Efficiently Calculating Sequential Numbers In Sorted Order In Arcmap

The Problem of Assigning Ordered IDs in GIS

A common task in GIS is to assign sequential integer identifiers (IDs) to features based on some attribute, ensuring the features are properly sorted before assigning the IDs. However, ArcMap lacks built-in tools to easily perform this operation. Manually ensuring features are sorted and then calculating sequential IDs is tedious and error-prone.

The difficulty arises because the default behavior when adding data to ArcMap does not guarantee features will be displayed in a sorted order. For example, loading a shapefile of parcel polygons will display the parcels based on their internal storage order in the shapefile, not by an attribute like parcel ID. Visually sorting the features based on an attribute before assigning ordered IDs can result in mistakes if some features are still unsorted.

Difficulty Ensuring Features are Truly Sorted Before Assigning IDs

The typical workflow to assign sequential IDs involves visually checking that features are sorted in the table of contents and layers panel by the desired attribute. However, it can be easy to miss unsorted features, leading to improper ID assignment. Relying on visual checks does not guarantee correct sort order.

Likewise, even if the layer visually appears sorted, there can still be internal inconsistencies in the data ordering. Certain attributes like text values are sorted alphabetically, which may not match the desired numeric sequence. Manually handling these intricacies around data sorting and ordering is impractical at scale.

ArcMap Lacks Built-in Tools to Assign Ordered IDs

ArcMap provides several tools for field calculations and data management, but lacks native tools for the distinct use case of assigning incremental IDs based on a sort order. The sequential ID calculation needs to happen after ensuring correct feature order in a separate step.

This increases the complexity when trying to automate the workflow. Without built-in methods, the burden falls on manual scripts or custom tools to first validate ordering, then safely calculate sequential IDs based on that order. Ad-hoc solutions may fail to handle all edge cases around feature sorting and ordering.

Using Python to Assign Sequential IDs

Python scripting provides an effective method to programmatically assign ordered, sequential IDs to features in ArcMap. Python can sort feature data, validate ordering, then safely calculate incremental IDs while handling errors and exceptions. Wrapped into a Python script tool, this functionality greatly simplifies ordered ID management.

Overview of Python Scripting to Automate ID Assignment

At a high level, the Python workflow to assign ordered IDs involves:

  1. Load feature layer into a Python list and sort by desired attribute
  2. Iterate through list and populate ID field with incremented integer value
  3. Write updates back to feature layer

Additional steps validate sort order before calculating IDs, and wrap logic into an ArcToolbox script tool with user parameters. Key Python modules used include arcpy for data access, numpy for array handling, and native math module for incremental values.

Walkthrough of Sample Script to Assign Ordered IDs

Below demonstrates a sample Python script to parameterize and automate assigning sequential IDs to features:

"""Assign Ordered IDs tool"""

# Import modules
import arcpy, numpy, math

# User inputs
in_features = arcpy.GetParameterAsText(0) 
id_field = arcpy.GetParameterAsText(1)
order_by_field = arcpy.GetParameterAsText(2)  

# Verify inputs 
if not arcpy.Exists(in_features):
    arcpy.AddError("Invalid input feature layer")

# Logic functions  
def sort_features(in_features, order_by_field):
    """Sort features by provided attribute"""
    # Details...

def assign_sequence(sorted_features, id_field): 
    """Calculate sequence IDs on sorted list"""
    # Details...     

# Execution logic
sorted_list = sort_features(in_features, order_by_field)  
assign_sequence(sorted_list, id_field)  

print "Ordered IDs assigned successfully!"

This demonstrates structuring a Python script tool for ArcGIS, retrieving user parameters, validating inputs, encapsulating logic into separate functions, and calling blocks of code to execute the ordered ID workflow. The full details…

Verifying Ordered IDs Were Assigned Correctly

After running automated Python scripts to assign ordered sequential IDs, it is critical to verify the IDs were calculated properly. Testing methodologies should validate both the sort order and sequence to catch any potential script bugs or logic errors.

Techniques to Validate Proper ID Assignment

Some best practices to confirm ordered ID assignment include:

  • Visually inspecting first and last IDs match expectations
  • Checking for duplicate IDs which may indicate skips in sequence
  • Programmatically comparing results against benchmark dataset
  • Identifying gaps in the ID sequence indicating sort anomalies
  • Spot checking middle ID values follow proper incrementing

For large datasets, automated testing comparing results against an expected sequence are required. Unit testing each function ensures code handles sort order exceptions properly before full workflow integration.

Troubleshooting Tips for Improper ID Ordering

Some common issues that can disrupt properly sequenced ID values include:

  • Unexpected text values in sort attribute field
  • Changes in underlying data source altering feature order
  • Incorrect field data type causing value sort anomalies
  • Null values disrupting the sort sequence
  • Changes in feature selection order after ID assignment

Code should handle unavoidable real-world data issues like text strings in numeric fields. Defensive coding practices alert users to unanticipated sort order changes for troubleshooting.

Optimizing Script Performance When Processing Large Datasets

When assigning sequential IDs to feature classes containing large numbers of records, script performance and processing time is a consideration. Targeted optimizations can drastically improve script speed and efficiency.

Strategies to Speed Up ID Assignment for Big Data

For large datasets, options to improve script performance include:

  • Batch Processing: Split into chunks by geometry or attributes
  • Query Filtering: Apply definition queries to limit records
  • Create Index: Index sort fields to optimize order
  • Reduce Writes: Delay saving changes to end

Processing big datasets in batches or blocks avoids memory issues and reduces locks. Optimized queries filter records before ordering and ID steps.

Hardware Considerations for Big Data Scripting

Apart from optimized code, processing large datasets depends significantly on:

  • CPU Cores: Support parallelized processing
  • RAM: Avoid swapping slow virtual memory
  • Fast Disks: Solid-state drives accelerate reads/writes
  • 64-bit Geoprocessing: Critical for large memory issues

Upgrading CPU, memory, storage and 64-bit hardware components drastically increases speed when scripting against big feature class data.

Applying Sequential IDs in Real-World GIS Workflows

Automating sequential ID management in ArcMap enables widespread workflow integrations based on ordered features. The scripts can drive core business processes relying on properly sorted attribute identifiers.

Use Cases Dependent on Ordered ID Values

Some example applications leveraging sorted sequential IDs include:

  • Coordinate Numbering: Label points or vertices with sorted coordinates
  • Address Geocoding: Support linear reference like 1000 – 2000 Main St
  • Work Order Tracking: Sequence locations requiring site visits
  • Parcel Numbering: Consistent attribute IDs for identification

Any process involving order-dependent enumeration or lookup benefits from automated assignment of incremental IDs.

Integrating ID Scripting into ArcGIS Workflows

Standard techniques to integrate orderly ID management into geoprocessing workflows include:

  • Importing scripts into ModelBuilder as script tool nodes
  • Calling scripts from Python AddIns in the ArcMap UI
  • Executing scripts from ArcGIS Pro notebooks and tasks
  • Chaining together scripts using ArcPy modules

Models and scripts performing spatial analysis or data maintenance can incorporate ordered ID capability for enhanced reporting and analytics.

Additional Resources

For further learning on calculating sequential IDs within ArcGIS, consult these additional resources:

References for Advancing Scripting and Automation Skills

Source Code Repository and Script Examples

See GitHub repo arcmap-sequential-id-scripts for additional script samples covered in this article.

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