Documentation

Using griddap to Request Data and Graphs from Gridded Datasets

griddap lets you request a data subset, graph, or map from a gridded dataset (for example, sea surface
temperature data from a satellite), via a specially formed URL. griddap uses the
OPeNDAP Data Access Protocol (DAP) and its projection constraints.

The URL specifies what you want: the dataset, a description of the graph or the subset of the data, and the file
type for the response.

(easy) You can get data by using the dataset's Data Access Form. It makes the URL for you.
(easy) You can make a graph or map by using the dataset's Make A Graph form. It makes the URL for you.
(not hard) You can bypass the forms and get the data or make a graph or map by generating the URL by
hand or with a computer program or script.

griddap request URLs must be in the form
http://upwell.pfeg.noaa.gov/erddap/griddap/datasetID.fileType{?query}
For example,
http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?sst[(2007-10-21T00:00:00)][0][(-75):100:(75)][(180):100:(360)]
Thus, the query is often a variable name (e.g., sst),
followed by a set of [] for each of the variable's dimensions
(for example, [time][altitude][latitude][longitude]),
with your desired (start):stride:(stop) values within each [].

Details:

Requests must not have any internal spaces.
Requests are case sensitive.
{} is notation to denote an optional part of the request.
datasetID identifies the name that ERDDAP assigned to the dataset (for example, erdBAssta5day).
You can see a list of datasetID options available via griddap.

fileType specifies the type of grid data file that you want to download (for example, .htmlTable).
The actual extension of the resulting file may be slightly different than the fileType (for example,
.smallPdf returns a small .pdf file).
The fileType options for downloading gridded data are:

Data fileTypes	Description	Info	Example
.asc	View OPeNDAP-style comma-separated ASCII text.	info	example
.csv	Download a comma-separated ASCII text table (line 1: names; line 2: units; ISO 8601 times).	info	example
.csvp	Download a .csv file with line 1: name (units). Times are ISO 8601 strings.	info	example
.das	View the data's metadata via an OPeNDAP Dataset Attribute Structure (DAS).	info	example
.dds	View the data's structure via an OPeNDAP Dataset Descriptor Structure (DDS).	info	example
.dods	OPeNDAP clients use this to download the data in the DODS binary format.	info	example
.esriAscii	Download an ESRI ASCII file (for lat lon data only; lon must be all below or all above 180).	info	example
.fgdc	View the data's FGDC .xml metadata.	info	example
.graph	View a Make A Graph web page.	info	example
.help	View a web page with a description of griddap.	info	example
.html	View an OPeNDAP-style HTML Data Access Form.	info	example
.htmlTable	View a .html web page with the data in a table. Times are ISO 8601 strings.	info	example
.iso19115	View the data's ISO 19115-2/19139 .xml metadata.	info	example
.json	View a table-like JSON file (missing value = 'null'; times are ISO 8601 strings).	info	example
.mat	Download a MATLAB binary file.	info	example
.nc	Download a NetCDF-3 binary file with COARDS/CF/THREDDS metadata.	info	example
.ncHeader	View the header (the metadata) for the NetCDF-3 file.	info	example
.odvTxt	Download time,latitude,longitude,otherVariables as an ODV Generic Spreadsheet File (.txt).	info	example
.tsv	Download a tab-separated ASCII text table (line 1: names; line 2: units; ISO 8601 times).	info	example
.tsvp	Download a .tsv file with line 1: name (units). Times are ISO 8601 strings.	info	example
.xhtml	View an XHTML (XML) file with the data in a table. Times are ISO 8601 strings.	info	example

For example, here is a request URL to download data formatted as an HTML table:
http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?sst[(2007-10-21T00:00:00)][0][(-75):100:(75)][(180):100:(360)]

ArcGIS .esriAsc - ArcGIS is a family of Geographical Information Systems (GIS) products from ESRI:
ArcView, ArcEditor, and ArcInfo.
If you have ArcGIS 10 or higher, you can download a subset of a dataset in a .nc file and open it in ArcGIS.

