Chapter 10- Map Overlays and Geoprocessing Copyright © 2015 by Maribeth H. Price 10-1

Map overlay Joins features based on common location Forces features to split when they overlap each other, creating new features Enforces one-to-one cardinality between features in order to join attributes

Why overlay? We found that spatial joins are not always a good solution to a problem. Features don’t always fall neatly inside others. Overlay provides a way to split features so that there is always a one to one correspondence.

The problem with spatial joins Task: Gather statistics on the total length of roads in each land use category. We want to associate the land use type with the road—an inside spatial join problem. BUT… The road does not stop at the land use boundary. A summarized join won’t work with categorical data. Three different land use categories apply to this single road segment.

Map overlay splits features A map overlay forces the road features to split at the land use boundaries. Each new segment falls inside one land use category. The tables can be now joined, providing a basis for calculating total road lengths in each land use category.

Lines and polygons Use intersect to associate the land use category with each road. Sum to give total length of each land use.

Types of overlays Point-in-polygon overlay Line-in-polygon overlay Polygon-on-polygon overlay

Point-in-polygon overlay Use to assign polygon attributes to points Assign each house point the associated school district with education statistics Allow buyers to compare schools

Points and polygons Attaches land use polygon attributes to each school. For points and polygons, overlay does the same thing as a spatial join.

Line-in-polygon overlay Use to assign polygon attributes to lines Associate a geologic unit with each section of stream Use rock type to model infiltration

Lines and polygons Each stream acquires the geologic unit from the polygon it lies inside. Streams are split at geologic unit boundaries.

Polygon-on-polygon overlay 1. Combine spatial features by producing all possible polygons Geology Slope class

Steps to overlay Combine features spatially, producing all possible new features Combine attribute tables, bringing original values from each table and assigning to each new feature New spatial data set is created with features and attribute table +

Geology Slope class 2. Copy original values from both tables for each new polygon

Types of overlay Union—combines and keeps all features –Polygon-on-polygon overlay only Intersect—combines and keeps common features –Points, lines, or polygons intersect union

Intersect: keeps common areas Union: keeps all areas Polygon-on-polygon overlay only Overlay types

Union example Geology Slope class Find combinations of geology and slope that are high risk for landslides

Intersect example Find residential areas that are at risk of radon infiltration from the Opeche formation Residential areas Opeche formation

Hazards mapping Residential areas Opeche formation Areas of both

Habitat analysis with Intersect 1. Use queries to isolate features of interest from input layers. 2. Intersect selection layers to find common areas.

Intersecting different geometries Intersecting polygons: choose polygons, lines, or points for output Intersecting lines: choose lines or points Output dimension must be less than or same as lowest input dimension.

Intersecting different geometries Intersecting polygons and lines: choose lines or points Intersecting points with anything else always yields points.

Overlay operations summary union clip erase intersect output overlay input Attributes not joined Attributes joined

Extraction functions: Clip- THINK COOKIE CUTTER with the inside of the cutter being what is preserved and everything outside is clipped away, leaving only the attributes inside Erase- THINK DONUTS with the part that we are interested in being the donut, and the hole being erased from the set of attributes that we are dealing with Extraction functions: Clip- THINK COOKIE CUTTER with the inside of the cutter being what is preserved and everything outside is clipped away, leaving only the attributes inside Erase- THINK DONUTS with the part that we are interested in being the donut, and the hole being erased from the set of attributes that we are dealing with

Length/area changes Clip/erase can change the lengths or areas of features on the boundary. Geodatabase feature classes have Shape_Length and Shape_Area fields that are automatically updated. All other length or area-based fields are NOT updated. A user-defined MILES field would contain incorrect lengths after the clip.

Dissolving Dissolve polygons on habitat class Dissolve lines on street name Main St Choose an attribute to dissolve on…

Dissolving eliminates all of the attributes in the table except the dissolved one… However, you can choose to summarize the other attributes… Avg_ crown_cov_percent Sum_acres

Buffering Constructs polygon areas within a specified distance of features. Single buffers around linesMultiple buffers around points

Append Places features from one feature class into an existing feature class of the same type. Unlike union, it does not attempt to split or change features. The features are simply brought in. If they overlap, you’ll have double features. To bring attributes with the features, the attribute tables of the two input layers should match exactly. This function has many caveats and issues. Read the Help before attempting for the first time.

Merge Combines two feature classes of the same type into a new combined feature class. Merge does not attempt to split or change the features. Overlaps are permitted. You can use a Field Map option to control which input fields are included in the output feature class. The attribute tables can be different. This function has many caveats and issues. Read the Help before attempting for the first time.

Efficient overlay Overlay is time- and memory-intensive Minimize number of features –Dissolve vegetation before intersecting with other layers

Slivers Tiny polygons created during geoprocessing Result of slight differences in boundaries Can build up as a result of multiple operations Nuisance for some applications

Slivers Tolerances can be used to reduce problems with slivers. Tricky to set. No good if they are too small. Will degrade accuracy if too large. Affects output layer only, not the input layer. Multiple geoprocessing steps can progressively degrade the accuracy even with an appropriate tolerance—so be careful. Measure to see typical size of slivers. If 5 meters, then an XY tolerance of 8-10 meters or so might work well.

