The existing AHPS netCDF files available here: https://water.weather.gov/precip/p_download_new/

do not include a projection...instead they're described as being in an HRAP coordinate system. Further the data is offseted via hrap_xor/hrap_yor. If you open the file in QGIS you need to provide a projection.

What is the proj4 for AHPS netCDF files to be able to display properly in QGIS?

2 Answers 2


By opening the newly formatted files available: http://preview-water.weather.gov/precip/downloads/2017/

and tweaking the projection those include I was able to create a custom projection that seems to more or less get the two files to overlay:

+proj=stere +lat_0=90 +lat_ts=60 +lon_0=-105 +k=1 +x_0=1842796.9875 +y_0=3705005.475 +a=6371200 +b=6371200 +to_meter=4762.5 +no_defs

Hopefully this can help someone and we can get a truly correct one!

  • wouldn't it be nice to have acronyms "explained" for noobs (like me) ? ... sure it is easy to google them .... but ...
    – Snaileater
    Jun 8, 2017 at 18:23
  • welcome to world of corporate/government :)
    – amohr
    Jun 8, 2017 at 19:11

This is rather belated, but it speaks to a question I wrestled with and have (so far) not been able to get NOAA to address. But I think I can add something to the discussion.

The NetCDF files throught June 27, 2017, were Stage III files and contained no projection information. In the archive folder, where all the pre-06-28-2017 files are stored, there is a small C-code program that lets you obtain lat/lon from the [y,x] grid points. It also has a shapefile (allpoints.xxx) that contains HRAPy,HRAPx,lat,lon values. Starting on June 28,2017, the NetCDF files are Stage IV, and do contain projection strings. However, the Stage IV files use a larger grid, and no longer contain the offsets mentioned above. What I wanted to do was obtain precip data for each gridpoint, and you cannot use the archived program (directly) to convert the grid points to lat/lon with this dataset. But you CAN modify the archived code slightly to do this. The Stage IV files have values for each y and x. These are the easting/northing values for each gridpoint. They take into account the offsets that had to be applied in the Stage III version. In order to use the embedded y,x values, you need to divide them by the mesh length (4762.5 m) in order to obtain a true grid value, and pass that to the HrapToLatLong() function. FWIW, the Stage IV NetCDFs can be properly displayed by QGIS -- just select it using the option to add Mesh datasets.

  • ya i ended up doing something similar so the cells would mostly overlap
    – amohr
    Jun 16 at 1:44

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