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Determine area of cell in raster (QGIS)

SUM Area irregular polygons per grid cellProjecting a Raster from WGS84 to planar Equal Area to Calculate Area: How to Select the Cell Size?Geographic vs. projected raster cell size areaCalculating area of polygon within square mile in ArcGIS?Sum area of specific poly attributes within raster grid cellHow to get unique values and cell counts of a raster in QGIS 2.6?Calculate area of raster grid cells in an unprojected raster using ArcGISChanging cell size of raster in QGIS during extraction (clip)QGIS 2D Raster Area CalculationCalculating the area of raster within polygon

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1

I have a raster layer of which I would need to find the area (m2) per cell. Any suggestions for a simple approach to achieve this?

qgis raster cell-size

share|improve this question

asked Apr 1 at 17:15

A.NA.N

261

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Project your raster layer into an appropriate coordinate reference system (CRS) that uses meters. E.g. find the correct UTM zone and use that CRS.
  
  – Jon
  Apr 1 at 17:17
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Jon that's going to change the grid basis. Probably not a good idea. If the raster is already in a cartesian system then multiply the height by the width. If the grid is lat-long then there's some calculations to be done...
  
  – Spacedman
  Apr 1 at 17:34
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I was assuming the data were in lat/long. Otherwise I thought it was obvious to just look at the pixel resolution, but maybe not.
  
  – Jon
  Apr 1 at 17:53
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks. To clarify - with height and width, are you referring to the cell size?
  
  – A.N
  Apr 1 at 17:53
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Then the unit is degrees and area of a pixel in hectares depends on where on the Earth the pixel is located. The answer by @Jon is relevant. If your raster is rather small you can calculate the length of one degree latitude and longitude around your location with some online tool like csgnetwork.com/degreelenllavcalc.html and use the result as an estimate.
  
  – user30184
  Apr 1 at 19:44
  

  
  
  
  

 | 
show 3 more comments

1

I have a raster layer of which I would need to find the area (m2) per cell. Any suggestions for a simple approach to achieve this?

qgis raster cell-size

share|improve this question

asked Apr 1 at 17:15

A.NA.N

261

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Project your raster layer into an appropriate coordinate reference system (CRS) that uses meters. E.g. find the correct UTM zone and use that CRS.
  
  – Jon
  Apr 1 at 17:17
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @Jon that's going to change the grid basis. Probably not a good idea. If the raster is already in a cartesian system then multiply the height by the width. If the grid is lat-long then there's some calculations to be done...
  
  – Spacedman
  Apr 1 at 17:34
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I was assuming the data were in lat/long. Otherwise I thought it was obvious to just look at the pixel resolution, but maybe not.
  
  – Jon
  Apr 1 at 17:53
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Thanks. To clarify - with height and width, are you referring to the cell size?
  
  – A.N
  Apr 1 at 17:53
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  Then the unit is degrees and area of a pixel in hectares depends on where on the Earth the pixel is located. The answer by @Jon is relevant. If your raster is rather small you can calculate the length of one degree latitude and longitude around your location with some online tool like csgnetwork.com/degreelenllavcalc.html and use the result as an estimate.
  
  – user30184
  Apr 1 at 19:44
  

  
  
  
  

 | 
show 3 more comments

I have a raster layer of which I would need to find the area (m2) per cell. Any suggestions for a simple approach to achieve this?

qgis raster cell-size

share|improve this question

asked Apr 1 at 17:15

A.NA.N

261

share|improve this question

asked Apr 1 at 17:15

A.NA.N

261

share|improve this question

asked Apr 1 at 17:15

A.NA.N

261

asked Apr 1 at 17:15

A.NA.N

261

asked Apr 1 at 17:15

A.NA.N

261

1 Answer
1

active

oldest

votes

0

Your question is not well constrained. If you are wondering how to find the area of a cell from a raster that is unprojected and aligned to WGS84 grid (e.g. epsg:4326), then the following Python code should help.

