Write a TiePointGrid to a S3 product

development python
#1

TL;DR: how do I build a TiePointGrid in snappy?

Hi all,

I’m struggling a little with the SNAP API.

I am trying to modify a TiePointGrid (SZA) in a Sentinel 3 OLCI scene for some testing purposes. I need it to be a TiePointGrid because the operator I use in the next step reads in the TiePointGrid. Therefore I cannot follow the easy path of converting it to a band.

Until now I have managed to open the Product and read in the TiePointGrid as a numpy array on which I can do as such:

 sza = prod.getRasterDataNode("SZA")
 height = prod.getSceneRasterHeight()  # Product band height
 width = prod.getSceneRasterWidth()  # Product band width

# Create array of zeros
array = np.zeros((height, width), dtype=np.float32)

# Read values into the array
szaraster = sza.readPixels(0, 0, width, height, array)

I use prod.getRasterDataNode, because I cannot access the array if I use prod.getTiePointGrid (see my comment in this post).

From what I read on this forum, I believe it’s better to create a new product and write to that one. I am able to do this with bands, however I can’t figure out how to create a TiePointGrid to add to the product.

I would like to create a new raster by performing operations on the szaraster variable shown above. Then write the raster to a TiePointGrid, that I would add to the new Product. How do I build this?

All the best!

Max

#2

Solution:

from snappy import TiePointGrid

""" Get a TiePointGrid from an existing product (previously opened, 
I won't repeat the code here)"""
szatp = prod.getTiePointGrid("SZA")

# Build a new TiePointGrid
new_szatp = TiePointGrid('newname',
                         szatp.getGridWidth(),
                         szatp.getGridHeight(),
                         szatp.getOffsetX(),
                         szatp.getOffsetY(),
                         szatp.getSubSamplingX(),
                         szatp.getSubSamplingY(),
                         tiePoints)

You can then pass an array into the tiePoints argument, as long as the size matches the GridWidth and GridHeight.
In the S3 product it seems that the TiePointGrids need to be consistent in dimensions. I have rewritten them to match the size of the bands in my case.