The current dataset is appropriate for displaying the overall pattern of WUI development at the county level, and comparing counties in terms of development patterns. Until the dataset is refined through a field review and quality assurance process, it is not suited for WUI designations for individual houses or neighborhoods.

The data can be downloaded as a file geodatabase here:  GIS Mapping and Data Analytics | CAL FIRE

Three WUI classes are mapped:

Wildland Urban Interface – dense housing adjacent to vegetation that can burn in a wildfire,

Wildland Urban Intermix - housing development interspersed in an area dominated by wildland vegetation subject to wildfire, and 

Wildfire Influence Zone - wildfire susceptible vegetation up to 1.5 miles from Wildland Urban Interface or Wildland Urban Intermix.  

Wildland Urban Interface, Wildland Urban Intermix, and Wildfire Influence Zones. The model uses residential structure density and vegetative cover to define areas within the Fire Hazard Severity Zones. 

Primary Data Inputs:

• Fire Hazard Severity Zones (FHSZ_Assessment25_1)
• Housing Unit Density (HousingUnit_Density2020_DEN4)

Secondary Data Inputs:(used to determine vegetation dominance)

• State Wildland Zones (FHSZ_State_Wildland_Zones_v17_1)
• Canopy Cover (CanopyCoverSALO2020)

Fire Hazard Severity Zones: This source raster dataset represents Fire Management Analysis Zones as adopted officially on April 1, 2024 for State Responsibility Area (SRA) and as distributed to local governments in February and March of 2025 for Local Responsibility Areas (LRA). The source data for the LRA release also contains FHSZ coverage for Federal Responsibility Areas, but these data are used to fill out the state for assessment purposes only. Data sources: FHSZALL_v25_1 (SRA Approved and LRA Recommended) Statewide_v17_4 (Federal Responsibility Areas)

Housing unit density classes for California derived from 30-m rasters extracted from Wildfire Risk to Communities: Spatial datasets of wildfire risk for populated areas in the United States and reprojected to California Teale Albers NAD87. 

DEN4 Description

1. Less than 1 Housing Unit per 20 acres
2. 1 Housing Unit per 20 acres to 1 Housing Unit per 5 acres
3. More than 1 Housing Unit per 5 acres to 1 Housing Unit per acre 
4. More than 1 Housing Unit per acre

After classifying, clusters of DEN4 values less than 80 cells (just under 18 acres) were nibbled to the nearest adjacent DEN4 class.

State Wildland Zones and Canopy Cover: State Wildland Zones are used to determine the dominance of vegetation. Areas which would otherwise be classed as Wildland Urban Interface.area reclassed to Intermix if the vegetation cover is 50% or more. These canopy cover data are used in concert with SALO Canopy Cover to determine vegetation dominance in areas both within and outside of the the extent of the wildland zones.

• State Wildland Zones: State wildland zones are determined by the attribute flame class, which was derived as part of fire hazard modeling used in the determination of Fire Hazard Severity Zones and represent areas where wildland fire behavior can be assessed using common fire behavior tools. It is derived from fuel model attributes slope, and local fire weather conditions as processed through the NEXUS Fire Behavior platform, and reflects flame front characterization of intensity (flame length) that was then aggregated to fire environment polygons, averaged across the polygon area, and finally classified nominally according to quantile distributions with some threshold adjustments to reflect realistic class breaks for marginal areas of widely accepted hazard levels. All wildland zones are 50% or more vegetated.
• Canopy Cover: The horizontal cover fraction occupied by tree canopies. 2020 SALO Canopy cover data was downloaded for all California counties from here: https://forestobservatory.com/ on 5/17/2022 and mosaiced into one statewide dataset, reprojected from UTM 10 to Teale Albers NAD83 and resampled to 30m. 

Note: Vegetation dominance is determined as either FHSZ Wildland Zone 1-3 or SALO > 50% cover. A 3X3 cell circular focal mean is applied and areas with 0.5 or greater are considered at least 50% vegetated.

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These data are analyzed according to the following parameters:

Interface:

• DEN4 Class 3 or 4 
• In Moderate, High, or Very High Fire Hazard Severity Zone
• Less than 50% vegetation cover
• Spatially contiguous groups of 30m cells that are approximately 20 acres in size or larger

Intermix:

• DEN4 Class 2 or 3
• In Moderate, High or Very High Fire Hazard Severity Zone
• Reclassed Interface:
• Interface cell groups less than 20 acres
• Interface that is 50% or more vegetated and in spatially contiguous groups of 30m cells that are at least 20 acres in size 
• Intermix is spatially contiguous groups of 30m cells approximately 100 acres in size or larger

Interface and Intermix are then combined. After combining, any cell group with fewer than 80 cells is classed to the value of its neighbor.

Influence Zone:

• Up to 1.5 miles from Interface or Intermix
• In Moderate, High, or Very High Fire Hazard Severity Zone
• Not Interface or Intermix

Interface, Intermix and Influence Zones are then combined. After combining, any cell group with fewer than 80 cells is classed to the value of its neighbor.

A final step in the model addressed an inadvertent error invoked by the processing of potential interface conversion to Intermix for small fragments (<20 acres of Interface) and larger areas of Interface that were covered by a majority (>50%) vegetation within the areas otherwise defined as Interface because of meeting housing unit density and hazard requirements. When these lands were then subject to the final size minimum of 100 acres they then reverted to being potential buffer areas. This was remedied by selecting all lands that met the criteria of DEN4 values 3 and 4 (i.e.,all areas with housing density greater than 1 HU/ac) and reverting them to Interface designation. These previously eliminated but now reverted Intermix areas did not meet the 100 acre requirement and did not produce additional buffer zone influence areas from them. In most cases they are sufficiently embedded within Influence Zone buffers to be consistent with map objectives describing the land in terms of exposure and opportunity for community protection and risk mitigation.