Simplify Line (Cartography)

Summary

Simplifies lines by removing relatively extraneous vertices while preserving essential shape.

Illustration

Usage

• This tool employs different simplification algorithms for different purposes. To learn more about these algorithms, see How Simplify Line and Simplify Polygon work.

  • The Retain critical points (Douglas-Peucker) algorithm (algorithm='POINT_REMOVE' in Python) works by identifying and removing relatively redundant vertices to simplify data for display at smaller scales. It is the fastest of the simplification algorithms in this tool. This algorithm is often used for data compression or for coarse simplification. The angularity of the resulting lines increase significantly as the tolerance increases. This algorithm is based on the Douglas-Peucker algorithm: Douglas, David and Peucker, Thomas, "Algorithms for the reduction of the number of points required to represent a digitized line or its caricature," The Canadian Cartographer. 10(2), 112–22 (1973).
  • The Retain critical bends (Wang-Müller) algorithm (algorithm='BEND_SIMPLIFY' in Python) works by identifying and eliminating relatively insignificant bends to simplify data for display at smaller scales. It is typically more faithful to the input geometry than the Retain critical points (Douglas-Peucker) algorithm but can take more time to process. This algorithm is based on the algorithm defined in Wang, Zeshen and Müller, Jean-Claude, "Line Generalization Based on Analysis of Shape Characteristics," Cartography and Geographic Information Systems 25(1), 3–15 (1998).
  • The Retain weighted effective areas (Zhou-Jones) algorithm (algorithm='WEIGHTED_AREA' in Python) works by first identifying triangles of effective area for each vertex. Those triangles are then weighted by a set of metrics to compare the flatness, skewness, and convexity of each area. The weighted areas guide the removal of their corresponding vertices to simplify the line while retaining as much character as possible. This algorithm is based on the algorithm defined in Zhou, Sheng and Jones, Christopher B., "Shape-Aware Line Generalisation with Weighted Effective Area," in Fisher, Peter F. (Ed.), Developments in Spatial Handling: 11th International Symposium on Spatial Handling, 369–80 (2005).
  • The Retain effective areas (Visvalingam-Whyatt) algorithm (algorithm='EFFECTIVE_AREA' in Python) works by identifying triangles of effective area for each vertex to guide the removal of vertices to simplify the line while retaining as much character as possible. This algorithm is based on the algorithm defined in Visvalingam, M. and Whyatt, J. D., "Line Generalisation by Repeated Elimination of the Smallest Area," Cartographic Information Systems Research Group (CISRG) Discussion Paper 10, The University of Hull (1992).

• The Simplification Tolerance parameter value determines the degree of simplification. The larger the tolerance, the more coarse the resulting geometry is. Smaller tolerances generate geometry more faithful to the input. MinSimpTol and MaxSimpTol fields are both added to the output to store the tolerance that was used.

  Legacy:

  In the course of topology resolution, versions of the tool prior to ArcGIS Pro 1.4 modified tolerance on a per-feature basis and stored those values in the MinSimpTol and MaxSimpTol fields. Now the values in these fields will be the same and equal to the tolerance specified in the Simplification Tolerance parameter. Be sure to modify existing models or scripts that rely on any of these fields.

  • For the Retain critical points (Douglas-Peucker) algorithm, the tolerance is the maximum allowable perpendicular distance between each vertex and the new line created.
  • For the Retain critical bends (Wang-Müller) algorithm, the tolerance is the diameter of a circle that approximates a significant bend.
  • For the Retain weighted effective areas (Zhou-Jones) algorithm, the square of the tolerance is the area of a significant triangle defined by three adjacent vertices. The further a triangle deviates from equilateral, the higher weight it is given, and the less likely it is to be removed.
  • For the Retain effective areas (Visvalingam-Whyatt) algorithm, the square of the tolerance is the area of a significant triangle defined by three adjacent vertices.

