Overview of ExposR

10 July 2022

EXPOS Model

The EXPOS model uses a digital elevation model (DEM) to estimate exposed and protected areas for a given hurricane wind direction and inflection angle. The resulting topograhic exposure maps can be combined with output from the HURRECON model to estimate hurricane wind damage across a region.

EXPOS contains two main functions:

  1. The expos_model function estmates topopgrahic exposure for a specified wind direction and inflection angle. The input file is assumed to be a raster of elevation values in GeoTiff format with missing values represented by zero. Cells may be rectangular. The user can specify if coordinates are lat/long; otherwise lat/long is assumed if X values are between -180 and 180 and Y values are between -90 and 90. If coordinates are lat/long, horizontal and vertical units are assumed to be degrees and meters, respectively; otherwise horizontal and vertical units must be the same. Columns are assumed to be closely aligned with true North (0 degrees); if not, the map orientation (azimuth) must be specified in degrees. The name of the input file is assumed to be “dem.tif”.

The output file is a raster file in GeoTiff format with the following values: 0 = missing data, 1 = protected, 2 = exposed. Output files are named “expos-xxx-yy.tif” where xxx is the wind direction and yy is the inflection angle.

In previous studies, spatial resolutions of 30 or 60 meters and an inflection angle of 6 degrees were found to work well (see below). Note that increasing the inflection angle tends to decrease the size and number of protected areas while increasing the odds that these sites are protected.

  1. The expos_damage function estimates regional hurricane damage where topographic exposure at each location is determined by peak wind direction. If a location is protected, the enhanced Fujita scale (EF) rating from HURRECON is reduced by a specified number of EF ratings. This function requires a hurricane GeoTiff file created by HURRECON, exposure files created by EXPOS for the eight cardinal wind directions (N, NE, E, etc), and a reprojection file in CSV format (reproject.csv) that contains lat/long coordinates for the lower left and upper right corners of the digital elevation model (variables: name, lat_0, lon_0, lat_1, lon_1).

The output file is a raster file in GeoTiff format with the following values: 0 = missing, 1 = no damage, 2 = EF0 damage, 3 = EF1 damage, 4 = EF2 damage, 5 = EF3 damage, 6 = EF4 damage, 7 = EF5 damage. Output files are named “hhhh-damage-yy-z.tif” where hhhh is the hurricane ID, yy is the inflection angle, and z is the reduction in EF rating for protected areas.

Getting Started

Here are the basic steps for using the model. Please see below for more details.

  1. Install and load the R package for the model (ExposR).
  2. Create a directory for a particular study area with subdirectories as described below.
  3. Copy the digital elevation file to the dem subdirectory and rename it “dem.tif”.
  4. Download or create geographic and political boundary shapefiles for the desired study area. Copy these files to the vector subdirectory and rename so the first name of each file is “boundaries”.
  5. Use the expos_model function to create exposure maps for different wind directions and inflection angles.
  6. Use the expos_damage function to create maps of enhanced Fujita scale wind damage for particular hurricanes.

Details

All user functions begin with “expos”. The wind direction and inflection angle must be specified in degrees.

The user specifies a directory (exp_path) for a given study area. Input and output files are stored on the following subdirectories of this directory:

exp_path/dem
exp_path/exposure
exp_path/damage
exp_path/vector

The dem subdirectory contains the input elevation file. The exposure subdirectory contains output files from the expos_model function. The damage subdirectory contains input files from the HURRECON model, the reprojection file, and output files from the expos_damage function. Shapefiles that contain geographic and political boundaries for viewing results are stored on the vector subdirectory.

To run the model, create the above directories, copy the input files to their respective subdirectories, and use the library function to load ExposR.

Model Functions

expos_set_path
expos_get_path
expos_model
expos_damage
expos_summarize
expos_plot

The expos_set_path function sets the path for the current set of model runs. The expos_get_path function returns the current path. Use expos_set_path before using other functions.

The expos_model function creates a raster file of topographic wind exposure as a function of wind direction and inflection angle.

The expos_damage function uses output from EXPOS and HURRECON to create a raster file of wind damage where topograhic exposure at each location is determined by peak wind direction. If a location is protected, the enhanced Fujita scale rating is reduced by a specfied amount.

The expos_summarize function displays summary information for a specified raster file, including the number of rows and columns, spatial extent, cell height and width, and minimum and maximum value.

The expos_plot function creates a plot of a specified raster file.

Examples

expos_set_path("c:/expos/wach_30m")
expos_get_path()

expos_model(wind_direction=90, inflection_angle=6)
expos_damage(hurricane="AL1938-06", inflection_angle=6, protect=2)

expos_summarize("dem")

expos_plot("dem")
expos_plot("expos-090-06")
expos_plot("AL1938-06-damage-06-2")

History

The original EXPOS model was written in Borland Pascal and depended on Idrisi for spatial visualization. The model was used in published studies of the ecological impacts of historical hurricanes in New England and Puerto Rico: