Preparing Data for Analysis and Analyzing Spatial Data/ Geoprocessing Class 11 GISG 110

Objectives Preparing and Analyzing Data Geoprocessing tools and processes –Data Extraction –Overlay –Proximity Spatial Analysis Models

Geoprocessing A GIS operation used to manipulate GIS data Allows for definition, management, and analysis of information used to form decisions Accomplished with ArcToolbox Typical geoprocessing operation: 1.Takes input data 2.Performs operation on data 3.Returns the result of operation as output data

Preparing data Common issues when preparing data: Too detailed or generalized for your map scale Not enough attributes, need to create and populate Too much data

Geoprocessing tools Analysis tools fall into 3 categories: Data extraction Overlay Proximity Analysis

Data extraction Creating a new subset of features in one feature class based on the geographic extent of another feature class Two common processes include: –Clip –Dissolve

Clip Extracts data An operation that extracts the features from one layer that reside entirely within a boundary defined by features in another layer E.g., Clip roads to a county boundary

Clip Example Clip roads in the state roads input layer (line fc) using the county polygon as the cookie cutter to create an output layer (line fc) containing just roads in the county

Clip Usage Tips & Facts The feature class that is having its features clipped (INPUT) can contain points, lines, or polygons The clip feature class, must be a polygon feature class The Output feature class will have the attributes of the Input features Large datasets = memory issues

Dissolve Simplifies data Process of removing unnecessary boundaries between features, such as edges of adjacent map sheets, after data has been captured

Dissolve Use Dissolve when you want to aggregate features based on a specified attribute or attributes E.g., Create the sales regions layer by removing the boundaries between counties represented by the same salesperson

Dissolve Example US Counties Before Dissolve US Subregions After Dissolve Create new subregions from dissolved census tracts, ZIP codes, or counties

Dissolve Usage Tips & Facts Dissolve fields –Features with the same value (within a field) will be aggregated (dissolved) into a single feature Multipart features –May result from dissolve –A single feature that contains noncontiguous elements and is represented in the attribute table as one record (e.g., county does not share border with other counties being dissolved)

Dissolve Usage Tips & Facts (cont.) Summarizing attributes –Aggregated features can also include summaries of any of the attributes present in the input features Coordinate system of the output –The output from the dissolve function will be in the same coordinate system as the input feature class

Overlay Identify overlaps between features in two layers and create a data set in which the lines of overlap define new features Two common processes include: –Union –Intersect

Union This tool builds a new polygon feature class by combining the features and attributes of each feature class

Union This operation combines 1. Features of a polygon input layer + 2. Polygons from an overlay layer = 3. Output layer that contains the attributes and full extent of both layers (polygon fc) Non-overlapping areas are included in the output data set

Union example Union streams buffered 35 feet (polygon fc) + Timber stand boundaries (types of timber, oak, pine, etc. – also polygon fc) = Portions of stands located within and outside buffered streams (polygon fc)

Union Usage Tips & Facts All input feature classes and feature layers must have polygon geometry The output features will have the attributes of all the input features that they overlap Output may be multipart (can change if not desired) Large datasets = memory issues

Intersect Computes a geometric intersection of the Input Features Features or portions of features which overlap (intersect) in all layers and/or feature classes will be written to the Output Feature Class

Intersect This operation 1. Cuts an input layer + 2. Features from an overlay layer = 3. Output layer with features that have attribute data from both layers

Intersect Usage Tips & Facts The Input Features must be simple features: point, multipoint, line, or polygon If the inputs have different geometry types (e.g., point on line), the Output Feature Class geometry type will default to be the same as the Input Features with the lowest dimension geometry (point)

Intersect Usage Tips (cont.) The Output Type can be that of the Input Features with the lowest dimension geometry or lower Output may be multipart (can change if not desired) Large datasets = memory issues

Union and Intersect Output Output contains the attributes and full extent of both layers Only overlapping geometry is preserved Features have attributes from both input layers

Proximity Analysis Finds what is near or within a certain distance of one or more features Common process includes buffer

Buffer This tool creates a new feature class of buffer polygons around a specified input feature (polygon, line, or point)

