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Read other spatial formats

Source: R/spatial.R
tda_read_spatial.Rd

The same shape as tda_read_shapefile: TDA reads the file and writes its spatial data file, which the rest of the family works on. tda_read_e00 reads an uncompressed ARC/INFO export, tda_read_gshhs a GSHHS coastline file, and tda_read_dcw a Digital Chart of the World polygon point file.

Usage

tda_read_e00(file, dir = tempfile("tda"), ...)

tda_read_gshhs(
  file,
  level = NULL,
  centre = c("greenwich", "pacific"),
  dir = tempfile("tda"),
  ...
)

tda_read_dcw(file, dir = tempfile("tda"), ...)

Arguments

file

path to the input file.

dir

working directory for the run.

...

further options for the command. sde00 takes attr for how many attribute variables to keep; sdgshhs takes level to select polygons; sddcwp takes nc to drop centroids.

level

for tda_read_gshhs: which hierarchy levels to keep – 1 land, 2 lake, 3 island in lake, 4 pond in island in lake, and since GSHHG 2.3.0 5 the Antarctic ice front and 6 the Antarctic grounding line. Several may be given, level = c(1, 2) for land and lakes. NULL, the default, keeps all of them; level 1 alone is much the fastest way to draw a coastline.

Antarctica appears twice in a 2.3.x file, once as level 5 and once as level 6, and they are about six degrees of latitude apart near the dateline. Keeping all levels draws both, which is why a world map shows two near-parallel southern coastlines; GSHHG intends you to pick one. Neither is an artefact – the ice front runs along roughly 78 degrees south there (the Ross Ice Shelf edge, near-straight) and the grounding line along roughly 84.

centre

for tda_read_gshhs: where the world map is cut. "greenwich" (default) gives the familiar view, longitudes -180..180, cut at the dateline. "pacific" cuts at the Greenwich meridian instead, longitudes 0..360. Polygons spanning whichever seam is in force are split either way.

Value

A tda_spatial structure.

Details

All three target older formats from GIS's pre-shapefile era, and only shapefile really remains in wide, current use – e00 and DCW were both effectively superseded by it, and DCW's distribution site has been gone for years, which is worth knowing going in:

  • tda_read_e00 works, tested against a real ARC/INFO export (a point coverage).

  • tda_read_dcw works, tested against a .pnt file rebuilt from a real DCW coverage read via the sf package (st_read() on the original coverage directory, coordinates regrouped into rings and written out in the plain text form sddcwp expects). Coordinates round-trip exactly: the intermediate this reader writes uses fmt = "24.16", not TDA's default, which would cost six digits of each one.

  • tda_read_gshhs reads both GSHHS header layouts: the pre-2.2 one and GSHHG 2.x, which is what you would download today. The layout is detected from the file itself and the version reported in the run's output.

    Straight lines across a world map would come from the dateline; that is handled for you, and there is nothing to configure. GSHHS stores the polygons that span the dateline with longitudes running past 180 (Eurasia reaches 190). Those are cut at the dateline and come back as separate objects, so each piece is drawn where it belongs and nothing is drawn between them; a polygon that does not span it is untouched. The Antarctic coastline, which runs from one edge of the map to the other, is returned as a line rather than a closed area for the same reason – closing it would draw a straight segment back across the world.

    This is what the GSHHG maintainers do in their own shapefile distribution, where the dateline-straddling polygons – the Antarctic cap chief among them – are split into east and west components. The native binary files are left unsplit for the reader to handle.

    centre = "pacific" cuts the map at Greenwich instead, giving longitudes on 0..360; the splitting then happens at that seam rather than at the dateline. Either way no polygon is drawn across the map.

Further options

Anything else these commands accept can be passed through ... under TDA's option name. tda_help("sdshp"), tda_help("sde00"), tda_help("sddcwp") and tda_help("sdgshhs") print the full lists. One worth knowing is nc, which tells the DCW reader that the input has no comment lines.

df=, dtda= and fmt= are set by the reader itself: the first two name the intermediate files it writes and reads back, and fmt is fixed at "24.16" so that no coordinate is rounded on the way through.

See also

Other spatial analysis: plot.tda_spatial(), tda_map(), tda_polygons(), tda_read_dbf(), tda_read_shapefile(), tda_sd(), tda_sd_analyses, tda_sd_ps3(), tda_spatial()

Examples

# nc.shp ships with sf, so this one runs where sf is installed.
# The .dbf beside the geometry holds the attribute table -- county
# names and areas -- and is attached as $attributes.
if (requireNamespace("sf", quietly = TRUE)) {
  nc <- tda_read_shapefile(system.file("shape/nc.shp", package = "sf"))
  head(nc$attributes[, c("NAME", "AREA")])
}
#>          NAME  AREA
#> 1        Ashe 0.114
#> 2   Alleghany 0.061
#> 3       Surry 0.143
#> 4   Currituck 0.070
#> 5 Northampton 0.153
#> 6    Hertford 0.097

if (FALSE) { # \dontrun{
# the other readers need files that do not ship with any package
tda_read_e00("world.e00")
tda_read_gshhs("gshhs_f.b", level = 1)  # either header layout
tda_read_dcw("polygon.pnt", nc = TRUE)
} # }