Retrieve pixel value using SNAP-engine and Python for specific geographic coordinates

Question

I'm trying to retrieve pixel values in Sentinel-3 data for specific geographic coordinates using the SNAP-Engine. Until now, I transform the geographic coordinates to the corresponding X and Y with the following code using these links:

• https://stackoverflow.com/questions/51194477/converting-geographical-coordinates-long-lat-to-pixel-position-x-y-for-a-se
• https://senbox.atlassian.net/wiki/spaces/SNAP/pages/19300362/How+to+use+the+SNAP+API+from+Python)

---

import sys
sys.path.append('/path/to/.snap/snap-python')
from snappy import ProductIO
import numpy as np
from snappy import GeoPos

p = ProductIO.readProduct('/path/to/file.xml')
iwv = p.getBand('IWV')

g = p.getSceneGeoCoding()

def XY_from_LatLon(ProductSceneGeoCoding, latitude, longitude):
    #From Latitude, Longitude satellite image, get the x, y position in image
    pixelPos = ProductSceneGeoCoding.getPixelPos(GeoPos(latitude, longitude), None)
    x = pixelPos.getX()
    y = pixelPos.getY()
    if str(x)=='nan':
        raise ValueError('Latitude or Longitude out of this product')
     else:
         return x, y

How can I retrieve the data from a pixel using image coordinates (X, Y) in SNAP-Engine ?

---

After some more searching in SNAP-Engine docs, I found the method: getPixelFloat() that returns the pixel value given x, y. The code above was updated to the following :

import sys
sys.path.append('/path/to/.snap/snap-python')
from snappy import ProductIO
import numpy as np
from snappy import GeoPos

p = ProductIO.readProduct('/path/to/file.xml')
iwv = p.getBand('IWV')

g = p.getSceneGeoCoding()

def XY_from_LatLon(ProductSceneGeoCoding, latitude, longitude):
    #From Latitude, Longitude satellite image, get the x, y position in image
    pixelPos = ProductSceneGeoCoding.getPixelPos(GeoPos(latitude, longitude), None)
    x = pixelPos.getX()
    y = pixelPos.getY()
    if str(x)=='nan':
        raise ValueError('Latitude or Longitude out of this product')
     else:
         return x, y

x, y = XY_from_LatLon(sg, <lat>, <lon>)
print(iwv.getPixelFloat(x, y))

but it raises the following error:

Traceback (most recent call last):
File "snap.py", line 28, in <module>
print(iwv.getPixelInt(x, y))
RuntimeError: no matching Java method overloads found

Answer 1

I know it has been two years, but there are three things:

1. You have to load raster data after retrieving a band

   p=ProductIO.readProduct('/path/to/file.xml')

   iwv=p.getBand('IWV')

   iwv.loadRasterData() #add this line

otherwise you will get:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: java.lang.NullPointerException

2. In this line: x, y = XY_from_LatLon(sg, <lat>, <lon>) you should have 'g' as variable name is g = p.getSceneGeoCoding() #sg was in the original script

3. According to the SNAP Engine API doc, x and y need to be integers: getPixelInt(int x, int y) (https://step.esa.int/docs/v6.0/apidoc/engine/org/esa/snap/core/datamodel/AbstractBand.html)

I tested on my S3 SL 2 LST dataset, and x & y were returned : x = 729.5 y = 581.5

lst.getPixelFloat(x,y)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: no matching Java method overloads found
lst.getPixelFloat(int(x), int(y))
309.7099914550781