Troy Slide content courtesy of Leslie Morrissey

Introduction to GIS Overlay attribute errors All input layer attributes are preserved as is Population is attributed to each output polygon Count is not recalculated proportional to area Total population for output is wrong! Country States A B C D + Population = 9 million Output = A+ 9 M C+ 9 M B+ 9 M D+ 9 M Uh oh! Sum Pop. = 36 M? ? @ 2007 Austin Troy Slide content courtesy of Leslie Morrissey

Introduction to GIS Clip This uses one theme to “clip, ” or serve as the outer boundary of another theme Breaks down features into smaller units Preserves the input theme’s attributes Polygon only Point, line, or polygon @ 2007 Austin Troy

Introduction to GIS Tools: Clip Point, line, polygon Polygon @ 2007 Austin Troy

Introduction to GIS Clipping highways for Merced Note that the “use selected features only” option was used @ 2007 Austin Troy

Introduction to GIS Clipping roads @ 2007 Austin Troy

Introduction to GIS Dissolve Tool for aggregating polygons—making them bigger. Single layer operation @ 2007 Austin Troy

Introduction to GIS Tools: Dissolve @ 2007 Austin Troy

Introduction to GIS Dissolve: Example Dissolve zip codes (small) into counties (large) @ 2007 Austin Troy

Introduction to GIS Dissolve: Example Choose the dissolve field: e. g. Dissolve based on the County field @ 2007 Austin Troy

Introduction to GIS Dissolve : Example • Summarize the resulting field values. For instance, you could sum population for each county, or average size of ZIP code zones for each county @ 2007 Austin Troy

Introduction to GIS Dissolve : Example Now we have created a county map, and for each county we have an attribute containing the sum of population of the constituent zip codes @ 2007 Austin Troy

Introduction to GIS Merge Allows you to “join” two adjacent or nonadjacent themes into the same layer Like “tiling” Best when attributes match @ 2007 Austin Troy

Introduction to GIS Tool: Merge @ 2007 Austin Troy

Introduction to GIS Merge Often when you merge you will want to follow up by dissolving. @ 2007 Austin Troy

Introduction to GIS Tools: Buffering is when you draw a polygon around a feature (point, line or polygon) @ 2007 Austin Troy

Introduction to GIS Tools: Buffering Based on distance Based on attribute @ 2007 Austin Troy

Introduction to GIS Tools: Variable Width Buffering @ 2007 Austin Troy

Introduction to GIS More Overlay Tools Update – merge new features, e. g. , add new parcel to parcels layer Erase – remove some interior portion of a layer @ 2007 Austin Troy Some content courtesy of Leslie Morrissey

Introduction to GIS Combining Geoprocessing Tools Involve multiple tasks performed in sequence, such as those that clip, buffer, intersect, union, then select datasets. – Step by step – Build and run a model – Create and run a script @ 2007 Austin Troy

Introduction to GIS Combining Buffering and Geoprocessing: Example Question: How to find areas that are near deer wintering areas and water bodies but far from traffic? Geospatial Data Polygon layer for deer wintering areas Polygon layer for Water bodies Roads layer: line features @ 2007 Austin Troy

Introduction to GIS Combining Buffering and Geoprocessing: Example Question: How to find areas that are near deer wintering areas and water bodies but far from traffic? – “Near” or “Far from”: Buffering – Areas that are near deer wintering areas AND water bodies: Intersect – Combining the layers: Union – Selecting: Query for areas that are not within a traffic buffer @ 2007 Austin Troy

Introduction to GIS Combining Buffering and Geoprocessing: Example Buffering: Made fixed buffers around deer wintering areas and water bodies, and a variable buffer around roads, based on traffic @ 2007 Austin Troy

Introduction to GIS Combining Buffering and Geoprocessing: Example Intersecting : The intersection of deer wintering buffers and water buffers (the area in the red) @ 2007 Austin Troy

Introduction to GIS Combining Buffering and Geoprocessing: Example The union of that intersection with the traffic buffer: @ 2007 Austin Troy

Introduction to GIS Combining Buffering and Geoprocessing: Example Selecting: Query for polygons that are not within (far from) a traffic buffer @ 2007 Austin Troy

Introduction to GIS Combining Buffering and Geoprocessing: Example Create a new layer by exporting the selected features (polygons) @ 2007 Austin Troy

Introduction to GIS Flow Chart for Analysis Deer wintering areas Water Traffic Buffer Deer_buff Water_buff Traffic_buff Deer/water buffers away from roads Select Intersect Deer wintering & water buffers Union @ 2007 Austin Troy Deer/water buffers plus traffic buffer Export Deer/water buffers away from roads

Introduction to GIS Geoprocessing Summary Union Intersect Identity Clip Dissolve @ 2007 Austin Troy Merge Buffer