import numpy as np
import rasterio

fp = 'your_tiff_file.tif'
with rasterio.open(fp) as testif:
 rast = testif.read(1)
 gt = testif.transform
 pix_width = gt[0]
 ulX = gt[2]
 ulY = gt[5]
 rows = testif.height
 cols = testif.width
 lrX = ulX + gt[0] * cols
 lrY = ulY + gt[4] * rows

lats = np.linspace(ulY,lrY,rows+1)

a = 6378137
b = 6356752.3142

# Degrees to radians
lats = lats * np.pi/180

# Intermediate vars
e = np.sqrt(1-(b/a)**2)
sinlats = np.sin(lats)
zm = 1 - e * sinlats
zp = 1 + e * sinlats

# Distance between meridians
# q = np.diff(longs)/360
q = pix_width/360

# Compute areas for each latitude in square km
areas_to_equator = np.pi * b**2 * ((2*np.arctanh(e*sinlats) / (2*e) + sinlats / (zp*zm))) / 10**6
areas_between_lats = np.diff(areas_to_equator)
areas_cells = np.abs(areas_between_lats) * q

areagrid = np.transpose(np.matlib.repmat(areas_cells,cols,1))

This snippet returns the array areagrid that contains the area in square kilometers of each pixel in your 4326 raster. There are a number of considerations "under the hood" that I am not detailing here, but I will note that I have compared this method with reprojection methods and the results have always been very similar.

You asked for a QGIS solution, so you would have to modify this a bit to get it to run in the Python console. Maybe overkill for what you want.

share|improve this answer

answered Apr 1 at 18:02

JonJon

1,4041421

add a comment | 

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0

Your question is not well constrained. If you are wondering how to find the area of a cell from a raster that is unprojected and aligned to WGS84 grid (e.g. epsg:4326), then the following Python code should help.

import numpy as np
import rasterio

fp = 'your_tiff_file.tif'
with rasterio.open(fp) as testif:
 rast = testif.read(1)
 gt = testif.transform
 pix_width = gt[0]
 ulX = gt[2]
 ulY = gt[5]
 rows = testif.height
 cols = testif.width
 lrX = ulX + gt[0] * cols
 lrY = ulY + gt[4] * rows

lats = np.linspace(ulY,lrY,rows+1)

a = 6378137
b = 6356752.3142

# Degrees to radians
lats = lats * np.pi/180

# Intermediate vars
e = np.sqrt(1-(b/a)**2)
sinlats = np.sin(lats)
zm = 1 - e * sinlats
zp = 1 + e * sinlats

# Distance between meridians
# q = np.diff(longs)/360
q = pix_width/360

# Compute areas for each latitude in square km
areas_to_equator = np.pi * b**2 * ((2*np.arctanh(e*sinlats) / (2*e) + sinlats / (zp*zm))) / 10**6
areas_between_lats = np.diff(areas_to_equator)
areas_cells = np.abs(areas_between_lats) * q

areagrid = np.transpose(np.matlib.repmat(areas_cells,cols,1))

This snippet returns the array areagrid that contains the area in square kilometers of each pixel in your 4326 raster. There are a number of considerations "under the hood" that I am not detailing here, but I will note that I have compared this method with reprojection methods and the results have always been very similar.

You asked for a QGIS solution, so you would have to modify this a bit to get it to run in the Python console. Maybe overkill for what you want.

share|improve this answer

answered Apr 1 at 18:02

JonJon

1,4041421

add a comment | 

0

Your question is not well constrained. If you are wondering how to find the area of a cell from a raster that is unprojected and aligned to WGS84 grid (e.g. epsg:4326), then the following Python code should help.

import numpy as np
import rasterio

fp = 'your_tiff_file.tif'
with rasterio.open(fp) as testif:
 rast = testif.read(1)
 gt = testif.transform
 pix_width = gt[0]
 ulX = gt[2]
 ulY = gt[5]
 rows = testif.height
 cols = testif.width
 lrX = ulX + gt[0] * cols
 lrY = ulY + gt[4] * rows

lats = np.linspace(ulY,lrY,rows+1)

a = 6378137
b = 6356752.3142

# Degrees to radians
lats = lats * np.pi/180

# Intermediate vars
e = np.sqrt(1-(b/a)**2)
sinlats = np.sin(lats)
zm = 1 - e * sinlats
zp = 1 + e * sinlats

# Distance between meridians
# q = np.diff(longs)/360
q = pix_width/360

# Compute areas for each latitude in square km
areas_to_equator = np.pi * b**2 * ((2*np.arctanh(e*sinlats) / (2*e) + sinlats / (zp*zm))) / 10**6
areas_between_lats = np.diff(areas_to_equator)
areas_cells = np.abs(areas_between_lats) * q

areagrid = np.transpose(np.matlib.repmat(areas_cells,cols,1))

