Accessing Data Through API

class: middle, inverse

# Creating sf object

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Bon Woo Koo & Subhro Guhathakurta

9/8/2022
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## sfg, sfc, and sf

### How simple features in R are organized:
* **Simple feature geometry.**
  * `my_point <- st_point(c(-84.388, 33.749))`.
  * **my_point** is a point on an arbitrary plane.
  * It cannot be mapped on Earth.
* **Simple feature geometry list-column.**
  * `my_sfc <- st_sfc(my_point, crs = 4326)`.
  * It adds **crs** information to sfg object.
  * With CRS defined, now R knows where that point is on Earth.
  * **CRS** must match the coordinates used to create sfg object.
  * The geometry column you see in sf objects = sfc

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* **Simple feature** 
  * `my_sf <- st_sf(my_sfc)`
  * This is the combination of a **data frame** and **sfc** object.
  * e.g., dplyr verbs don't work for sfc, but they work for sf.

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To create (POINT) sf object from a pair of lng/lat coordinates, you need to do it in **sfg &rarr; sfc &rarr; sf**, in this order.
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1. Create an .red[**sfg**] using coordinates. **my_sfg <- st_point(c(-84.388, 33.749))**.
2. Convert the .red[**sfg**] object into .blue[**sfc**] object with CRS info. **my_sfc <- my_sfg %>% st_sfc(crs = 4326)**.
3. Convert the .blue[**sfc**] object into .green[**sf**] object using **my_sf <- st_sf(my_sfc)**.
]
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<img src="sf_structure.png" width="100%" />