If you have ArcGIS 9.x or lower:
in ERDDAP, choose the .esriAscii file type to save a latitude longitude subset of data for just one point in time.
The file's extension will be .asc. This file format was designed by ESRI to transfer coverage data between computers.
Then, import the data file into your ArcGIS program:

For ArcInfo v8 or higher:
1. Open ArcInfo.
2. Use ArcToolbox.
3. Use the Import to Raster : ASCII to Grid command
For ArcView's or ArcEditor's ArcMap:
1. Open ArcMap.
2. Choose a new empty map or load an existing map.
3. Click on the ArcToolbox icon.
  1. Open Conversion Tools : To Raster : ASCII to Raster
    1. Input Raster File - Browse to select the .esriAscii .asc file.
    2. Output Raster File - Keep the default (or change it).
    3. Output Data Type - change to FLOAT.
    4. Click on OK.
  2. Open Data Management Tools : Projections and Transformations : Define Projection
    1. Input Dataset or Feature Class - Browse to select the Raster (GRID) that was just
      created (on disk).
    2. Coordinate System - click the icon to the right of the text box.
      That will open the Spatial Reference Properties window.
      1. Click the Select button.
      2. Choose the Geographic Coordinate Systems folder. (Most data in ERDDAP uses a
        'geographic' projection, otherwise known as unprojected data or lat/lon data).
      3. Select on the Spheroid-based folder.
      4. Select the Clarke 1866.prj file and click Add.
      5. Click OK in the Spatial Reference Properties Window.
      6. Click OK in the Define Projection Window.
For older ArcView (v3) - You need the Spatial Analyst extension, which is sold separately.
1. Open ArcView
2. Open Spatial Analyst
3. Use Import Data Source : ASCII Raster

Shapefiles - Sorry, ERDDAP currently does not distribute grid data as shapefiles.

Ferret is a free program for visualizing and analyzing large and complex gridded
datasets. Ferret should work well with all datasets in griddap since griddap is
fully compatible with OPeNDAP. See the Ferret documentation.
Note that the griddap dataset's OPeNDAP base URL that you use with Ferret's
set data, for example, http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day ,
won't ever have a file extension at the end.

IDL - IDL is a commercial scientific data visualization program. To get data from ERDDAP
into IDL, first use ERDDAP to select a subset of data and download a .nc file.
Then, use these instructions to import the data from the .nc file into IDL.

JSON .json files are widely used to transfer data to JavaScript scripts running on web pages.
Griddap will format the data in a flat, table-like structure in the .json file.

JSONP .json - Requests for .json files may now include an optional jsonp request
by adding &.jsonp=functionName to the end of the query.
Basically, this just tells ERDDAP to add functionName( to the beginning of the
response and ")" to the end of the response.
If originally there was no query, leave off the "&" in your query.

MATLAB .mat users can use griddap's .mat file type to download data from within MATLAB.
Here is a one line example:

load(urlwrite('http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.mat?sst[(2007-10-21)][0][(-75):50:(75)][(180):50:(360)]&.draw=surface&.vars=longitude|latitude|sst&.colorBar=Rainbow|C|Linear|0|32|', 'test.mat'));

The data will be in a MATLAB structure. The structure's name will be the datasetID
(for example, erdBAssta5day).
The structure's internal variables will have the same names as in ERDDAP,
(for example, use fieldnames(erdBAssta5day)).
If you download a 2D matrix of data (as in the example above), you can plot it with
(for example):

imagesc(erdBAssta5day.longitude, erdBAssta5day.latitude, squeeze(erdBAssta5day.sst), [0 32])
set(gca, 'YDir', 'normal')

The numbers at the end of the first line specify the range for the color mapping.
The 'set' command flips the map to make it upright.

There are also Matlab Xtractomatic scripts for ERDDAP, which are particularly useful for
getting environmental data related to points along an animal's track (e.g., GTOPP data).

ERDDAP stores datetime values in .mat files as "seconds since 1970-01-01T00:00:00Z".
To display one of these values as a String in Matlab, you can use, e.g.,
datastr(cwwcNDBCMet.time(1)/86400 + 719529)
86400 converts ERDDAP's "seconds since" to Matlab's "days since". 719529 converts
ERDDAP's base time of "1970-01-01T00:00:00Z" to Matlab's "0000-01-00T00:00:00Z".

NetCDF .nc - Requests for .nc files return the requested subset of the dataset in a
standard, version 3, 32-bit, .nc file.