This snippet returns the array areagrid that contains the area in square kilometers of each pixel in your 4326 raster. There are a number of considerations "under the hood" that I am not detailing here, but I will note that I have compared this method with reprojection methods and the results have always been very similar.

You asked for a QGIS solution, so you would have to modify this a bit to get it to run in the Python console. Maybe overkill for what you want.

share|improve this answer

answered Apr 1 at 18:02

JonJon

1,4041421

add a comment | 

Your question is not well constrained. If you are wondering how to find the area of a cell from a raster that is unprojected and aligned to WGS84 grid (e.g. epsg:4326), then the following Python code should help.

import numpy as np
import rasterio

fp = 'your_tiff_file.tif'
with rasterio.open(fp) as testif:
 rast = testif.read(1)
 gt = testif.transform
 pix_width = gt[0]
 ulX = gt[2]
 ulY = gt[5]
 rows = testif.height
 cols = testif.width
 lrX = ulX + gt[0] * cols
 lrY = ulY + gt[4] * rows

lats = np.linspace(ulY,lrY,rows+1)

a = 6378137
b = 6356752.3142

# Degrees to radians
lats = lats * np.pi/180

# Intermediate vars
e = np.sqrt(1-(b/a)**2)
sinlats = np.sin(lats)
zm = 1 - e * sinlats
zp = 1 + e * sinlats

# Distance between meridians
# q = np.diff(longs)/360
q = pix_width/360

# Compute areas for each latitude in square km
areas_to_equator = np.pi * b**2 * ((2*np.arctanh(e*sinlats) / (2*e) + sinlats / (zp*zm))) / 10**6
areas_between_lats = np.diff(areas_to_equator)
areas_cells = np.abs(areas_between_lats) * q

areagrid = np.transpose(np.matlib.repmat(areas_cells,cols,1))

This snippet returns the array areagrid that contains the area in square kilometers of each pixel in your 4326 raster. There are a number of considerations "under the hood" that I am not detailing here, but I will note that I have compared this method with reprojection methods and the results have always been very similar.

You asked for a QGIS solution, so you would have to modify this a bit to get it to run in the Python console. Maybe overkill for what you want.

share|improve this answer

answered Apr 1 at 18:02

JonJon

1,4041421

import numpy as np
import rasterio

fp = 'your_tiff_file.tif'
with rasterio.open(fp) as testif:
 rast = testif.read(1)
 gt = testif.transform
 pix_width = gt[0]
 ulX = gt[2]
 ulY = gt[5]
 rows = testif.height
 cols = testif.width
 lrX = ulX + gt[0] * cols
 lrY = ulY + gt[4] * rows

lats = np.linspace(ulY,lrY,rows+1)

a = 6378137
b = 6356752.3142

# Degrees to radians
lats = lats * np.pi/180

# Intermediate vars
e = np.sqrt(1-(b/a)**2)
sinlats = np.sin(lats)
zm = 1 - e * sinlats
zp = 1 + e * sinlats

# Distance between meridians
# q = np.diff(longs)/360
q = pix_width/360

# Compute areas for each latitude in square km
areas_to_equator = np.pi * b**2 * ((2*np.arctanh(e*sinlats) / (2*e) + sinlats / (zp*zm))) / 10**6
areas_between_lats = np.diff(areas_to_equator)
areas_cells = np.abs(areas_between_lats) * q

areagrid = np.transpose(np.matlib.repmat(areas_cells,cols,1))

share|improve this answer

answered Apr 1 at 18:02

JonJon

1,4041421

answered Apr 1 at 18:02

JonJon

1,4041421

answered Apr 1 at 18:02

JonJon

1,4041421