Geoprocessing Executing analysis functions and tools Stringing together analysis functions to achieve a result

Geoprocessing Environment Familiar ways to execute functions –Menus and toolbars –ArcToolbox Environment settings New ways to execute functions –Command line –Creating new tools with ModelBuilder –Writing scripts

Menus and Toolbars Quick access to common functions Organized by topic Turn off and on as needed Customizable Many different toolbars and menus available

ArcToolbox features Hundreds of functions organized into toolsets Expandable by purchasing extensions. Runs in ArcMap or ArcCatalog Many functions in the toolbox are not accessible from the menus and toolbars. Create custom toolsets with your favorite tools Create your own tools

Environment Settings Environment Settings affect the output of tools and commands Set at application level or when running a tool Application level settings are saved with map documents

Setting Environment Settings Application level settings affect all tools and menus Tool settings affect one execution of one tool

Some useful settings Current and scratch workspace Output coordinate systems Processing tolerances Use sparingly at first, and experiment with tool settings until you gain experience.

The XY tolerance Feature class property set at time it is created Minimum allowed distance between two vertices Used to cluster nearby vertices during geometry operations such as clip, intersect, etc. Default setting intended to preserve accuracy meter feet degrees gap overlap a c b Clustering

Using tolerances You can increase the XY Tolerance during geoprocessing to help eliminate slivers. Value must be carefully chosen. Affects the output only.

Effect of tolerances 1500 m1000 m Default500 m Union performed on districts and counties using different tolerance settings

Coordinate Systems and Geoprocessing Issues Geoprocessing often involves areas or lengths, so using a projected coordinate system is usually required for best results. If not otherwise defined, the output CS defaults to the first data set input to the tool. Reprojections will be done during processing if needed, but it is better to control if they are done. A good analyst is always aware of the coordinate systems being used during processing. Best Practices for Analysis When developing a project, choose a coordinate system, instead leaving data in a GCS or a chance mixture. Select a projection with minimal distortion for the study region. Convert all data sets to the same coordinate system (see Chapter 11) before doing any analysis. Ensure that the data frame is always set to the same CS as the data it contains.

Background Processing Introduced with ArcGIS 10 Continue working while a tool runs On by default Slower for short tasks Potential for file conflicts Author recommends turning it off Completion notification boxes: Background processing Foreground processing

Geoprocessing options Turn off background processing here Recently used tools

Using a tool Access info and Help Error tips Double-click

Searching for tools Search window Find tools based on name or keyword See where tool lives Click to open tool Hover for description

The Intersect tool

Dissolving Choose to summarize other attributes One attribute preserved in output

The Buffer tool NONE optionALL option Careful! The default Dissolve option on this tool is often NOT the best choice.

Areas and lengths Areas or lengths are important –Total length of streams in each watershed? –Total snail habitat area? Often tools change areas or lengths of features Before After Dissolve

Remember Geodatabase Shape_Area and Shape_Length fields are automatically updated. –Other fields may contain areas or lengths which are NOT Shapefiles do NOT update any area or length fields automatically.

Updating lengths and areas After performing operations than can change geometry, such as clip or intersect, recalculate any user-defined length or area fields.

ModelBuilder Create models built from sequences of tools Store processing steps for later reference Execute models repeatedly with different inputs Share models with others

Building models 1.Open new model 2.Drag and drop a tool from the toolbox to the model. 3.Open the tool and enter the parameters. 5. Done. Add another step if needed 4. Run the model step

Creating a new toolbox Toolboxes may be created in folders, geodatabases, or in other toolboxes. Name toolbox

Add toolbox to ArcToolbox Although accessible from the Catalog, it is convenient to place the new toolbox in ArcToolbox.

Creating a model 1. Create new model 2. Open Model Properties 3. Set Model Properties

Adding tools to model 1. Drag ‘n’ drop tool to model 2. Right-click to open tool properties Manage model layout

Set tool arguments Colors indicate that tool is ready to run. inputtooloutput Run Shadow indicates the step has been run.

Set Intersect arguments Select blue icons to ensure chain of inputs, outputs

Set Intersect(2) arguments

Set output properties Add to Display puts the output in the Table of Contents for viewing. Intermediate indicates that the output is temporary. It will be deleted after tool completes running.

Model ready to run

Examine Output Shadows indicate model has run.

Running it again Tools > Options > Geoprocessing –Set ArcMap to overwrite existing datasets

Run a model as a tool Setting an input to a parameter lets the user enter a value each time it is run. Run as tool Open canvas and edit

Parameters from tool arguments

Models and Scripts Models can be converted to scripts (programs) Scripts can be written and edited separately from models Scripts can include looping, if-then control etc. Choose from several languages Python is used by ESRI for examples Another way to create your own tools