If you are using NetCDF-Java, don't try to directly access an ERDDAP dataset or subset
as a .nc file. (It won't work, mostly because that .nc file isn't a static, persistent file. It is a
virtual file.) Instead, use one of these two options:

Most situations: Open the ERDDAP dataset as an OPeNDAP dataset. For example:
NetcdfFile nc = NetcdfDataset.openFile("http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day", null);
(don't use NetcdfFile.open; it is for local files only) or
NetcdfDataset nc = NetcdfDataset.openDataset("http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day");
(NetcdfFiles are a lower level approach than NetcdfDatasets. It is your choice.)
Don't use a file extension (e.g., .nc) at the end of the dataset's name.
And don't specify a variable or a subset of the dataset at this stage.
Few situations: By hand in a browser or with a program like curl, download a .nc file
with a subset of the dataset. Then, use NetCDF-Java to open and access the data in
that local file, e.g.,
NetcdfFile nc = NetcdfFile.open("c:\downloads\theDownloadedFile.nc");
or
NetcdfDataset nc = NetcdfDataset.openDataset("c:\downloads\theDownloadedFile.nc");
(NetcdfFiles are a lower level approach than NetcdfDatasets. It is your choice.)
This approach makes more sense if you want a local copy of the data subset, so
that you can access it repeatedly (today, tomorrow, next week, ...) and quickly.

In both cases, you can then do what you want with the nc object, for example,
request metadata or request a subset of a variable's data.

.ncHeader - Requests for .ncHeader files will return the header information (text) that
would be generated if you used ncdump -h fileName on the corresponding .nc file.

Ocean Data View .odvTxt - ODV users can download data in a
ODV Generic Spreadsheet Format .txt file by requesting griddap's .odvTxt fileType.
The dataset MUST include longitude, latitude, and time dimensions.
Any longitude values (0 to 360, or -180 to 180) are fine.
After saving the resulting file (with the extension .txt) in your computer:

Open ODV.
Use File : Open.
- Change Files of type to Data Files (*.txt *.csv *.jos *.o4x).
- Browse to select the .txt file you created in ERDDAP and click on Open.
  The data locations should now be visible on a map in ODV.
If you downloaded a longitude, latitude slice of a dataset, use:
- Press F12 or View : Layout Templates : 1 SURFACE Window to view the
  default isosurface variable.
- If you want to view a different isosurface variable,
  - To define a new isosurface variable, use View : Isosurface Variables.
  - Right click on the map and choose Properties : Data : Z-axis to pick
    the new isosurface variable.
See ODV's Help menu for more help using ODV.

OPeNDAP Libraries - Since ERDDAP is an OPeNDAP-compatible data server, you can use
any OPeNDAP client library, such as NetCDF-Java, NetCDF-C, NetCDF-Fortran, NetCDF-Perl,
Java-DAP2, Ferret, or the Pydap Client, to get data from an ERDDAP griddap dataset.
When creating the initial connection to an ERDDAP griddap dataset from any OPeNDAP library:

Don't include a file extension (e.g., .nc) at the end of the dataset's name.
Don't specify a variable or a subset of the dataset.

Once you have made the connection to the dataset, you can request metadata or a subset
of a variable's data.

For example, with the NetCDF-Java library, you can use:
NetcdfFile nc = NetcdfDataset.openFile("http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day", null);
(don't use NetcdfFile.open; it is for local files only) or
NetcdfDataset nc = NetcdfDataset.openDataset("http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day");
(NetcdfFiles are a lower level approach than NetcdfDatasets. It is your choice.)
Once you have the nc object, you can request metadata or a subset of a
variable's data.

Pydap Client users can access griddap datasets via ERDDAP's standard OPeNDAP services.
See the Pydap Client instructions for accessing gridded data.
Note that the name of a dataset in ERDDAP will always be a single word,
e.g., erdBAssta5day in the OPeNDAP dataset URL
http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day
and won't ever have a file extension (unlike, for example, .nc for the
sample dataset in the Pydap instructions).

R Statistical Package - R is an open source statistical package for many operating systems.
In R, you can download a NetCDF version 3 .nc file from ERDDAP. For example:

  download.file(url="http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.nc?sst[(2007-10-21)][0][(-75):50:(75)][(180):50:(360)]&.draw=surface&.vars=longitude|latitude|sst&.colorBar=Rainbow|C|Linear|0|32|", destfile="/home/bsimons/test.nc")

Then import data from that .nc file into R with the RNetCDF, ncdf, or ncdf4 packages available
from CRAN. Or, if you want the data in tabular form, download and import the data in a .csv file.
For example,

  download.file(url="http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.csv?sst[(2007-10-21)][0][(-75):50:(75)][(180):50:(360)]&.draw=surface&.vars=longitude|latitude|sst&.colorBar=Rainbow|C|Linear|0|32|", destfile="/home/bsimons/test.csv")
  test<-read.csv(file="/home/bsimons/test.csv")

There are also R Xtractomatic scripts for ERDDAP, which are particularly useful for getting
environmental data related to points along an animal's track (e.g., GTOPP data).

