SMOS products (i.e. L1C brightness temperatures in X and Y pol., and L2 soil moisture) are defined on the ISEA 4H9 grid with a spatial resolution of 15 km. Each point (or node) of this grid is known as a DGG (Discrete Global Grid) that has fixed coordinates (i.e. Latitude, Longitude) and is assigned an identificator the “DGG Id”.
Some of the routine provided to extract data from the products are based on this DGG Id (i.e. XY2HV.m).But, how to find a DGG Id that is of interest for the user ?find_nearestDGG.m is the solution !
All the grid point informations (DGG Id, Latitude, Longitude) has been gathered in an ascii file isea4h9.txt.gz. Using the matlab function find_nearestDGG.m allows the user to extract the nearest DGG node to a given latitude/longitude. We also provide the ascii file isea4h9_land.txt.gz which contains the index of the DGG over land (of interest for L2 soil moisture).See the files and the readme that comes with them.You will find the matlab routine together with the grids in the software page.
Be careful not to mixed up the spatial resolution ! The grid of SMOS products is 15 km of spatial resolution and the radiometric resolution of the instrument is ~40 km.
Arnaud
The ISEA4H9 grid has exactly 2,621,442 grid cells. So why does the isea4h9.txt have 2,621,450 grid points? Please clarify.
It is true that the isea 4H9 has 2,621,442 grid cells. In the matrix provided here, there are 2,621,442 different geographical coordinates (i.e.. lat,lon).\n\n10 zones are distinguished : 2 have 262145 nodes and 8 have 262144. For our purpose we have repeated one node in every of the 8 zones having 262144 nodes (2 nodes having the same coordinates), so that all the 10 zones have 262145 nodes for all the zones.
Thank you Arnaud, it makes sense now.