Copyright © 2006 by Maribeth H. Price 8-1 Chapter 8 Geoprocessing
Copyright © 2006 by Maribeth H. Price 8-2 Outline Spatial analysis and overlays Types of overlays How to perform overlays Other geoprocessing tools –Clip, dissolve, buffer, append Tips for overlay Geoprocessing methods
Copyright © 2006 by Maribeth H. Price 8-3 Geoprocessing Executing analysis functions and tools Stringing together analysis functions to achieve a result
Copyright © 2006 by Maribeth H. Price 8-4 Geoprocessing Environment Familiar ways to execute functions –Menus –Tools in ArcToolbox Environment settings New ways to execute functions –Command line –Creating new tools with ModelBuilder –Writing scripts
Copyright © 2006 by Maribeth H. Price 8-5 Key analysis functions Overlay –Without attributes (clip, erase) –With attributes (intersect, union) Dissolve Buffer Append
Copyright © 2006 by Maribeth H. Price 8-6 Without attributes (extraction) Extracts portions of features based on an overlay layer. Clip keeps the features inside the feature boundary. Erase keeps the features outside the feature boundary. clip/erase layer Ignores interior boundaries; brings attributes through unchanged.
Copyright © 2006 by Maribeth H. Price 8-7 On-the-fly clipping Temporary clip applied to a map layout Does not create new layers Can be performed on many layers simultaneously Can be removed when no longer needed Set as a data frame property
Copyright © 2006 by Maribeth H. Price 8-8
Copyright © 2006 by Maribeth H. Price 8-9 Overlay with attributes Similar to a spatial join Combines attributes based on common location (inside join) Enforces one-to-one cardinality between features
Copyright © 2006 by Maribeth H. Price 8-10 The problem with spatial joins Associate the land use type with the road BUT The road does not stop at the land use boundary.
Copyright © 2006 by Maribeth H. Price 8-11 Map overlay A map overlay forces features to split at polygon boundaries.
Copyright © 2006 by Maribeth H. Price 8-12 Now a land use type can be associated with each road. Sum to give total length of each land use.
Copyright © 2006 by Maribeth H. Price 8-13 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 polygon New spatial data set is created with features and attribute table
Copyright © 2006 by Maribeth H. Price Combine spatial features by producing all possible polygons Geology Slope class
Copyright © 2006 by Maribeth H. Price 8-15 Geology Slope class 2. Combine tables, bringing original values from each table for each new polygon
Copyright © 2006 by Maribeth H. Price 8-16 Intersect: keeps common areas Union: keeps all areas Overlay types
Copyright © 2006 by Maribeth H. Price 8-17 Geology Slope class Union example Find combinations of geology and slope that are high risk for landslides
Copyright © 2006 by Maribeth H. Price 8-18 Intersect example Find residential areas that are at risk of radon infiltration from the Opeche formation Residential areas Opeche formation
Copyright © 2006 by Maribeth H. Price 8-19 Hazards mapping Residential areas Opeche formation Areas of both
Copyright © 2006 by Maribeth H. Price 8-20 Habitat analysis with Intersect Use of multiple input layers to define regions with specific characteristics
Copyright © 2006 by Maribeth H. Price 8-21 Other overlay examples Point-in-poly Wells in geology Line-in-poly Roads in landuse Poly-on-poly Zoning on geology
Copyright © 2006 by Maribeth H. Price 8-22 Output geometry choices Intersecting polygons: choose polygons, lines, or points Intersecting lines: choose lines or points Output dimension must be less than or same as lowest input dimension.
Copyright © 2006 by Maribeth H. Price 8-23 Output geometries Intersecting polygons and lines
Copyright © 2006 by Maribeth H. Price 8-24 Overlay operations summary union clip erase intersect output overlay input Attributes not joined Attributes joined
Copyright © 2006 by Maribeth H. Price 8-25 Geoprocessing Environment Familiar ways to execute functions –Menus –Tools in ArcToolbox Environment settings New ways to execute functions –Command line –Creating new tools with ModelBuilder –Writing scripts
Copyright © 2006 by Maribeth H. Price 8-26 Using a tool
Copyright © 2006 by Maribeth H. Price 8-27
Copyright © 2006 by Maribeth H. Price 8-28 The Intersect tool
Copyright © 2006 by Maribeth H. Price 8-29 Dissolving Choose to summarize other attributes Only attribute preserved in output layer
Copyright © 2006 by Maribeth H. Price 8-30 Dissolving Dissolve polygons on habitat class Dissolve lines on street name Main St Choose an attribute to dissolve on…
Copyright © 2006 by Maribeth H. Price 8-31 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
Copyright © 2006 by Maribeth H. Price 8-32 Buffering Constructs polygon areas within a specified distance of features. Single buffers around lines Multiple buffers around points
Copyright © 2006 by Maribeth H. Price 8-33 The Buffer tool
Copyright © 2006 by Maribeth H. Price 8-34 Buffering with the NONE option Buffering with the ALL option The Dissolve option
Copyright © 2006 by Maribeth H. Price 8-35 Append
Copyright © 2006 by Maribeth H. Price 8-36 Appending attributes Combines feature classes without overlay To bring attributes along, tables of input features classes must match exactly. –same classes, same order, same definitions
Copyright © 2006 by Maribeth H. Price 8-37 Efficient overlay Overlay is time intensive Minimize number of features –Intersect geology and elevation first –Dissolve vegetation before intersecting with other layers
Copyright © 2006 by Maribeth H. Price 8-38 Geoprocessing Environment Familiar ways to execute functions –Menus –Tools in ArcToolbox Environment settings New ways to execute functions –Command line –Creating new tools with ModelBuilder –Writing scripts
Copyright © 2006 by Maribeth H. Price 8-39 Geoprocessing Environment Settings that influence function inputs and outputs
Copyright © 2006 by Maribeth H. Price 8-40 Environment Settings Hierarchical—can be set for the document or models or individual tools Most specific setting takes precedence
Copyright © 2006 by Maribeth H. Price 8-41 Using environment settings You are clipping a series of layers in order to place them in a new geodatabase with a different coordinate system. Slow way: Clip each file Project each clipped file and put into final geodatabase Delete intermediate files Clever way: Set the current workspace and output geodatabase in Environments Clip each file. Output automatically projected into the final geodatabase.
