Hi All:
I meant to chime in on this, and it slipped through the cracks. We have run
into this also, mostly with GIS software, but with other visualization
software also. It has to do with how the grid is interpreted when the image
is rendered - are the data the center of the pixel or the edge of the pixels.
The difference in interpretation will cause an offset of one-half the lat-lon
grid size. I use to know which it was GIS software assumed, but I am old and
forgetful.
-Roy
> On Aug 23, 2016, at 10:00 AM, Sean Arms <sarms@xxxxxxxx> wrote:
>
> Greetings Simon,
>
> It almost looks like this is an issue of how the pixel is defined - it's like
> there is an offset of 0.5 pixel between the green and grey layer. I do not
> have any experience with any of the GIS packages. Anyone out there
> experienced this before?
>
> Sean
>
>
> On Tue, Aug 16, 2016 at 9:12 AM, Wright, Simon M. <smw@xxxxxxxxx> wrote:
> Dear THREDDS mailing list
>
>
>
> We have set THREDDS up to host a WMS for some netCDF files. These netCDF
> files have a 5 km resolution and are in the EPSG:3035 coordinate reference
> system. The metadata header for one of the netCDF files is as follows:
>
>
>
> netcdf Z:/thredds/edge/edge-mockup-20160711-RCP2_6-ECMWF-NOAA-indicator01.nc {
>
> dimensions:
>
> time = UNLIMITED; // (17 currently)
>
> x = 1000;
>
> y = 950;
>
> variables:
>
> float indicator(time=17, y=950, x=1000);
>
> :_FillValue = -9999.0f; // float
>
> :standard_name = "indicator01";
>
> :long_name = "indicator01";
>
> :units = "-";
>
> :RCP = "RCP2_6";
>
> :GCM = "ECMWF";
>
> :HYDROMODEL = "NOAA";
>
> :coordinates = "time y x";
>
> :grid_mapping = "lambert_azimuthal_equal_area";
>
> :missing_value = -9999.0f; // float
>
> :valid_min = 0.0; // double
>
> :valid_max = 22.8378; // double
>
> :_ChunkSizes = 6, 317, 334; // int
>
>
>
> int lambert_azimuthal_equal_area;
>
> :grid_mapping_name = "lambert_azimuthal_equal_area";
>
> :longitude_of_projection_origin = "10.0";
>
> :latitude_of_projection_origin = "52.0";
>
> :false_easting = "4321000.0";
>
> :false_northing = "3210000.0";
>
> :_CoordinateTransformType = "Projection";
>
> :_CoordinateAxisTypes = "GeoX GeoY";
>
>
>
> double lat(y=950, x=1000);
>
> :standard_name = "latitude";
>
> :long_name = "latitude coordinate";
>
> :units = "degrees_north";
>
> :_ChunkSizes = 475, 500; // int
>
>
>
> double lon(y=950, x=1000);
>
> :standard_name = "longitude";
>
> :long_name = "longitude coordinate";
>
> :units = "degrees_east";
>
> :_ChunkSizes = 475, 500; // int
>
>
>
> double time(time=17);
>
> :_FillValue = -9999.0; // double
>
> :long_name = "time";
>
> :units = "seconds since 2019-01-01 23:59:59";
>
> :calendar = "gregorian";
>
> :_ChunkSizes = 17; // int
>
> :_CoordinateAxisType = "Time";
>
>
>
> double x(x=1000);
>
> :standard_name = "projection_x_coordinate";
>
> :long_name = "x coordinate of projection";
>
> :units = "Meter";
>
> :_ChunkSizes = 1000; // int
>
> :_CoordinateAxisType = "GeoX";
>
>
>
> double y(y=950);
>
> :standard_name = "projection_y_coordinate";
>
> :long_name = "y coordinate of projection";
>
> :units = "Meter";
>
> :_ChunkSizes = 950; // int
>
> :_CoordinateAxisType = "GeoY";
>
> }
>
>
>
> Displaying the netCDF file via the Godiva 2 viewer in THREDDS seems to work
> as expected, but when overlaying other datasets in other software packages
> (e.g. QGIS, ArcGIS, etc.) there is a noticeable offset/error in the WMS
> images generated by THREDDS.
>
>
>
> To spot the offset, we load the WMS from THREDDS and the original source data
> from the netCDF file as layers in QGIS. In the attached image
> (QGIS-error.png), you can clearly see that the grey (local netCDF file) layer
> is not completely hidden by the green (THREDDS WMS) layer.
>
>
>
> We assume that this is something to do with re-projection of the data?
> Pulling an image from the THREDDS WMS in the EPSG:3035 projection system
> yields the same sort of offset. A call with WMS parameters
> SRS=EPSG:3035&BBOX=3615000,3550000,3620000,3555000 in our mind should return
> a single pixel from the netCDF data, however it doesn't.
>
>
>
> One other thing to note is that QGIS reports the source projection definition
> as '+proj=laea +lat_0=52 +lon_0=10 +x_0=4321000 +y_0=3210000 +ellps=WGS84
> +towgs84=0,0,0,0,0,0,0 +units=m +no_defs' which differs from EPSG:3035 in
> its datum. However we don't think this should matter as the same netCDF file
> is being processed by QGIS and THREDDS.
>
>
>
> Therefore, should the coordinate reference system in the netCDF file be
> defined in a different way or is there a bug in the ncWMS used by the THREDDS?
>
>
>
>
>
>
>
> Many thanks for any help that people can provide, best wishes, Simon.
>
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**********************
"The contents of this message do not reflect any position of the U.S.
Government or NOAA."
**********************
Roy Mendelssohn
Supervisory Operations Research Analyst
NOAA/NMFS
Environmental Research Division
Southwest Fisheries Science Center
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