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Digital Elevation Model Data Preparation Process - GIS toolbox method
6. Assets Layer
Phoenix incorporates an asset impact model that can calculate asset impact during the fire simulation process. Impacts can calculated for up to 99 asset types and evaluated against up to 99 loss functions, these functions can express loss as a percentage of any of the fire characteristics calculated during the simulation process.
Characterising fires by asset impact provides an additional means of comparing fires. This can be useful for prioritising fires in a tactical scenario or strategically for quantifying the effectiveness of a specific treatment. Asset impact can be a more meaningfully measure than traditional characteristics like fire area, average intensity, perimeter length, etc.
Assets are incorporated as a standard Phoenix dataset. The major limitation of this process is that only one Asset can be recorded against a 30 m cell which requires assets to be in a priority order.
Asset Code Description
The Phoenix asset code is an unsigned 9 digit integer (max integer length supported by shapefile .dbf files). The code is composed of the following 4 parts:
Some worked examples:
| Asset Id | Impact Type Code | Asset Value | Phoenix Integer Scientific Notation | Asset Code |
|---|---|---|---|---|
| 23 | 03 | 1234 | 1234E+0 | 230312340 |
| 11 | 02 | 1.234567 | 1234E-3 | 110212343 |
| 3 | 17 | .0012345 | 0012E-4 | _31700124 |
| 1 | 43 | 50 | 0050E+0 | _14300500 |
Asset Id
The 2-digit asset id is used to report loss against. The current list of assets for Phoenix with their corresponding Impact Types is:
| Asset Id | Description | Impact Type |
|---|---|---|
| 1 | Housing | 2 |
| 2 | Infrastructure | 5 |
| 3 | Plantation | 5 |
| 4 | Catchment Tributaries | 4 |
| 5 | Catchment | 3 |
| 6 | Rainforest | 5 |
Impact Type Code
| Impact Type | Loss Description |
|---|---|
| 1 | Record loss if fire present |
| 2 | HouseLossRatio (Intensity, Ember Density) |
| 3 | Intensity > 3,000 kW/m |
| 4 | Intensity > 10,000 kW/m |
| 5 | Intensity > 30,000 kW/m |
Asset Value
Assets values are all expressed as units per square metre; point assets such as houses will need to be converted to their equivalent density/m2 value. Area based assets such as catchments are given a asset value of 1, indicating a 1 to 1 relationship with burnt area i.e. 1 unit/m2.
Asset values are limited to a maximum of 4 digits and 4 decimal places giving an effective range of .0001 to 9999. Care needs to be taken to ensure suitable units are selected for assets to accommodate this limited range.
Asset Layer Preparation Process
Housing
Step 1.
Starting with a house point layer in the correct projection, creates a house point density raster using the ‘Point Density’ tool in the ‘Spatial Analyst Tools – Density’ toolbox. Ensure the settings match the image below, this will create a 3m raster with a per metre housing density value. (Alternatively, Neighbourhood Settings could be 1 x 1 Cells)
NOTE : Ensure 'Area Units' are in m2
Step 2.
The housing density raster is next converted to a house count raster by multiplying the housing density value by each cells area (30 x 30 = 900m) and add 0.5 to round up. This is done using the ‘Single Output Map Algebra’ tool in the ‘Spatial Analyst Tools – Map Algebra’ toolbox with the following expression:
Int(D:\Otways2030\SourceShapefiles\Asset\house_density * 900 + 0.5)
Step 3.
The house count raster is next converted to an intermediate house count polygon shapefile using the ‘Raster to Polygon’ tool in the ‘Conversion Tools – From Raster’ toolbox.
NOTE: Uncheck the 'Simplify polygons' check box
Step 4.
NOTE : Delete all polygons with a house count of 0 after conversion to polygon. This significantly reduced the size of the file and speeds later processing.
Step 5.
Add a ‘House Density’ column (Double Precision) and using the "Calculate Field" option on the source table, convert the house count value back to a metre square density value by dividing the house count by 900.
Step 6.
Using the Phoenix asset classification tool for density values, generate the asset code
Step 7.
Finally use the Asset tool in Data Preparation toolbox to convert the housing layer into Phoenix Format.
NOTE : Convert to raster doesn’t seem to work from within a file geodatabase, export to shapefile before converting.
7. Wind Modifiers Layer
The "Wind Modifiers" data layer assists in making changes to local wind speed and direction due to the influences of the topography. Phoenix uses a "Mass Conservation" model called "Wind Ninja" developed by Jason Forthofer in 2007. A mass conservation model allows for winds to speed up over hills and be channeled up valleys to some extent, but it does not reproduce the full fluid dynamics of wind flow. However, it is considered worthwhile to take some account of topography on local wind speed and direction rather than assume that it is uniform across the landscape.
The only input data needed to create the Wind Modifiers layer is the Phoenix DEM (topography) layer. Winds of all speeds and directions are modelled across this terrain to determine the appropriate wind modifiers for all situations.
Use the "Wind Modifiers" tool in the Phoenix Toolbox to create this data layer for Phoenix. The output file will be in the same folder as the DEM layer used. This process can take a very long time.
