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Data support in ArcGIS > NetCDF: multidimensional, time series data > Working with netCDF data in ArcGIS
Data support in ArcGIS > NetCDF: multidimensional, time series data > Working with netCDF data in ArcGIS
FAQs about working with netCDF data |
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Release 9.3
Last modified November 19, 2008 |
Print all topics in : "Working with netCDF data in ArcGIS" |
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When reading netCDF data in ArcGIS, you may run into often-encountered issues and have questions such as Why doesn't the netCDF raster look right? or Why is the spatial reference undefined? If you've reached a stumbling block in reading your netCDF file and making it work with other GIS data, the following list of frequently asked questions (FAQs) may provide an answer for you.
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I dragged and dropped a netCDF file into ArcMap. Why doesn't the netCDF raster look right?
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The error message, "One or both dimensions have variable spacing in their coordinate values" occurs when attempting to create a netCDF raster layer. Why do I get this error message?
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Why is the spatial reference undefined?
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The netCDF file contains longitude and latitude information. Why are the auxiliary coordinate variables longitude and latitude not available in the X and Y dimension lists when I select a variable?
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Why are the values in the time dimension displayed as numbers but not as dates?
The data type is determined from the units specified by the units attribute (where units is the name of the attribute), not from the name of the variable. Times are stored as numbers in the coordinate variable. If CF-compliant units are not specified, then the values are listed as numbers. When appropriate units such as time:units = "days since 1895-01-01" are specified, then the numbers are displayed as dates. -
Why can't I fix the data source of a netCDF layer or table after the map document is relocated relative to a netCDF data file?
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How can I see netCDF files in ArcCatalog?
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How can I see the metadata of a netCDF file?
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What is the best way for storing temporal data—a netCDF file or a relational database? Which one is faster?
ArcGIS looks for the first variable in the file with two or more dimensions and uses it to create a raster. Sometimes, the default variable chosen isn't the one you want to display.
To display a particular variable, change the variable and other parameters on the netCDF raster layer properties dialog box or use the Make NetCDF Raster Layer tool and specify the variable you want to display.
To create a netCDF raster layer from a netCDF variable, the spacing between x-coordinates must be equal and the spacing between y-coordinates must be equal. If the coordinates are unequally spaced, create a netCDF feature layer, then interpolate to raster. In most cases, you encounter this problem when the spacing between latitudes is not equal. The differences between latitudes decreases as you go toward poles.
The spatial reference of a netCDF layer depends on various parameters. Make sure that the appropriate dimensions, coordinate variables, and auxiliary coordinate variables are specified as x and y dimensions. If you are expecting a geographic coordinate system (GCS), check the units attribute. The units for longitude and latitude should be degrees_east and degrees_north or one of the acceptable combinations. If you are expecting a projected coordinate system (PCS), check the grid_mapping attribute. The projection defined by the grid_mapping attribute must be supported by CF conventions. Also check the standard_name and units attributes for x and y dimensions.
Views like latitude vs. longitude, y vs. x, longitude vs. time, x vs. z, etc., will always result in an undefined spatial reference.
Only the dimensions and the auxiliary coordinate variables specified by the coordinate attributes are listed in the X and Y dimension lists. The coordinate attribute defines the relationship between a variable and its auxiliary coordinate variables. Check the coordinates attribute of the variable you have selected.
The normal process of repairing a broken data link does not work with a netCDF file. Check the data source path and copy the netCDF file into the correct folder. Alternatively, make the netCDF layer or table again.
Precaution: Set the data source option to store relative path names and copy the data and the map document together.
You can add the .nc extension to the ArcCatalog file extension options so that you can drag and drop .nc files from ArcCatalog.
There are third-party utilities and APIs available to extract metadata from a netCDF file. You can download ncdump from the NetCDF Installation and Porting Guide.
Storing temporal data in a relational database is just as viable as using a netCDF file. ESRI's support of netCDF is primarily to support the existing community of netCDF data and users, not to force people to learn about a new file format. The decision should be made based on how you want to create and manage data in your organization.