Making an Image File with a Graph or Map of Gridded Data
If a griddap request URL specifies a subset of data which is suitable for making
a graph or a map, and the fileType is an image fileType, griddap will return an image
with a graph or map.
griddap request URLs can include optional graphics commands which let you
customize the graph or map.
As with other griddap request URLs, you can create these URLs by hand or have a
computer program do it. Or, you can use the Make A Graph web pages, which simplify
creating these URLs (see the "graph" links in the table of griddap datasets).

The fileType options for downloading images of graphs and maps of grid data are:

Image fileTypes	Description	Info	Example
.geotif	View a GeoTIFF .tif image file (for lat lon data only; lon must be all below or all above 180).	info	example
.kml	View a Google Earth .kml file (for lat, lon data only).	info	example
.smallPdf	View a small .pdf image file with a graph or map.	info	example
.pdf	View a standard, medium-sized .pdf image file with a graph or map.	info	example
.largePdf	View a large .pdf image file with a graph or map.	info	example
.smallPng	View a small .png image file with a graph or map.	info	example
.png	View a standard, medium-sized .png image file with a graph or map.	info	example
.largePng	View a large .png image file with a graph or map.	info	example
.transparentPng	View a .png image file (just the data, without axes, landmask, or legend).	info	example

Image Size - ".small" and ".large" were ERDDAP's original system for making
different-sized images. Now, for .png and .transparentPng images (not other
image file types), you can also use the &.size=width|height parameter to request
an image of any size.

.transparentPng - The .transparentPng file type will make a graph or map without
the graph axes, landmask, or legend, and with a transparent (not opaque white)
background. This option can be used for any type of graph or map.
When &.draw= is set to anything other than surface, the default image size
is 360x360 pixels. Use the &.size=width|height parameter to specify a different size.
When &.draw= is set to surface, ERDDAP makes an image where each
data point becomes one pixel, which may result in a huge image. If the request
takes too long or fails (perhaps for an odd apparent reason, like a Proxy Error)
either use a stride (see below) value greater than 1 (e.g., 5) for the x and y axis
variables, or use the &.size=width|height parameter to restrict the image size,
so that the image isn't huge.

Incompatibilities
Some results file types have restrictions. For example, Google Earth .kml is only
appropriate for results with longitude and latitude values. If a given request is
incompatible with the requested file type, griddap throws an error.

Command Line Downloads with curl
If you want to download a series of files from ERDDAP, you don't have to request each file's
ERDDAP URL in your browser, sitting and waiting for each file to download.
If you are comfortable writing computer programs (e.g., with C, Java, Python, Matlab, r)
you can write a program with a loop that imports all of the desired data files.
Or, if are comfortable with command line programs (just running a program, or using bash or tcsh
scripts in Linux or Mac OS X, or batch files in Windows), you can use curl to save results files
from ERDDAP into files on your hard drive, without using a browser or writing a computer program.
ERDDAP+curl is amazingly powerful and allows you to use ERDDAP in many new ways.
On Linux or Mac OS X, curl is probably already installed as /usr/bin/curl.
On Windows, or if your computer doesn't have curl already, you need to download curl
and install it. To get to a command line in Windows, use "Start : Run" and type in "cmd".
("Win32 - Generic, Win32, binary (without SSL)" worked for me on Windows XP and Windows 7.)
Instructions for using curl are on the curl man page and in this curl tutorial.
But here is a quick tutorial related to using curl with ERDDAP:

To download and save one file, use
curl -g "erddapUrl" -o fileDir/fileName.ext
where -g disables curl's globbing feature,
erddapUrl is any ERDDAP URL that requests a data or image file, and
-o fileDir/fileName.ext specifies the name for the file that will be created.
For example,
```
curl -g "http://coastwatch.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.png?sst[%282010-09-01T12:00:00Z%29][][][]&.draw=surface&.vars=longitude|latitude|sst&.colorBar=|||||" -o BAssta5day20100901.png
```
The erddapUrl must be percent encoded.
If you get the URL from your browser's address textfield, this may be already done.
If not, in practice, this can be very minimal percent encoding: all you usually
have to do is convert % into %25, & into %26, " into %22, + into %2B,
space into %20 (or +), < into %3C, > into %3E, ~ into %7E, and convert
all characters above #126 to their %HH form (where HH is the 2-digit hex value).
Unicode characters above #255 must be UTF-8 encoded and then each byte
must be converted to %HH form (ask a programmer for help).
To download and save many files in one step, use curl with the globbing feature enabled:
curl "erddapUrl" -o fileDir/fileName#1.ext
Since the globbing feature treats the characters [, ], {, and } as special, you must also
percent encode them in the erddapURL as %5B, %5D, %7B, %7D, respectively.
Then, in the erddapUrl, replace a zero-padded number (for example 01) with a range
of values (for example, [01-05] ),
or replace a substring (for example 5day) with a list of values (for example,
{5day,8day,mday} ).
The #1 within the output fileName causes the current value of the range or list
to be put into the output fileName.
For example,
```
curl "http://coastwatch.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.png?sst%5B%282010-09-[01-05]T12:00:00Z%29%5D%5B%5D%5B%5D%5B%5D&.draw=surface&.vars=longitude|latitude|sst&.colorBar=|||||" -o BAssta5day201009#1.png
```

