Subject: Marine Geospatial Ecology Tools (MGET) help
Text archives
| From: | "Jason Roberts" <> |
|---|---|
| To: | <> |
| Cc: | <> |
| Subject: | RE: [mget-help] Help with "Convert 2D variable in NetCDF to ArcGIS raster" tool |
| Date: | Fri, 5 Nov 2010 11:17:14 -0400 |
Hello Corinne,
We're sorry you're having trouble with this. NetCDF files can be difficult
to work with but I should be able to help you.
First, a little background. Forgive me if you already know this.
The origin of your difficulty lies in how the netCDF format was defined and
how it is used by organizations that publish netCDF files. In its simplest
usage, netCDF provides a means to transfer bare data values, such as lists
or grids of numbers. The numbers might represent something, such as sea
surface temperatures in degrees Celsius for the Mediterranean Sea, but the
meaning of them is not evident from the numbers themselves.
In order for the numbers to be used in a GIS system, some additional
information is needed, such as where the grid is located on the earth, what
the cell size is, and so on. This information is called metadata. The netCDF
format defines a mechanism for including metadata in netCDF files, in
addition to the raw data values. But there are two problems:
1. Metadata is not required.
2. The syntax for metadata is very flexible, allowing many different ways
for representing "cell size", for instance, and the users of netCDF have not
been able to agree on a single standard for representing the different kinds
of metadata.
The end result is that different netCDF publishers include different amounts
and kinds of metadata, in different syntaxes, and sometimes not at all. It
has gotten better over the years, with groups of organizations agreeing to
adhere to one standard or another (these are often called "conventions"),
but it is still too difficult to write a single tool that can take any
netCDF file as input and automatically extract the needed metadata from it.
We opted not to attempt that. Instead, it is the user's responsibility to
somehow find out the metadata and input it into our tool.
Often, some or all of the needed metadata is included in the netCDF file. By
extracting the header, you can see the metadata. But it is often not obvious
how to see it. For example, what ArcGIS calls the "NoData Value" may be
called "_FillValue" in the netCDF header. Unfortunately there is no easy way
to learn what to do. You just have to work with netCDFs from different
organizations and learn how their terminology differs from ArcGIS and from
each other. It is a slow and often frustrating process.
To make matters worse, the netCDF header is usually missing important
metadata. For example, you rarely see full details (if any) about the
coordinate system for the data. To find these things out, you have to resort
to documentation provided by the data publisher, or even email them.
Now regarding your specific problem. Unfortunately I cannot help you
determine the three parameters you mention without knowing more about the
exact netCDFs you're trying to convert. This is because, as I discussed
above, there is no standard way for netCDFs to represent them in their
metadata. But if you tell me a bit more about the file you want to convert
(send me a link to the place where you download it), I can help you figure
them out. I know you referenced Unidata's example netCDF files but I doubt
these are the ones you ultimately want to use. Let me know what those are
and we'll try to figure it out.
Best regards,
Jason
-----Original Message-----
From:
[mailto:]
Sent: Friday, November 05, 2010 9:54 AM
To:
Subject: [mget-help] Help with "Convert 2D variable in NetCDF to ArcGIS
raster" tool
Hello,
I am trying to use the "Convert 2D variable in NetCDF to ArcGIS raster" tool
from the MGET toolbox in ArcGIS 9.2. I am new to NetCDF format and have
trouble
finding the correct information to input in some of the tool's boxes. I have
tried with several example netCDF files (available at
http://www.unidata.ucar.edu/software/netcdf/examples/files.html),
unsuccessfully. Specifically, I have trouble with the following boxes:
- X coordinate of lower-left corner
- Y coordinate of lower-left corner
- Cell size
I have extracted the header of NetCDF files in search for this information,
but
I could not find anything that worked. Any help on where I can find the
information to input in the boxes would be greatly apreciated.
Best wishes,
Corinne
We're sorry you're having trouble with this. NetCDF files can be difficult
to work with but I should be able to help you.
First, a little background. Forgive me if you already know this.
The origin of your difficulty lies in how the netCDF format was defined and
how it is used by organizations that publish netCDF files. In its simplest
usage, netCDF provides a means to transfer bare data values, such as lists
or grids of numbers. The numbers might represent something, such as sea
surface temperatures in degrees Celsius for the Mediterranean Sea, but the
meaning of them is not evident from the numbers themselves.
In order for the numbers to be used in a GIS system, some additional
information is needed, such as where the grid is located on the earth, what
the cell size is, and so on. This information is called metadata. The netCDF
format defines a mechanism for including metadata in netCDF files, in
addition to the raw data values. But there are two problems:
1. Metadata is not required.
2. The syntax for metadata is very flexible, allowing many different ways
for representing "cell size", for instance, and the users of netCDF have not
been able to agree on a single standard for representing the different kinds
of metadata.
The end result is that different netCDF publishers include different amounts
and kinds of metadata, in different syntaxes, and sometimes not at all. It
has gotten better over the years, with groups of organizations agreeing to
adhere to one standard or another (these are often called "conventions"),
but it is still too difficult to write a single tool that can take any
netCDF file as input and automatically extract the needed metadata from it.
We opted not to attempt that. Instead, it is the user's responsibility to
somehow find out the metadata and input it into our tool.
Often, some or all of the needed metadata is included in the netCDF file. By
extracting the header, you can see the metadata. But it is often not obvious
how to see it. For example, what ArcGIS calls the "NoData Value" may be
called "_FillValue" in the netCDF header. Unfortunately there is no easy way
to learn what to do. You just have to work with netCDFs from different
organizations and learn how their terminology differs from ArcGIS and from
each other. It is a slow and often frustrating process.
To make matters worse, the netCDF header is usually missing important
metadata. For example, you rarely see full details (if any) about the
coordinate system for the data. To find these things out, you have to resort
to documentation provided by the data publisher, or even email them.
Now regarding your specific problem. Unfortunately I cannot help you
determine the three parameters you mention without knowing more about the
exact netCDFs you're trying to convert. This is because, as I discussed
above, there is no standard way for netCDFs to represent them in their
metadata. But if you tell me a bit more about the file you want to convert
(send me a link to the place where you download it), I can help you figure
them out. I know you referenced Unidata's example netCDF files but I doubt
these are the ones you ultimately want to use. Let me know what those are
and we'll try to figure it out.
Best regards,
Jason
-----Original Message-----
From:
[mailto:]
Sent: Friday, November 05, 2010 9:54 AM
To:
Subject: [mget-help] Help with "Convert 2D variable in NetCDF to ArcGIS
raster" tool
Hello,
I am trying to use the "Convert 2D variable in NetCDF to ArcGIS raster" tool
from the MGET toolbox in ArcGIS 9.2. I am new to NetCDF format and have
trouble
finding the correct information to input in some of the tool's boxes. I have
tried with several example netCDF files (available at
http://www.unidata.ucar.edu/software/netcdf/examples/files.html),
unsuccessfully. Specifically, I have trouble with the following boxes:
- X coordinate of lower-left corner
- Y coordinate of lower-left corner
- Cell size
I have extracted the header of NetCDF files in search for this information,
but
I could not find anything that worked. Any help on where I can find the
information to input in the boxes would be greatly apreciated.
Best wishes,
Corinne
Archives powered by MHonArc.