Buffer A zone around a map feature measured in units of distance (or time) Area drawn at a specified distance around a feature Features lying inside the buffer have different status from features lying outside the buffer

How buffer works Buffer works in Euclidean space and uses a two-dimensional algorithm A buffer will be the same width no matter what the coordinate system is It will not reflect the curvature or the shape of the earth For the best results, generate the buffer in a map projection that minimizes distortion in the area of interest

How buffer works (cont.) The buffer width can be specified in two ways: Fixed distance: Specify a constant distance to apply to the Input Features (OR) From field: Choose the name of a numeric distance field from the specified feature class. Each feature will be buffered according to its associated value in the chosen field.

Fixed distance Buffer a line feature a distance of 20 (e.g. 20 feet) Because the buffer distance is a constant, all features are buffered to the same width (20 feet)

From field Buffer a line feature using a numeric field with values of 10, 20, and 30 for Distance Because the buffer distances are dependent on the field values, various buffer widths can be applied in the same operation

Buffer Example Oceanside Unified School District

Data analysis steps Typical analysis with geoprocessing operations: 1. Determine which geoprocessing tools you need (plan) 2. Determine the order in which the geoprocessing tools should be used (plan more – use workflow diagram) 3. Locate the first tool and open its dialog box 4. Enter the tool parameters, including the input and output datasets 5. Run the tool, examine output and repeat if necessary

Spatial Analysis Problem solving in GIS Process of examining the locations, attributes, and relationships of features in spatial data through overlay and other analytical techniques in order to address a question or gain useful knowledge –Use when need to solve a spatial or statistical problem Extracts or creates new information from spatial data Two very useful spatial analysis tools in ArcToolbox: buffers and overlays Range from simple to complex

Sample spatial analysis applications Overlaying layersOverlaying layers — Overlaying different layers of features to create new information identifying the spatial relationships between the inputs (union, intersect) Overlaying layers Identifying what is nearbyIdentifying what is nearby — Finding what is near a feature or the distance between different types of spatial features (buffer) Identifying what is nearby Quantifying the resultsQuantifying the results — Presenting the results of your analysis through statistical analysis or frequency count Quantifying the results

Spatial Analysis Example Locating the number of public schools within 30 miles of a hospital 1. Buffer hospitals by 30 miles 2. Select by location 3. Output: New layer or selected set

Models

Models A collection of geoprocessing operations that automatically execute in sequence when the model is run to produce a final output Any geoprocessing operation in a model can be modified –The model can be run again to quickly refine an analysis or produce new data

Workflow diagrams and models Workflow diagrams can be turned into models

Modelbuilder ModelBuilder is the graphical environment in which you create and interact with models The building block of a model is called a process The building block of a model is called a process –A process consists of a geoprocessing tool

Modelbuilder Color coded by elements (input, values, and output) and processes (or geoprocessing tool) –White elements = not ready to run –If all elements in process are colored = ready to run

Modelbuilder To add a process to a model, find a tool in ArcToolbox or ArcCatalog and drag it into the model Can also drag and drop data into a model from ArcCatalog or ArcMap

Review 1. Analysis tools fall into what three categories? 2. Use Clip when you want to aggregate features based on a specified attribute or attributes. (T/F) 3. The clip feature class, must be a polygon. (T/F) 4. To perform a ______ operation, all input feature classes and feature layers must have polygon geometry. 5. List an example (or operation) for a proximity analysis? 6. In most cases, a buffer distance will vary depending on the type of coordinate system. (T/F) 7. The first step in a typical analysis with geoprocessing operations is: Determine the order in which to run the geoprocessing tools. (T/F) 8. __________ diagrams (white in color) can be turned into models. 9. The Modelbuilder is color coded by ________ (input, values, and output) and ________ (or geoprocessing tool). 10. You can drag and drop tools and data into a model from ArcCatalog and ArcMap. (T/F)

Remainder of Class Break ESRI Chapter 11 Preparing data for analysis (Dissolving, Creating graphs, Clipping, Exporting) ESRI Chapter 12 Analyzing spatial data (Buffering, Overlaying, Calculating attribute values)