obliqueGSD.r

# Calculate the ground sample distance (cm/pixel) for oblique camera position.
# Sensor defaults for a Nikon D5600 with 35mm lens
# Inputs:
#   Sw = sensor width in mm
#   Sh = sensor height in mm
#   f = focal length of lens in mm
#   Pw = sensor width in pixels
#   Ph = sensor height in pixels
#   phi = off-nadir (directly below sensor) angle
#   h = height of sensor above ground in metres
# Outputs:
#   gsd.inner = closest band of image ground resolution in cm / pixel
#   gsd.outer = furthest band of image ground resolution in cm / pixel
#   width.inner = image width at closest point
#   width.outer = image width at furthest point
#   strip.width = perpendicular distance at centre of image

gsdOblique2 <- function(Sw = 23.2, f = 35, Sh = 15.4, phi = 45 * pi/180, h = 450/3.28, Pw = 6016) {
  ## Angle of view ----
  A.h <- 2 * atan((Sw / 2) / f) # horizonal angle of view°
  A.v <- 2 * atan((Sh / 2) / f) # vertical angle of view°
  
  ## 
  h.1 <- h / cos(phi - A.v / 2) # length to lower edge of view
  w.1 <- h.1 * Sw / f
  h.2 <- h / cos(phi + A.v / 2)
  w.2 <- h.2 * Sw / f
  
  gsd.1 <- w.1 / Pw
  gsd.2 <- w.2 / Pw
  
  stripwidth <- sqrt(-h^2 + h.2^2) - sqrt(-h^2 + h.1^2)
  
  return(data.frame(gsd.inner = gsd.1, gsd.outer = gsd.2, width.inner = w.1, width.outer = w.2, stripwidth))
}