Automating Unique Id Assignment Within Groups In Arcgis

The Problem of Duplicate IDs

In any GIS dataset, unique identifiers (IDs) are essential for distinguishing between features during analysis and data management. However, manually assigning unique IDs can be tedious and error-prone, often resulting in duplicate values.

Duplicate IDs can create numerous issues in ArcGIS workflows:

• Joins and relates may associate records incorrectly
• Spatial analysis results such as proximity or overlay operations may be inaccurate
• Attribute queries and selections may return unexpected features
• Symbolizing and labeling becomes problematic when features can’t be distinguished
• Errors occur when exporting or loading data into other systems expecting unique IDs

Tracking down the source of duplicate IDs means thoroughly reviewing attributes to locate the features with shared values. As datasets grow in size and complexity, manually ensuring unique IDs across all features and attribute combinations becomes infeasible.

Automated Solutions

Fortunately, ArcGIS provides built-in tools to automate the assignment of unique, sequential IDs while grouping features based on user-defined attributes.

The Calculate Field tool allows calculating ID values using Python parsers and functions. By leveraging the unique combination of attributes for each feature, scripts can assign IDs uniquely across the entire dataset or relative to subgroups sharing common attributes.

Using the ArcGIS Calculate Field tool to assign unique IDs

1. Add a new Text field that will hold the assigned unique IDs
2. Open the Calculate Field tool from the Attribute Table
3. Select the new ID field as the Field to calculate
4. Choose “Python Parser” as the Parser
5. Construct an UpdateCursor to iterate through features
• Set a counter variable to increment for each ID
• Use a unique value from other attributes to classify groupings
• Assign the incremented counter to the new ID field
6. Validate results and handle errors

Example code for calculating sequential unique IDs grouped by attributes

This script demonstrates sequentially assigning unique IDs by neighborhood groups:

rec=0  
def autoIncrement():  
 global rec  
 pStart = 1  
 pInterval = 1  
 if (rec == 0):  
  rec = pStart  
 else:  
  rec += pInterval  
 return "NH" + str(rec)

The key steps it executes:

1. Initialize a global rec counter variable to track IDs
2. Start IDs at 1 and increment by 1 for each feature
3. Prefix IDs with “NH” grouping acronym
4. Combine rec counter with acronym to build ID text
5. Reset rec to the starting number to restart sequence by group

By inserting the above function into a cursor that checks a Neighborhood field value, unique IDs can be assigned sequentially within groups sharing that attribute.

Customizing the ID format

The returned ID text from a calculate field script can be customized in different ways:

• Prefix, suffix or sequence letter/number patterns
• Specify exactly where repeating values restart
• Set variable ID length or fixed length by padding zeros
• Combine multiple attribute values into a single concatentated ID

Choosing recognizable patterns makes interpreting and validating IDs easier for workflows like manual data edits or join/relates.

Additional Options

For more advanced unique ID needs, ArcGIS provides additional approaches to explore.

Alternative tools like UpdateCursor for unique ID assignment

The UpdateCursor tool in the ArcPy Data Access module gives another option for unique field calculations:

• Directly updates values without a separate tool
• Offers coarser control with row-by-row iteration
• Supplementary compared to Calculate Field methods

Cursors let you leverage the same type of Python scripts and attribute-based groups described earlier.

Options when working with complex data or workflows

For organisms expecting much editing or complex analysis chains, more advanced setups may be useful:

• Load initial dataset into an enterprise geodatabase to enable database-driven ID rules
• Use globally unique identifiers (GUIDs) instead of sequential values
• Execute processes via custom script tools or Python add-ins for reusability

IT collaboration helps apply industry standards for enterprise systems and STDM needs.

Verifying and Maintaining Unique IDs

Checking that automated processes succeeded and handling ongoing edits to data requires additional steps.

SQL queries to validate unique IDs after assignment

Uniquely-indexed SQL queries provide the most reliable way to validate IDs:

 
CREATE UNIQUE INDEX [IDX_UniqueIDs] ON [LayerName] ([IDFieldName])

With the unique index, any query searching for duplicate IDs will fail if found.

Handling new features or changes to groupings

As edits occur adding new features or altering grouping attributes, reassess ID values:

• Re-run the Calculate ID process on updated fields or full layer
• Custom script tools detecting changes can trigger automated recalculation

Periodic checks for duplicate values are encouraged even once setup.