Copyright © 2006 by Maribeth H. Price 8-42 Coordinate systems Output coordinate system rules –If output is placed in a feature dataset, the output CS matches the dataset CS –If the environment setting is set to a coordinate system, that CS is used. –If the environment is not set, the CS of the first input data set to the tool will be used. Using projected coordinate systems is best –More convenient when areas/lengths are needed –Fewer distortions to worry about
Copyright © 2006 by Maribeth H. Price 8-43 Managing slivers with settings Tiny areas of overlap created during geoprocessing Result of slight differences in boundaries Can build up as a result of multiple operations Nuisance for some applications You can reduce sliver problems by specifying an XY tolerance in the Environment Settings
Copyright © 2006 by Maribeth H. Price 8-44 Tolerances During geometry operations (like buffering, intersect, editing, etc) tolerances are used to determine whether two features should be the same. The tolerance is the maximum distance a feature vertex can be moved to coincide with another vertex. Tolerance a c b a c b gap overlap Coincident lines? Clustering
Copyright © 2006 by Maribeth H. Price 8-45 The XY tolerance XY Tolerance is a property of a feature class –Set at time feature class is created –Used during geoprocessing operations such as clip, buffer, intersect, etc. –Avoids tiny slivers and dangles –Can override it using geoprocessing environment settings if needed (General Settings) Default setting –0.001 meter – feet ( inches) – degrees Defaults are calibrated to preserve coordinate precision, not resolve differences between data sets during geoprocessing.
Copyright © 2006 by Maribeth H. Price 8-46 Setting the tolerance If slivers are a problem for your analysis, you may wish to increase the XY Tolerance in the Environments settings to several meters or more. The XY Tolerance box is way down the list of settings under the General Settings heading. Measure to see typical size of slivers. If 5 meters, then an XY tolerance of 8-10 meters or so might work well.
Copyright © 2006 by Maribeth H. Price 8-47 Geoprocessing Environment Familiar ways to execute functions –Menus –Tools in ArcToolbox Environment settings New ways to execute functions –Command line –Creating new tools with ModelBuilder –Writing scripts
Copyright © 2006 by Maribeth H. Price 8-48 Other ways to execute functions Menus Tools Command line ModelBuilder Scripts
Copyright © 2006 by Maribeth H. Price 8-49 Command line geoprocessing Type command instead of using tool or menu Can be faster than using the mouse Smart interface helps you complete commands correctly with right syntax Type commands here Messages/progress here
Copyright © 2006 by Maribeth H. Price 8-50 ModelBuilder Create models built from sequences of tools Store processing steps for later reference Execute models repeatedly with different inputs Share models with others
Copyright © 2006 by Maribeth H. Price 8-51 Scripts Programs to execute geoprocessing steps Include looping, if-then control etc. Choose from several languages Python used by ESRI for examples Can convert models to scripts and then modify them
Copyright © 2006 by Maribeth H. Price 8-52 Areas and lengths Often analysis involves areas or lengths –Total length of streams in each watershed? –Total snail habitat area?
Copyright © 2006 by Maribeth H. Price 8-53 Geodatabase geometry measures Automatically created and maintained –Usually appear at end of table Shape_Area field Shape_Length field –Units will match units of the coordinate system
Copyright © 2006 by Maribeth H. Price 8-54 Coverage geometry measures –Automatically constructed –Updated whenever the BUILD or CLEAN command is used to update topology LENGTH field in arc tables AREA field in polygon tables PERIMETER field in polygon tables
Copyright © 2006 by Maribeth H. Price 8-55 Shapefile geometry measures Shapefiles DO NOT create or maintain these fields automatically! –Must be created and updated manually
Copyright © 2006 by Maribeth H. Price 8-56 Be careful! Before After AREA/LENGTH/PERIMETER fields in shapefiles are NOT automatically updated if features change. Don’t use one of these fields unless you are CERTAIN that they are correct. These area/perimeter fields are likely from a coverage
Copyright © 2006 by Maribeth H. Price 8-57 User geometry calculations Create field to hold values Choose type, coordinate system, and units Not automatically updated if features change!!!
Copyright © 2006 by Maribeth H. Price 8-58 Test yourself: True or False? A field named AREA in a shapefile will always have the correct area? –False. Shapefile area fields are not automatically maintained. A field named AREA in a geodatabase will always have the correct area? –False. Automatically updated fields in a geodatabase are called Shape_Area. A field named AREA in a coverage will always have the correct area? –True. Automatically updated area fields in a coverage ARE called AREA.