I'm working with Google Earth Engine images from ERA5-Land Hourly and am trying to take the mean or the sum of the hourly data on a daily basis. I've come across an issue where taking the mean seems to multiply the resolution by 10 and I'm at a loss trying to understand it.
Below is some Python code for a minimum reproducible example of the issue. Can anyone tell me what I'm doing wrong here?
import ee # Select the ImageCollection: polygon = ee.Geometry.Polygon([ (-119.6863362607843, 37.91032228872993), (-119.24681336572189, 37.91032228872993), (-119.24681336572189, 37.58602618425783), (-119.6863362607843, 37.58602618425783), (-119.6863362607843, 37.91032228872993) ]) ic = ee.ImageCollection("ECMWF/ERA5_LAND/HOURLY").filterDate("2020-01-01", "2020-01-10").filterBounds(polygon).select("total_precipitation") # Take a sample image: num_img = ic.size().getInfo() img_list = ic.toList(num_img) img = ee.Image(img_list.get(0)) # Print the scale: print(img.projection().nominalScale().getInfo())
This outputs 11131.949079327358 - which is the expected resolution for ERA-5 Land.
Now if I take the mean of the first day's images, and print the scale, it has multiplied by 10:
img = ic.filterDate("2020-01-01", "2020-01-02").mean() print(img.projection().nominalScale().getInfo())
Note that this happens if I take the first day, 2 days, etc. What am I doing wrong?