# Introduction to arcpullr

#### 08 March, 2022

To see more complete package documentation check out: https://pfrater.github.io/arcpullr/

## URL’s for examples

Example Source Data

    wdnr_base_url <- "https://dnrmaps.wi.gov/arcgis/rest/services"
    streams_layer_url <- "WT_SWDV/WT_Inland_Water_Resources_WTM_Ext_v2/MapServer/2"
    streams_url <- paste(wdnr_base_url, streams_layer_url, sep = "/")

    # the mke_county polygon is available as an exported object in arcpullr
    open_water_layer_url <- "WT_SWDV/WT_Inland_Water_Resources_WTM_Ext_v2/MapServer/3"
    hydro_url <- paste(wdnr_base_url, open_water_layer_url, sep = "/")

    wis_county_layer_url <- "https://datcpgis.wi.gov/arcgis/rest/services/Base/PolygonsExternal/MapServer/0"

# Feature (i.e. vector) Layers

## The `get_spatial_layer()` function

This is one of the core functions of the package, and allows users to pull Feature Service data from an ArcGIS REST API.

    # url <- some url from an ArcGIS REST API layer
    layer <- get_spatial_layer(url)

The URL should be a specific layer from an ArcGIS REST API, such as the Wisconsin Dep’t. of Natural Resources Musky Streams layer, for example.

The `get_spatial_layer()` function will retrieve data from this layer, and format it to an object of class (i.e. of the R package sf: Simple Features for R).

This function is also query-able both using both SQL code and ArcGIS’s Query (Feature Service) functionality.

### Querying Spatially via ArcGIS Feature Service

Along with SQL, layers from an ArcGIS REST API may be queried spatially. This is accomplished with the `get_layer_by_*` family of functions. These functions are essentially a wrapper around `get_spatial_layer` that removes the abstraction of the spatial query syntax used by ArcGIS. These functions require a spatial object of class `sf` to be passed to the `geometry` argument. To test this out you can also quickly create simple `sf` objects using `sf_lines()`, `sf_points()`, or `sf_polygons()` to test out the service feature.

    mke_waters <- get_layer_by_poly(url = streams_url, mke_county)

Spatial queries can be done with polygons, lines, or points…just use the respective `get_layer_by_*` function. See the vignette on spatial querying for more detailed examples.

### Querying via SQL

To query via SQL within the function the field name for the query of interest must be known. For example,

    wi_river <- get_spatial_layer(hydro_url, where = "WATERBODY_ROW_NAME = 'Wisconsin River'")

For multiple WHERE clauses see the `?sql_where` function, which automates the creation of a WHERE clause.

# Raster Layers

arcpullr is capable of pulling raster layers as well (denoted as map and image layers by ArcGIS REST APIs). Utilize the `get_map_layer` and `get_image_layer` functions to pull these respective layers. See more information in the vignette on raster layers

# Plotting Layers

Layers retrieved via any of the `get_*_layer` functions may be plotted using an appropriate method which corresponds to the object type. For quick plotting we’ve also included the `plot_layer` function. This function will plot layers using `ggplot2` by default or can be switched to base graphics using the `plot_pkg` argument.