query is the part of the request after the "?".
It specifies the subset of data that you want to receive.
In griddap, it is an optional OPeNDAP DAP projection constraint query which can request:
- One or more dimension (axis) variables, for example
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?latitude[0:10:100] .
- One or more data variables, for example
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?sst[656][0][0:100:1500][1800:100:3600] .
  To request more than one data variable, separate the desired data variable names by commas.
  If you do request more than one data variable, the requested subset for each variable
  must be identical (see below).
  (In griddap, all data variables within a grid dataset share the same dimensions.)
- The entire dataset. Omitting the entire query is the same as requesting all of the data
  for all of the variables. Because of the large size of most datasets, this is usually
  only appropriate for fileTypes that return information about the dataset (for example,
  .das, .dds, and .html), but don't return all of the actual data values. For example,
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.dds
  griddap is designed to handle requests of any size but trying to download all of the
  data with one request will usually fail (for example, downloads that last days usually
  fail at some point). If you need all of the data, consider breaking your big request
  into several smaller requests. If you just need a sample of the data, use the largest
  acceptable stride values (see below) to minimize the download time.
Using [start:stride:stop]
When requesting dimension (axis) variables or data variables, the query may
specify a subset of a given dimension by identifying the [start{{:stride}:stop}]
indices for that dimension.
- start is the index of the first desired value. Indices are 0-based.
  (0 is the first index. 1 is the second index. ...)
- stride indicates how many intervening values to get: 1=get every value,
  2=get every other value, 3=get every third value, ...
  Stride values are in index units (not the units of the dimension).
- stop is the index of the last desired value.
- Specifying only two values for a dimension (i.e., [start:stop]) is interpreted
  as [start:1:stop].
- Specifying only one value for a dimension (i.e., [start]) is interpreted
  as [start:1:start].
- Specifying no values for a dimension (i.e., []) is interpreted as [0:1:max].
- Omitting all of the [start:stride:stop] values (that is, requesting the
  variable without the subset constraint) is equivalent to requesting the entire variable.
  For dimension variables (for example, longitude, latitude, and time) and for fileTypes
  that don't download actual data (notably, .das, .dds, .html, and all of the graph and
  map fileTypes) this is fine. For example,
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.dds?sst
  For data variables, the resulting data may be very large.
  griddap is designed to handle requests of any size. But if you try to download all of
  the data with one request, the request will often fail for other reasons (for example,
  downloads that last for days usually fail at some point). If you need all of the data,
  consider breaking your big request into several smaller requests.
  If you just need a sample of the data, use the largest acceptable stride values
  to minimize the download time.
- last - ERDDAP extends the OPeNDAP standard by interpreting
  a start or stop value of last as the last available index value.
  You can also use the notation last-n (e.g., last-10)
  to specify the last index minus some number of indices.
  You can use '+' in place of '-'. The number of indices can be negative.
- griddap extends the standard OPeNDAP subset syntax by allowing the start
  and/or stop values to be actual dimension values (for example, longitude values
  in degrees_east) within parentheses, instead of array indices.
  This example with [time][altitude][latitude][longitude] dimension values
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?sst[(1192924800)][0][(-75):100:(75)][(180):100:(360)]
  is (at least at the time of writing this) equivalent to this example with dimension indices
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?sst[656][0][0:100:1500][1800:100:3600]
  The value in parentheses must be within the range of values for the dimension.
  If the value in parentheses doesn't exactly equal one of the dimension values, the
  closest dimension value will be used.
- griddap does not allow parentheses around stride values.
  The reasoning is: With the start and stop values, it is easy to convert the value in
  parentheses into the appropriate index value by finding the nearest dimension value.
  This works if the dimension values are evenly spaced or not.
  If the dimension values were always evenly spaced, it would be easy to use a similar
  technique to convert a stride value in parentheses into a stride index value.
  But dimension values often aren't evenly spaced. So for now, ERDDAP doesn't support the
  parentheses notation for stride values.
- griddap always stores date/time values as numbers (in seconds since 1970-01-01T00:00:00Z).
  Here is an example of a query which includes date/time numbers:
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?sst[(1192924800)][0][(-75):100:(75)][(180):100:(360)]
  Some fileTypes (notably, .csv, .tsv, .htmlTable, .odvTxt, and .xhtml) display date/time values as
  ISO 8601:2004 "extended" date/time strings (e.g., 2002-08-03T12:30:00Z).
  ERDDAP has a utility to Convert a Numeric Time to/from a String Time.
  See also: How ERDDAP Deals with Time.
- For the time dimension, griddap extends the OPeNDAP standard by allowing you to specify an
  ISO 8601:2004 "extended" date/time string in parentheses, which griddap then converts to the
  internal number (in seconds since 1970-01-01T00:00:00Z) and then to the appropriate
  array index. The ISO date/time value should be in the form: YYYY-MM-DDThh:mm:ssZ,
  where Z is 'Z' or a ±hh:mm offset from UTC.
  If you omit Z (or the ±hh:mm offset), :ssZ, :mm:ssZ, or Thh:mm:ssZ from the ISO date/time
  that you specify, the missing fields are assumed to be 0.
  The example below is equivalent (at least at the time of writing this) to the examples above:
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?sst[(2007-10-21T00:00:00)][0][(-75):100:(75)][(180):100:(360)]
- (last) - ERDDAP interprets a start or stop value of (last) as the last
  available index.
  You can also use the notation (last-d) (e.g., last-10.5) to specify
