vapour_read_geometry: Read GDAL feature geometry

In vapour: Lightweight Access to the 'Geospatial Data Abstraction Library' ('GDAL')

Description

Read GDAL geometry as binary blob, text, or numeric extent.

Usage

vapour_read_geometry(
  dsource,
  layer = 0L,
  sql = "",
  limit_n = NULL,
  skip_n = 0,
  extent = NA
)

vapour_read_geometry_text(
  dsource,
  layer = 0L,
  sql = "",
  textformat = "json",
  limit_n = NULL,
  skip_n = 0,
  extent = NA
)

vapour_read_extent(
  dsource,
  layer = 0L,
  sql = "",
  limit_n = NULL,
  skip_n = 0,
  extent = NA
)

Arguments

dsource	data source name (path to file, connection string, URL)
layer	integer of layer to work with, defaults to the first (0) or the name of the layer
sql	if not empty this is executed against the data source (layer will be ignored)
limit_n	an arbitrary limit to the number of features scanned
skip_n	an arbitrary number of features to skip
extent	apply an arbitrary extent, only when 'sql' used (must be 'ex = c(xmin, xmax, ymin, ymax)' but sp bbox, sf bbox, and raster extent also accepted)
textformat	indicate text output format, available are "json" (default), "gml", "kml", "wkt"

Details

vapour_read_geometry will read features as binary WKB, vapour_read_geometry_text as various text formats (geo-json, wkt, kml, gml), vapour_read_extent a numeric extent which is the native bounding box, the four numbers (in this order) xmin, xmax, ymin, ymax. For each function an optional SQL string will be evaluated against the data source before reading.

vapour_read_geometry_cpp will read a feature for each of the ways listed above and is used by those functions. It's recommended to use the more specialist functions rather than this more general one.

Note that limit_n and skip_n interact with the affect of sql, first the query is executed on the data source, then while looping through available features skip_n features are ignored, and then a feature-count begins and the loop is stopped if limit_n is reached.

Note that extent applies to the 'SpatialFilter' of 'ExecuteSQL': https://gdal.org/user/ogr_sql_dialect.html#executesql.

Examples

file <- "list_locality_postcode_meander_valley.tab"
## A MapInfo TAB file with polygons
mvfile <- system.file(file.path("extdata/tab", file), package="vapour")
## A shapefile with points
pfile <- system.file("extdata/point.shp", package = "vapour")

## raw binary WKB points in a list
ptgeom <- vapour_read_geometry(pfile)
## create a filter query to ensure data read is small
SQL <- "SELECT FID FROM list_locality_postcode_meander_valley WHERE FID < 3"
## polygons in raw binary (WKB)
plgeom <- vapour_read_geometry_text(mvfile, sql = SQL)
## polygons in raw text (GeoJSON)
txtjson <- vapour_read_geometry_text(mvfile, sql = SQL)

## polygon extents in a list xmin, xmax, ymin, ymax
exgeom <- vapour_read_extent(mvfile)

## points in raw text (GeoJSON)
txtpointjson <- vapour_read_geometry_text(pfile)
## points in raw text (WKT)
txtpointwkt <- vapour_read_geometry_text(pfile, textformat = "wkt")

vapour documentation built on July 2, 2020, 2:32 a.m.