• Use the Keep collapsed points parameter (collapsed_point_option in Python) to create an output point feature class to store the endpoints of any lines that are smaller than the spatial tolerance of the data. The point output is derived; it will use the same name and location as the Output feature class (out_feature_class in Python) but with a _Pnt suffix. The output line feature class contains all the fields present in the input feature class. The output point feature class does not contain any of these fields.

• The output line feature class is topologically correct. Any topological errors in the input data are flagged in the output line feature class. The output feature class includes two additional fields: InLine_FID and SimLnFlag that contain the input feature IDs and topological errors of the input, respectively. A SimLnFlag value of 1 indicates a topological error is present; 0 (zero) indicates that no errors are present.

  Legacy:

  Prior to the ArcGIS Pro 1.4 version of this tool, topological errors could be generated during processing. The Check for topological errors (error_checking_option in Python) and the Resolve topological errors (error_resolving_option in Python) parameters were included to identify and optionally resolve these errors. These parameters are still included in the tool's syntax for compatibility in scripts and models but are now ignored and are hidden from the tool's dialog box. The SimLnFlag field was used to flag topological errors introduced by the tool in processing. Now this field flags errors present in the input.

• Use the Input barrier layers parameter to identify features that must not be crossed by simplified lines. Barrier features can be points, lines, or polygons.

• Domains and subtypes are copied to the output even if the Transfer field domain, subtypes and attributes rules environment is unchecked.

• Processing large datasets may exceed memory limitations. In such cases, consider processing input data by partition by identifying a relevant polygon feature class in the Cartographic Partitions environment setting. Portions of the data, defined by partition boundaries, will be processed sequentially. The resulting feature class will be seamless and consistent at partition edges. See Generalizing large datasets using partitions for more information.

Syntax

arcpy.cartography.SimplifyLine(in_features, out_feature_class, algorithm, tolerance, {error_resolving_option}, {collapsed_point_option}, {error_checking_option}, {in_barriers})

Parameter	Explanation	Data Type
in_features	The input line features to be simplified.	Feature Layer
out_feature_class	The simplified output line feature class. It contains all fields present in the input feature class. The output line feature class is topologically correct. The tool does not introduce topology errors, but topological errors in the input data are flagged in the output line feature class. The output feature class includes two additional fields: InLine_FID and SimLnFlag to contain the input feature IDs and the input topological errors, respectively. A SimLnFlag value of 1 indicates an input topological error is present; 0 (zero) indicates that no input error is present.	Feature Class
algorithm	Specifies the line simplification algorithm. POINT_REMOVE — Retains critical points that preserve the essential shape of a line and removes all other points (Douglas-Peucker).This is the default. BEND_SIMPLIFY — Retains the critical bends and removes extraneous bends from a line (Wang-Müller). WEIGHTED_AREA —Retains vertices that form triangles of effective area that have been weighted by triangle shape (Zhou-Jones). EFFECTIVE_AREA — Retains vertices that form triangles of effective area (Visvalingam-Whyatt).	String
tolerance	The tolerance determines the degree of simplification. You can choose a preferred unit; otherwise, the units of the input will be used. MinSimpTol and MaxSimpTol fields are added to the output to store the tolerance that was used when processing occurred. For the POINT_REMOVE algorithm, the tolerance is the maximum allowable perpendicular distance between each vertex and the new line created. For the BEND_SIMPLIFY algorithm, the tolerance is the diameter of a circle that approximates a significant bend. For the WEIGHTED_AREA algorithm, the square of the tolerance is the area of a significant triangle defined by three adjacent vertices. The further a triangle deviates from equilateral, the higher weight it is given, and the less likely it is to be removed. For the EFFECTIVE_AREA algorithm, the square of the tolerance is the area of a significant triangle defined by three adjacent vertices.	Linear Unit
error_resolving_option (Optional)	Legacy: This is a legacy parameter that is no longer used. It was formerly used to dictate how topological errors, possibly introduced during processing, were handled. This parameter is still included in the tool's syntax for compatibility in scripts and models but is hidden from the tool's dialog box.	Boolean
collapsed_point_option (Optional)	Dictate whether to create an output point feature class to store the endpoints of any lines that are smaller than the spatial tolerance. The point output is derived; it will use the same name and location as the out_feature_class but with a _Pnt suffix. KEEP_COLLAPSED_POINTS —Record the endpoints of zero-length lines in a derived output point feature class. This is the default. NO_KEEP —Do not create a derived output point feature class.	Boolean
error_checking_option (Optional)	Note: This is a legacy parameter that is no longer used. It was formerly used to dictate how topological errors, possibly introduced during processing, were handled. This parameter is still included in the tool's syntax for compatibility in scripts and models but is hidden from the tool's dialog box.	Boolean
in_barriers [in_barriers,...] (Optional)	Inputs containing features to act as barriers for simplification. Resulting simplified lines will not touch or cross barrier features. For example, when simplifying contour lines, spot height features input as barriers ensure that the simplified contour lines will not simplify across these points. The output will not violate the elevation as described by measured spot heights.	Feature Layer

