[gdal-dev] Questions on working with HYCOM data in netcdf format

Mon Jun 6 04:18:15 PDT 2016

Hi Stephen,

I sounds like you should be asking on
https://groups.google.com/a/hycom.org/forum/#!forum/forum rather than
gdal-dev.

-kurt

On Sun, Jun 5, 2016 at 2:10 PM, Stephen Woodbridge <woodbri at swoodbridge.com>
wrote:

> Hi Even, Kurt,
>
> Thank you for the suggestions.
>
> I'm reading through stuff and searching for solutions. I need access to
> the 2d layer which is only available via the GLBa0.08 dataset and not in
> the GLBu0.08 dataset which is easier to deal with. I'll start with the
> GLBu0.08 dataset for the other data following Kurt suggestions.
>
> This page describes the problem of georeferencing the GLBa0.08 dataset but
> the solution is based on windows python script and using ArcGIS, but the
> idea might be adapted to writing some C/C++ or script to handle creating
> those artifacts. I want to script the download and processing of the data
> into geotiff's running on Linux.
>
> http://code.nicholas.duke.edu/projects/mget/wiki/HYCOM
>
> One problem is that the HYCOM site does not have *.grid.[ab] files that I
> can find. They are supposed to be in the topo directory:
>
> Index of ftp://ftp.hycom.org/datasets/GLBa0.08/expt_91.2/topo/
>
> Name    Size    Last Modified
> depth_GLBa0.08_09.a   57984 KB  8/24/2013       12:00:00 AM
> depth_GLBa0.08_09.b       1 KB  8/24/2013       12:00:00 AM
> depth_GLBa0.08_09.nc 173950 KB  11/12/2013      12:00:00 AM
>
> and it is not obvious how to read and incorporate these files as I think
> they only work with the HYCOM related executables.
>
> I did notice the the lat/lon info is already in the .nc for the "u"
> dataset which can be seen with ncdump. In fact it is also there for the "a"
> dataset also, but the values 1-3298 and 1-4500 which is the size of the
> file in pixels, so not helpful.
>
> I've looked on the HYCOM forum but that has not been very helpful yet.
>
> Thanks,
>   -Steve
>
>
> On 6/5/2016 2:04 PM, Kurt Schwehr wrote:
>
>> Stephen,
>>
>> Take a look at these two discussions for starters on working with
>> hycom:
>>
>>
>> http://gis.stackexchange.com/questions/167155/how-can-i-make-geotiffs-of-individual-depth-layers-in-the-hycom-ocean-circulatio
>>
>>
>>
>> http://gis.stackexchange.com/questions/170882/how-can-a-make-a-netcdf-with-subgroups-smaller
>>
>>  You should start with the regular grids.  e.g.
>>
>>
>> ftp://ftp.hycom.org/datasets/GLBu0.08/expt_91.1/data/hindcasts/2015/hycom_glb_911_2015093000_t000_uv3z.nc
>>
>>  Make an 80 layer geotiff from the u/v and then use mapserver as
>> Even suggested.
>>
>> -kurt
>>
>>
>> On Sat, Jun 4, 2016 at 6:29 AM, Even Rouault
>> <even.rouault at spatialys.com <mailto:even.rouault at spatialys.com>>
>>
>> wrote:
>>
>> Hi,
>>
>>>
>>> I want to convert some HYCOM data into GTiff format and I'm a
>>> little lost on how to get started, whether I can do what I need
>>> just using a vrt or if I'm going to have to write code to process
>>> the data.
>>>
>>> To start with I have placed gdalinfo on one file here:
>>> http://imaptools.com:8080/dl/hycom-gdalinfo.txt
>>>
>>> The data comes from http://hycom.org/dataserver/glb-analysis and I
>>> will need to be using GLBa0.08 dataset.
>>>
>>> I have two use cases:
>>>
>>> 1) The simple one is to extract say Band 1 into a georeferenced
>>> GTiff and apply color to using color ranges without loosing the
>>> NODATA. I think I can do later using .vrt, correct?
>>>
>>
>> gdaldem color-relief can output to a VRT, but given that this netCDF
>> file is georeferenced through a geolocation array, you'll likely have
>> first to run gdalwarp -geoloc to have something useful at the end.
>>
>>
>>> I'm not sure how to deal with the georeferencing, because the
>>> download site says: "Native hycom .[ab] data converted to NetCDF on
>>> native Mercator-curvilinear HYCOM horizontal grid" but that does
>>> not seem in sync with the Metadata in the file.
>>>
>>> 2) The more complicated case is that there are two files: a)
>>> eastward_sea_water_velocity (aka: u, u-velocity) b)
>>> northward_sea_water_velocity (aka: v, v-velocity) that I would like
>>> to use to create a grid of vectors with arrowheads using u and v to
>>> define the vector and where the color of the vector is related to
>>> the magnitude of its length. Ultimately this data will get rendered
>>> via mapserver so I'm wondering if I can do this via a GTiff or if I
>>> will have to resort to creating a massive point shapefile with
>>> attributes of angle and magnitude and use a symbol. It seems like
>>> it would be best if I can keep the data as a GTiff and then sample
>>> the points and render the vectors on the fly for when you zoom
>>> in/out.
>>>
>>
>> See http://mapserver.org/input/vector/vector_field.html
>>
>> You don't need to create a vector file.
>>
>> You'll have to preprocess your file to get the u and v bands in the
>> same raster.
>>
>> Even
>>
>> -- Spatialys - Geospatial professional services
>> http://www.spatialys.com
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>>
>>
>> -- -- http://schwehr.org
>>
>
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