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# Introduction to Geospatial Raster and Vector Data with R: References

## Key Points

 Intro to Raster Data in R The GeoTIFF file format includes metadata about the raster data. To plot raster data with the `ggplot2` package, we need to convert it to a dataframe. R stores CRS information in the Proj4 format. Be careful when dealing with missing or bad data values. Plot Raster Data in R Continuous data ranges can be grouped into categories using `mutate()` and `cut()`. Use built-in `terrain.colors()` or set your preferred color scheme manually. Layer rasters on top of one another by using the `alpha` aesthetic. Reproject Raster Data in R In order to plot two raster data sets together, they must be in the same CRS. Use the `projectRaster()` function to convert between CRSs. Raster Calculations in R Rasters can be computed on using mathematical functions. The `overlay()` function provides an efficient way to do raster math. The `writeRaster()` function can be used to write raster data to a file. Work With Multi-Band Rasters in R A single raster file can contain multiple bands or layers. Use the `stack()` function to load all bands in a multi-layer raster file into R. Individual bands within a stack can be accessed, analyzed, and visualized using the same functions as single bands. Open and Plot Shapefiles in R Shapefile metadata include geometry type, CRS, and extent. Load spatial objects into R with the `st_read()` function. Spatial objects can be plotted directly with `ggplot` using the `geom_sf()` function. No need to convert to a dataframe. Explore and Plot by Shapefile Attributes Spatial objects in `sf` are similar to standard data frames and can be manipulated using the same functions. Almost any feature of a plot can be customized using the various functions and options in the `ggplot2` package. Plot Multiple Shapefiles in R Use the `+` operator to add multiple layers to a ggplot. Multi-layered plots can combine raster and vector datasets. Use the `show.legend` argument to set legend symbol types. Use the `scale_fill_manual()` function to set legend colors. Handling Spatial Projection & CRS in R `ggplot2` automatically converts all objects in a plot to the same CRS. Still be aware of the CRS and extent for each object. Convert from .csv to a Shapefile in R Know the projection (if any) of your point data prior to converting to a spatial object. Convert a data frame to an `sf` object using the `st_as_sf()` function. Export an `sf` object as text using the `st_write()` function. Manipulate Raster Data in R Use the `crop()` function to crop a raster object. Use the `extract()` function to extract pixels from a raster object that fall within a particular extent boundary. Use the `extent()` function to define an extent. Raster Time Series Data in R Use the `list.files()` function to get a list of filenames matching a specific pattern. Use the `facet_wrap()` function to create multi-paneled plots with `ggplot2`. Use the `as.Date()` function to convert data to date format. Create Publication-quality Graphics Use the `theme_void()` function for a clean background to your plot. Use the `element_text()` function to adjust text size, font, and position. Use the `brewer.pal()` function to create a custom color palette. Use the `gsub()` function to do pattern matching and replacement in text. Derive Values from Raster Time Series Use the `cellStats()` function to calculate summary statistics for cells in a raster object. The pipe (`|`) operator means `or`. Use the `rbind()` function to combine data frames that have the same column names.