Reading bands and Tie Point Grids in snappy

development python
#1

Hi there,

I have noted a difference in the output between raster bands and tie point grids, and was wondering how I could get around it. Using snappy, I do the following to extract a band to a raster:

# Open product
prod = ProductIO.readProduct(indir)

# Get width and height
w = prod.getSceneRasterWidth()
h = prod.getSceneRasterHeight()

     
# Initialise 
array = np.zeros((w,h),dtype=np.float32)  # Empty array

# Extract raster
currentband = prod.getBand(stringname)
bandraster = currentband.readPixels(0, 0, w, h, array)

With this method, bandraster is returned as an array. If I look at the SNAP Engine API the documentation for the Tie Point Grids says the following:

readPixels(int x, int y, int w, int h, float[] pixels, ProgressMonitor pm)
Retrieves an array of tie point data interpolated to the product with and height as float array.

However, using the same method as above, replacing the line:

currentband = prod.getBand(stringname)

with

currentband = prod.getTiePointGrid(stringname)

I don’t get an array anymore but instead a Java object:

In: tiepointraster
Out: [F(objectRef=0x9acec50)
In: tiepointraster.getClass()
Out: java.lang.Class(objectRef=0x9acec20)

How can I retrieve the Tie Point Grid as an array?

Cheers,

Ground control points
#2

Hi Maxim,

currentTpGrid = prod.getTiePointGrid(stringname)

this return the tiepoint grid. Afterwards you can call

currentTpGrid.readPixels(0, 0, w, h, array)

You can generify this by

currentRaster = prod.getRasterDataNode(stringname)
currentRaster.readPixels(0, 0, w, h, array)

Doing it like this you can treat both bands and tie-point grids in the same way, as long you don’t need to specify methods of these classes.

1 Like
#3

Hi Marco,

You can generify this by

currentRaster = prod.getRasterDataNode(stringname)
currentRaster.readPixels(0, 0, w, h, array)

Thanks for the tip, this method populates the array with the values from the TiePointGrid.

currentTpGrid = prod.getTiePointGrid(stringname)
this return the tiepoint grid. Afterwards you can call

currentTpGrid.readPixels(0, 0, w, h, array)

However, this method doesn’t work for me: the array remains full of zeros and the call returns a java object. See below:

IN: currentTpGrid.readPixels(0,0,w,h,array)
Out: [F(objectRef=0x9c329a8)

Write a TiePointGrid to a S3 product
#5

Hi there,

If this is of any use to other people, I found a way of reading in the TiePointGrids: