How extract data from a SNODAS file

Question

I'm trying to extract data from a raster file from "Snow Data Assimilation System (SNODAS)" and convert it to a MMQGIS (plugin) grid layer (Hexagon) with QGIS. This raster file stores the snow depth of the whole of the United States.

1. I load my raster (tif)
2. I extract the data with a terrain analysis (Aspect)
3. I create my grid layer as a hexagon
4. I run Zonal statistics from my Aspect to my Grid
5. At this point my grid has "count", "sum" and "mean" attributes. I use the "sum" attribute.
6. I load it on TileMill and I style the "sum" attribute.

I would like to know if I have the correct process and the right information about snow depth on my Grid layer? Is "Sum" the right attribute to print?

Thank you for your help.

Answer 1

I work for NOHRSC, the weather service office that creates the SNODAS rasters. Each pixel in a snow depth raster represents the model's best estimate for the value over the 1km x 1km cell. So, there is an implicit 3000x7000 grid in the data. Depth is measured in millimeters, in 16-bit big-endian integers. Raster data is available at http://www.nohrsc.noaa.gov/archived_data .

The respondent wants to re-aggregate the data to much larger hexagons.
MEAN would be the proper Zonal parameter.

Note: SNODAS models Snow Water Equivalent, the liquid water content of the snowpack. SWE is a much more robust statistic than Depth, which changes the minute the snowflakes hit the ground. A twelve-inch snowfall slumps to eight inches after a day of sunshine and wind as the snowpack compresses and metamorphs, without any change to the SWE value. Depth is computed after that fact, by multiplying SWE against an independent estimate of Snow Density.

Tim Szeliga, NOHRSC