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Download ground elevation along a path

Source: R/path2elevation.R
path2elevation.Rd

This function is a wrapper of the GeoPressureAPI elevationPath entry point. It queries the ground elevation from SRTM (SRTM90_V4) along a polyline specify by path.

Because the position are often defined on a relative coarse scale (e.g., 0.25° ~ 29km), you can request the elevation at a specify resolution defined by scale and return pre-defined percentile of the elevation at this resolution.

The returned data.frame provide the ground elevation along the path with a resolution defined by sampling_scale.

Usage

path2elevation(
  path,
  scale = 4,
  sampling_scale = scale,
  percentile = c(10, 50, 90),
  timeout = 60 * 5,
  debug = FALSE
)

Arguments

path

a GeoPressureR path data.frame

scale

spatial resolution of the SRTM to use to query the elevation. scale is defined as the number of pixels per 1° latitude-longitude (see tag_set_map() for details). Native resolution of SRTM is 30m.

sampling_scale

spatial resolution of the point along the polyline on which the SRTM is estimated. Same unit as scale.

percentile

percentile of the ground elevation distribution found within each grid cell of the SRTM at the resolution defined by scale. 50 corresponds to the median.

timeout

maximum duration to make the httr request (see httr2::req_timeout())

debug

logical to display additional information to debug a request

Value

A data.frame containing

  • stap_id numeric value corresponding to the ratio of distance between position of known stap

  • lon

  • lat

  • distance distance in km along the path starting at the first stap_id

See also

Other path: ind2path(), path2edge(), plot_path()

Examples

# Create a path
path <- data.frame(
  lon = c(8.47, 9.41, 9.01, -0.91, 14.24, 27.30, 34.39, 30.00),
  lat = c(48.89, 44.78, 40.07, 37.68, 17.33, 7.32, 8.09, -23.13),
  start = as.POSIXct(
    c(
      "2017-05-01 00:42", "2017-05-03 01:22", "2017-05-03 22:47", "2017-05-06 03:32",
      "2017-05-07 01:12", "2017-05-07 22:32", "2017-05-09 21:52", "2017-05-10 21:12"
    ),
    tz = "UTC"
  ),
  end = as.POSIXct(
    c(
      "2017-05-02 22:12", "2017-05-03 20:12", "2017-05-06 02:47", "2017-05-06 23:22",
      "2017-05-07 17:42", "2017-05-09 20:27", "2017-05-10 19:57", "2017-05-11 21:17"
    ),
    tz = "UTC"
  ),
  stap_id = seq(1, 8)
)


plot_path(path)



elevation <- path2elevation(path)

plot(elevation$distance, elevation$X50,
  type = "l",
  ylab = "Elevation (m)", xlab = "Distance from start (km)"
)
lines(elevation$distance, elevation$X10, lty = 5)
lines(elevation$distance, elevation$X90, lty = 5)
id <- elevation$stap_id %% 1 == 0
points(elevation$distance[id], elevation$X90[id], col = "red")