fire_exp_dir() returns a SpatVector of linear transects toward
a value. Transects are assessed as viable wildfire pathways by intersecting
them with areas of high exposure.
Usage
fire_exp_dir(
exposure,
value,
t_lengths = c(5000, 5000, 5000),
interval = 1,
thresh_exp = 0.6,
thresh_viable = 0.8,
table = FALSE
)Arguments
- exposure
SpatRaster (e.g. from
fire_exp())- value
Spatvector of value as a point or simplified polygon
- t_lengths
(Optional) A vector of three numeric values. The length of each transect starting from the value in meters. The default is
c(5000, 5000, 5000).- interval
(Optional) Numeric. The degree interval at which to draw the transects for analysis. Can be one of 0.5, 1, 2, 3, 4, 5, 6, 8, or 10. The default is
1.- thresh_exp
(optional) Numeric. The exposure value to use to define high exposure as a proportion. Must be between 0-1. The default is
0.6.- thresh_viable
(optional) Numeric. The minimum intersection of a transect with a high exposure patch to be defined as a viable pathway as a proportion. Must be between 0-1. The default is
0.8.- table
Boolean, when
TRUE: returns a table instead of a feature. The default isFALSE.
Value
a SpatVector of the transects with the attributes: 'deg' = degree, 'seg' = segment, and 'viable'. The transects will be returned with the same CRS as the input features.
If table = TRUE: a data frame is returned instead with an additional
attribute 'wkt', which is a Well Known Text (WKT) string of transect
geometries (coordinate reference system: WGS84).
Details
fire_exp_dir() automates the directional vulnerability assessment
methods documented in Beverly and Forbes (2023). This analysis is used to
assess linear wildfire vulnerability on the landscape in a systematic radial
sampling pattern. This is a landscape scale process, so the exposure raster
used should also be landscape scale. Use tdist = "l" when preparing data
with fire_exp() for use with this function. See fire_exp() details
for more information.
The output line features will have the attribute 'viable'
which can be used to visualize the pathways. Outputs can be visualized with
fire_exp_dir_plot(), fire_exp_dir_map(), or exported as a spatial
feature.
Spatial Reference
The inputs for the exposure and value layer must have the same coordinate reference system (CRS) defined. The transects will be returned in the same CRS as the inputs.
This function draws the transects by calculating the end point of a
transect by finding the shortest path along the WGS84
(EPSG:4326) ellipsoid at a given bearing and
distance. The value input is reprojected from the input CRS to latitude and
longitude using WGS84 for the calculations. After the transects are created,
they are projected to match the CRS of the input exposure and value layer.
The lengths of the projected transects will be effected by the scale factor
of the input CRS; however, the geodesic lengths are maintained.
Value feature
The value feature can be provided as a point or a simplified polygon.
If using a point feature the analysis can be sensitive to the placement of a point. For example, if using a point to represent a large town a point placed at the centroid will likely have different outputs than a point placed at the edge of the community due to the arrangement of lower exposure values typical of a built environment.
An option to use a simplified polygon has been added to this function for values that may be too large to represent with a single point. The polygon should be drawn with the consideration of the following or the function will not be able to run. The polygon must be a single part polygon with no holes. The polygon should have a smooth and simple shape, ideally circular or ellipsoidal. The polygon should also have an approximate radius of less than 5000 meters from the center. If the area of interest is larger than this then consider using multiple assessments.
Default Values
The default values are based on the methods used and validated in Alberta, Canada by Beverly and Forbes (2023). Options have been added to the function to allow these values to be adjusted by the user if required. Adjusting these values can lead to unexpected and uncertain results.
Adjusting the transects
The drawing of the transects can be customized by varying the intervals and
segment lengths if desired. Adjustments to the interval and t_lengths
parameters will effect how much of the exposure data is being captured by
the transects. If both parameters are being adjusted some trigonometry might
be required to find the optimal combination of the parameters to ensure
distances between the transects make sense for the intended application. The
resolution of the exposure raster may also be a factor in these decisions.
Interval
The interval parameter defines how many transects are drawn. The default of
1 draws a transect at every whole degree from 1-360. This outputs a total
of 1080 transects, 360 for each segment. Increasing the interval will
output less transects and process faster. When the interval is
increased, the distance between the ends of the transects will also be
increased. For example: the terminus of 15000 meter transects (the default)
drawn every 1 degree (the default) will be approximately 250 meters apart,
but if drawn at 10 degree intervals will be approximately 2500 meters
apart. Larger intervals will increase speed and processing time, but might
not capture potential pathways between transects farther from the value.
Adjusting the thresholds
Threshold adjustments should only be considered if validation within the
area of interest have been done. The function fire_exp_validate() has been
included with this package to make the process easier, but still requires
significant time, data, and understanding.
High exposure
The thresh_exp parameter can be adjusted to define the minimum exposure
value to be considered for the directional assessment. The default value of
0.6 is based on the findings of Beverly et al. (2021) who showed that
observed fires burned preferentially in areas where wildfire exposure values
exceed 60%. Adjusting this value is only recommended after a validation of
wildfire exposure has been conducted in the area of interest.
Viability
The thresh_viable parameter defines the minimum intersection with high
exposure areas to be considered a viable pathway. The default value of
0.8 was determined by Beverly and Forbes (2023) by drawing continuous
linear transects within burned areas to represent observed pathways. It was
found that the average intersection with patches of pre-fire high exposure
was 80%. This methodology could be repeated in the users area of interest.
References
Beverly JL, Forbes AM (2023) Assessing directional vulnerability to wildfire. Natural Hazards 117, 831-849. DOI
Beverly JL, McLoughlin N, Chapman E (2021) A simple metric of landscape fire exposure. Landscape Ecology 36, 785-801. DOI
Examples
# read example hazard data
hazard_file_path <- "extdata/hazard.tif"
hazard <- terra::rast(system.file(hazard_file_path, package = "fireexposuR"))
# generate an example point
point_wkt <- "POINT (400000 6050000)"
point <- terra::vect(point_wkt, crs = hazard)
# compute exposure metric
exposure <- fire_exp(hazard)
# assess directional exposure
fire_exp_dir(exposure, point)
#> class : SpatVector
#> geometry : lines
#> dimensions : 1080, 3 (geometries, attributes)
#> extent : 385010.1, 414989.5, 6035010, 6064990 (xmin, xmax, ymin, ymax)
#> coord. ref. : NAD83 / Alberta 10-TM (Forest) (EPSG:3400)
#> names : deg seg viable
#> type : <num> <chr> <num>
#> values : 1 seg1 0
#> 1 seg2 1
#> 1 seg3 0