Derived Output

Name	Explanation	Data Type
out_point_feature_class	When the Keep collapsed points parameter (collapsed_point_option in Python) is used, an output point feature class is created to store the endpoints of any lines that are smaller than the spatial tolerance of the data.	Feature Class

Code sample

SimplifyLine example (Python window)

The following Python window script demonstrates how to use the SimplifyLine tool in immediate mode.

import arcpy
import arcpy.cartography as CA
arcpy.env.workspace = "C:/data"
CA.SimplifyLine("roads.shp", 
                "C:/output/output.gdb/simplified_roads", 
                "POINT_REMOVE", 
                20)

SimplifyLine example 2 (stand-alone script)

The following stand-alone script demonstrates how to use the SimplifyLine tool.

# Name: SimplifyLine_Example2.py
# Description: Simplify line features from two feature classes, rivers and coastlines,
# while maintaining their connections

# Import system modules
import arcpy
import arcpy.management as DM
import arcpy.cartography as CA
 
# Set environment settings
arcpy.env.workspace = "C:/data/Portland.gdb/Hydrography"
 
# Set local variables
inRiverFeatures = "rivers"
inCoastlineFeatures = "coastlines"

mergedFeatures = "C:/data/PortlandOutput.gdb/merged_lines"
simplifiedFeatures = "C:/data/PortlandOutput.gdb/merged_lines_simplified"
tempLayer = "tempLyr"

outRiverFeatureClass = "C:/data/PortlandOutput.gdb/rivers_final"
outCoastlineFeatureClass = "C:/data/PortlandOutput.gdb/coastlines_final"

# Merge rivers and coastlines into one feature class, 
# assuming that they have a common f-code field 
# with value 40 for rivers and 80 for coastlines.
DM.Merge(inRiverFeatures, inCoastlineFeatures, mergedFeatures)

# Simplify all lines.
CA.SimplifyLine(mergedFeatures, 
                simplifiedFeatures, 
                "BEND_SIMPLIFY", 
                100, 
                "KEEP_COLLAPSED_POINTS")
 
# Select rivers and coastlines by their f-code values 
# and put them in separate feature classes.
DM.MakeFeatureLayer(simplifiedFeatures, tempLayer, "f-code = 40")
DM.CopyFeatures(tempLayer, outRiverFeatureClass)

DM.MakeFeatureLayer(simplifiedFeatures, tempLayer, "f-code = 80")
DM.CopyFeatures(tempLayer, outCoastlineFeatureClass)

Environments

Cartographic Partitions, Current Workspace, Default Output Z Value, Output M Domain, Output XY Domain, Output Coordinate System, Extent, Output has M values, Output has Z values, Scratch Workspace, XY Tolerance

Licensing information

• Basic: No
• Standard: Yes
• Advanced: Yes