[R-sig-Geo] Calculate the area of pixel in square kilometers from a raster in lat/long projection

[Thiago V. dos Santos]

raster::area would calculate the area in square kilometers, and from there one can convert to hectare or square meters.

I was wondering: how do those calculations differ from raster::area's? 
Greetings,
 -- Thiago V. dos Santos

PhD student
Land and Atmospheric Science
University of Minnesota


On Tuesday, November 22, 2016 8:15 PM, Isaque Daniel <isaquedanielre at hotmail.com> wrote:



Hi there,
I found a example in stackoverflow of distance calculation and convertion, so I modificate that to compute the area by pixel in hectares using the raster in degrees.

#####
# Function to compute the area  in hectares

measureAreahec <- function(imageRaster) {
  R <- 6378.137                                # radius of earth in Km
  areaImagetemp <- area(ceiImg)
  xresVal <- xres(areaImagetemp)
  yresVal <- yres(areaImagetemp)
  coords <- data.frame(coordinates(areaImagetemp))
  colnames(coords) <- c("lon","lat")
  lon1 <- coords$lon - (xresVal/2)
  lon2 <- coords$lon + (xresVal/2)
  lat1 <- coords$lat - (yresVal/2)
  lat2 <- coords$lat + (yresVal/2)

  # width
  dLat <- (lat1-lat1)*pi/180
  dLon <- (lon2-lon1)*pi/180
  a <- sin((dLat/2))^2 + cos(lat1*pi/180)*cos(lat1*pi/180)*(sin(dLon/2))^2
  c <- 2 * atan2(sqrt(a), sqrt(1-a))
  dlongitude <- R * c * 1000

  # heigth
  dLat <- (lat1-lat2)*pi/180
  dLon <- (lon1-lon1)*pi/180
  a <- sin((dLat/2))^2 + cos(lat1*pi/180)*cos(lat1*pi/180)*(sin(dLon/2))^2
  c <- 2 * atan2(sqrt(a), sqrt(1-a))
  dlatitude <- R * c * 1000
  return (dlatitude * dlongitude / 10000)                            # distance in meters
}
##########################


It's a very simple one, but ,maybe can help somebody.

Best
Isaque


------------------------------------------------------------------------------------------------------------------
Agronomist engineer
Master in Remote Sensing - National  Institute for Space Research (INPE) - Brazil
PHD Student in Transport - Brazlia University (UNB)


________________________________
De: R-sig-Geo <r-sig-geo-bounces at r-project.org> em nome de Isaque Daniel <isaquedanielre at hotmail.com>
Enviado: ter�-feira, 22 de novembro de 2016 17:48
Para: r-sig-geo
Assunto: [R-sig-Geo] Calculate the area of pixel in square kilometers from a raster in lat/long projection

Hi,


I'm looking for some advice to convert the area calculated by raster::area() in a raster image as the projection in lat/long.

[[elided Hotmail spam]]
Best
Isaque


------------------------------------------------------------------------------------------------------------------
Agronomist engineer
Master in Remote Sensing - National  Institute for Space Research (INPE) - Brazil
PHD Student in Transport - Brazilia University (UNB)

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