Google Earth Engine – Understanding Image Dimensions

banddimensionsgoogle-earth-engineimage

How can one get image (band) width and height on the server side?

On the user side you do it with image.getInfo().bands[0].dimensions, following Extracting information from Google Earth Engine image by row and column position?

However, it is not possible to map such a function, since getInfo() is a user-side function.

The task sounds extremely simple, since you always see band dimensions whenever you print an image, but how to get them as a list or a numbers?

Best Answer

You can use ee.Algorithms.Describe() (bit hidden in the documentation)

this should work :

var numberOfPixels = function(img){
  
  var imgDescription = ee.Algorithms.Describe( img );
  var height = ee.List( ee.Dictionary( ee.List(  ee.Dictionary( imgDescription ).get("bands") ).get(0) ).get("dimensions") ).get(0);
  var width = ee.List( ee.Dictionary( ee.List(  ee.Dictionary( imgDescription ).get("bands") ).get(0) ).get("dimensions") ).get(1);
  
  //print("width", width )
  //print("height", height )
  
  return  ee.Number( width ).multiply( ee.Number( height ) );
};

Related Solutions

Google Earth Engine – Extract Rows and Columns from Raster in Google Earth Engine

I modified your code to print the dimensions of a specified region (ROI).

var ROI = ee.Geometry.Rectangle(-82.5574, 35.5958, -82.5563, 35.59515); // define area

var collection = ee.ImageCollection('LANDSAT/LC08/C01/T1')
    .filterDate('2014-05-01', '2015-10-01')
    .filterBounds(ROI)
    .limit(1);

var composite = ee.Algorithms.Landsat.simpleComposite(collection);
var image = composite.clip(ROI);

Map.centerObject(ROI);
Map.addLayer(composite, {bands: 'B4,B3,B2', min:0, max:100}, "composite", false);
Map.addLayer(image, {bands: 'B4,B3,B2', min:0, max:100},"image");

// Extract projection information from the first band of a sample image in the collection.
var sample = ee.Image(collection.first());
var projection = sample.select('B1').projection();

// Reproject the composite image to the selected projection.
image = composite.reproject(projection).clip(ROI);

print('image information', image);

Note that:

Because image collections may contain images with many different projections, you need to specify the projection of your desired output in order to get back meaningful dimensions. In the example above I used the projection information from the first band of a sample image.
Bands within an image may have different projections. (This is the case for Landsat 8 images.)
I made the ROI smaller, so you can see the individual pixels when added to the map.
I limited the collection to a single image that intersects the ROI, so it is easier to interpret the results.

If you want to access the information on the client side, you can do the following:

// Return the image metadata to the client.
var image_info = image.getInfo();

// On the clientside, using Javascript, print out the dimensions.
print('band B1 dimensions: ' + image_info.bands[0].dimensions);

While it is possible to construct images in Earth Engine that store pixel information as multi-dimensional arrays, most images (including the Landsat collection shown in your example) have scalar values (i.e. 0-dimensional arrays) so it is not necessary to use ee.Array object methods to extract information.

[GIS] Writing code for monthly NDVI medians in Google Earth Engine

For the record, here is a good way to do this:

var imageCollection = ee.ImageCollection("LANDSAT/LT05/C01/T1");
var months = ee.List.sequence(1, 12);

var composites = ee.ImageCollection.fromImages(months.map(function(m) {
  var filtered = imageCollection.filter(ee.Filter.calendarRange({
    start: m,
    field: 'month'
  }));
  var composite = ee.Algorithms.Landsat.simpleComposite(filtered);
  return composite.normalizedDifference(['B4', 'B3']).rename('NDVI')
      .set('month', m);
}));
print(composites);

var check = ee.Image(composites.first());
Map.addLayer(check, {min: 0, max: 1}, 'check');

For the search robots, this is a "temporally grouped reduction."

Best Answer

Related Solutions

Google Earth Engine – Extract Rows and Columns from Raster in Google Earth Engine

[GIS] Writing code for monthly NDVI medians in Google Earth Engine

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