Available Operations

exactextract provides numerous built-in operations for summarizing gridded data. This page provides a list of the Built-in operations and arguments tuning their behavior. If a built-in operation is not suitable, custom operations can be defined when using either the Python or R bindings.

Note

All of the operations listed in this page are supported in the command-line interface and Python bindings. Some operations are not currently available in R.

Built-in operations

• \(x_i\) represents the value of the ith raster cell,

• \(c_i\) represents the fraction of the ith raster cell that is covered by the polygon (unless directed otherwise by the coverage_weight argument), and

• \(w_i\) represents the weight of the ith raster cell.

Values in the “example result” column refer to the value and weighting rasters shown below. In these images, values of the “value raster” range from 1 to 4, and values of the “weighting raster” range from 5 to 8. The area covered by the polygon is shaded purple.

Example Value Raster	Example Weighting Raster

Name	Formula	Description	Typical Application	Example Result
cell_id		Array with 0-based index of each cell that intersects the polygon, increasing left-to-right.		[ 0, 2, 3]
center_x		Array with cell center x-coordinate for each cell that intersects the polygon. Each cell center may or may not be inside the polygon.		[ 0.5, 0.5, 1.5 ]
center_y		Array with cell center y-coordinate for each cell that intersects the polygon. Each cell center may or may not be inside the polygon.		[ 1.5, 0.5, 0.5 ]
coefficient_of_variation	stdev / mean	Population coefficient of variation of cell values that intersect the polygon, taking into account coverage fraction.		0.41
count	\(\Sigma{c_i}\)	Sum of all cell coverage fractions.		0.5 + 0 + 1 + 0.25 = 1.75
coverage		Array with coverage fraction of each cell that intersects the polygon		[ 0.5, 1.0, 0.25 ]
frac		Fraction of covered cells that are occupied by each distinct raster value.	Land cover summary
majority		The raster value occupying the greatest number of cells, taking into account cell coverage fractions but not weighting raster values.	Most common land cover type	3
max		Maximum value of cells that intersect the polygon, not taking coverage fractions or weighting raster values into account.	Maximum temperature	4
max_center_x		Cell center x-coordinate for the cell containing the maximum value intersected by the polygon. The center of this cell may or may not be inside the polygon.	Highest point in watershed	1.5
max_center_y		Cell center y-coordinate for the cell containing the maximum value intersected by the polygon. The center of this cell may or may not be inside the polygon.	Highest point in watershed	0.5
mean	\(\frac{\Sigma{x_ic_i}}{\Sigma{c_i}}\)	Mean value of cells that intersect the polygon, weighted by the percent of each cell that is covered.	Average temperature	4.5/1.75 = 2.57
median		Median value of cells that intersect the polygon, weighted by the percent of each cell that is covered	Average temperature	2
min		Minimum value of cells that intersect the polygon, not taking coverage fractions or weighting raster values into account.	Minimum elevation	1
min_center_x		Cell center x-coordinate for the cell containing the minimum value intersected by the polygon. The center of this cell may or may not be inside the polygon.	Lowest point in watershed	0.5
min_center_y		Cell center y-coordinate for the cell containing the minimum value intersected by the polygon. The center of this cell may or may not be inside the polygon.	Lowest point in watershed	1.5
minority		The raster value occupying the least number of cells, taking into account cell coverage fractions but not weighting raster values.	Least common land cover type	4
quantile		Specified quantile of cells that intersect the polygon, weighted by the percent of each cell that is covered. Quantile value is specified by the q argument, 0 to 1.
stdev	\(\sqrt{\frac{\Sigma{c_i}(x_i - \bar{x})^{2}}{\Sigma{c_i}}}\)	Population standard deviation of cell values that intersect the polygon, taking into account coverage fraction.		1.05
sum	\(\Sigma{x_ic_i}\)	Sum of values of raster cells that intersect the polygon, with each raster value weighted by its coverage fraction.	Total population	0.5*1 + 0&2 + 1.0*3 + 0.25*4 = 4.5
unique		Array of unique raster values for cells that intersect the polygon		[ 1, 3, 4 ]
values		Array of raster values for each cell that intersects the polygon		[ 1, 3, 4 ]
variance	\(\frac{\Sigma{c_i}(x_i - \bar{x})^{2}}{\Sigma{c_i}}\)	Population variance of cell values that intersect the polygon, taking into account coverage fraction.		1.10
variety		The number of distinct raster values in cells wholly or partially covered by the polygon.	Number of land cover types	3
weighted_frac		Fraction of covered cells that are occupied by each distinct raster value, weighted by the value of a second weighting raster.	Population-weighted land cover summary
weighted_mean	\(\frac{\Sigma{x_ic_iwi}}{\Sigma{c_iw_i}}\)	Mean value of cells that intersect the polygon, weighted by the product over the coverage fraction and the weighting raster.	Population-weighted average temperature	31.5 / (0.5*5 + 0*6 + 1.0*7 + 0.25*8) = 2.74
weighted_stdev		Weighted version of stdev.
weighted_sum	\(\Sigma{x_ic_iw_i}\)	Sum of raster cells covered by the polygon, with each raster value weighted by its coverage fraction and weighting raster value.	Total crop production lost	0.5*1*5 + 0*2*6 + 1.0*3*7 + 0.25*4*8 = 31.5
weighted_variance		Weighted version of variance
weights		Array of weight values for each cell that intersects the polygon		[ 5, 7, 8 ]

Operation arguments

The behavior of these statistics may be modified by the following arguments:

• coverage_weight - specifies the value to use for \(c_i\) in the above formulas. The following methods are available:

  • fraction - the default method, with \(c_i\) ranging from 0 to 1

  • none - \(c_i\) is always equal to 1; all pixels are given the same weight in the above calculations regardless of their coverage fraction

  • area_cartesian - \(c_i\) is the fraction of the pixel multiplied by it x and y resolutions

  • area_spherical_m2 - \(c_i\) is the fraction of the pixel that is covered multiplied by a spherical approximation of the cell’s area in square meters

  • area_spherical_km2 - \(c_i\) is the fraction of the pixel that is covered multiplied by a spherical approximation of the cell’s area in square kilometers

• default_value - specifies a value to be used for NODATA pixels instead of ignoring them

• default_weight - specifies a weighing value to be used for NODATA pixels instead of ignoring them

• min_coverage_frac - specifies the minimum fraction of the pixel (0 to 1) that must be covered in order for a pixel to be considered in calculations. Defaults to 0.

Examples

• quantile(q=0.33): the 33% quantile of the pixels that intersect the polygon, weighted by the coverage fraction of each pixel.

• count(default_value=0): the sum of all pixel coverage fractions, including NODATA pixels, as they will be treated as having value 0 instead of being ignored.

• mean(min_coverage_frac=0.5): the mean value of pixels having 50% or more of their area covered by the polygon. Pixel values will be weighted by the coverage fraction.

• mean(min_coverage_frac=0.5,coverage_weight=none): the mean value of pixels having 50% or more of their area covered by the polygon. Pixel values will be weighted equally (all included pixels treated as having 100% coverage).

• mean(coverage_weight=none): the mean value of pixels that intersect the polygon. All pixels with any coverage will be counted as having 100% coverage.