  the last index's value minus some number, which is then converted to the nearest
  index. You can use '+' in place of '-'. The number can be negative.
  For a time axis, the number is interpreted as some number of seconds.
- Graphics Commands - griddap extends the OPeNDAP standard by allowing graphics commands
  in the query.
  The Make A Graph web pages simplify the creation of URLs with these graphics commands
  (see the "graph" links in the table of griddap datasets).
  So we recommend using the Make A Graph web pages to generate URLs, and then, when
  needed, using the information here to modify the URLs for special purposes.
  These commands are optional.
  If present, they must occur after the data request part of the query.
  These commands are used by griddap if you request an image fileType (PNG or PDF) and are
  ignored if you request a data file (e.g., .asc).
  If relevant commands are not included in your request, griddap uses the defaults and tries
  its best to generate a reasonable graph or map.
  All of the commands are in the form &.commandName=value .
  If the value has sub-values, they are separated by the '|' character.
  The commands are:
  - &.colorBar=palette|continuous|scale|min|max|nSections
    This specifies the settings for a color bar. The sub-values are:
    - palette - All ERDDAP installations support a standard set of palettes:
      BlackBlueWhite, BlackRedWhite, BlackWhite, BlueWhiteRed, LightRainbow,
      Ocean, OceanDepth, Rainbow, RedWhiteBlue, ReverseRainbow, Topography,
      WhiteBlack, WhiteBlueBlack, WhiteRedBlack.
      Some ERDDAP installations support additional options. See a Make A Graph
      web page for a complete list.
      The default varies based on min and max: if -1*min ~= max, the default
      is BlueWhiteRed; otherwise, the default is Rainbow.
    - continuous - must be either no value (the default), 'C' (for Continuous),
      or 'D' (for Discrete). The default is different for different datasets.
    - scale - must be either no value (the default), Linear, or Log.
      The default is different for different datasets.
    - min - The minimum value for the color bar.
      The default is different for different datasets.
    - max - The maximum value for the color bar.
      The default is different for different datasets.
    - nSections - The preferred number of sections (for Log color bars,
      this is a minimum value). The default is different for different datasets.
    If you don't specify one of the sub-values, the default for the sub-value will be used.
  - &.color=value
    This specifies the color for data lines, markers, vectors, etc. The value must
    be specified as an 0xRRGGBB value (e.g., 0xFF0000 is red, 0x00FF00 is green).
    The default is 0x000000 (black).
  - &.draw=value
    This specifies how the data will be drawn, as lines, linesAndMarkers,
    markers, sticks, surface, or vectors.
  - &.font=scaleFactor
    This specifies a scale factor for the font (e.g., 1.5 would make the font
    1.5 times as big as normal).
  - &.land=value
    This specifies whether the landmask should be drawn under or over the data.
    The default is different for different datasets (under the ERDDAP administrator's
    control via a drawLandMask setting in datasets.xml, or via the fallback drawLandMask
    setting in setup.xml).
    Terrestrial researchers usually prefer under.
    Oceanographers often prefer over.
  - &.legend=value
    This specifies whether the legend on PNG images (not PDF's) should be at the
    Bottom (default), Off, or Only (which returns only the legend).
  - &.marker=markerType|markerSize
    markerType is an integer: 0=None, 1=Plus, 2=X, 3=Dot, 4=Square,
    5=Filled Square (default), 6=Circle, 7=Filled Circle, 8=Up Triangle,
    9=Filled Up Triangle.
    markerSize is an integer from 3 to 50 (default=5)
  - &.size=width|height
    For PNG images (not PDF's), this specifies the desired size of the image, in pixels.
    This allows you to specify sizes other than the predefined sizes of .smallPng, .png,
    and .largePng, or the variable size of .transparentPng.
  - &.trim=trimPixels
    For PNG images (not PDF's), this tells ERDDAP to make the image shorter by removing
    all whitespace at the bottom, except for trimPixels.
  - &.vars='|'-separated list
    This is a '|'-separated list of variables names. Defaults are hard to predict.
    The meaning associated with each position varies with the &.draw value:
    - for lines: xAxis|yAxis
    - for linesAndMarkers: xAxis|yAxis|Color
    - for markers: xAxis|yAxis|Color
    - for sticks: xAxis|uComponent|vComponent
    - for surface: xAxis|yAxis|Color
    - for vectors: xAxis|yAxis|uComponent|vComponent
    If xAxis=longitude and yAxis=latitude, you get a map; otherwise, you get a graph.
  - &.vec=value
    This specifies the data vector length (in data units) to be scaled to the
    size of the sample vector in the legend. The default varies based on the data.
  - &.xRange=min|max
    This specifies the min|max for the X axis. The default varies based on the data.
  - &.yRange=min|max
    This specifies the min|max for the Y axis. The default varies based on the data.
  A sample graph URL is
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.png?sst[(2007-07-01):(2007-10-21)][0][(29)][(225)]&.draw=linesAndMarkers&.vars=time|sst|&.marker=1|3&.color=0xFF9900&.colorBar=|C|Linear|||
  Or, if you change the fileType in the URL from .png to .graph, you can see a Make A Graph
  web page with that request loaded:
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.graph?sst[(2007-07-01):(2007-10-21)][0][(29)][(225)]&.draw=linesAndMarkers&.vars=time|sst|&.marker=1|3&.color=0xFF9900&.colorBar=|C|Linear|||
  That makes it easy for humans to modify an image request to make a similar graph or map.
  Or, if you change the fileType in the URL from .png to a data fileType (e.g., .htmlTable), you can download the data that was graphed:
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?sst[(2007-07-01):(2007-10-21)][0][(29)][(225)]&.draw=linesAndMarkers&.vars=time|sst|&.marker=1|3&.color=0xFF9900&.colorBar=|C|Linear|||
  A sample map URL is
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.png?sst[(2007-10-21)][0][(-75):50:(75)][(180):50:(360)]&.draw=surface&.vars=longitude|latitude|sst&.colorBar=Rainbow|C|Linear|0|32|
  Or, if you change the fileType in the URL from .png to .graph, you can see a Make A Graph
  web page with that request loaded:
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.graph?sst[(2007-10-21)][0][(-75):50:(75)][(180):50:(360)]&.draw=surface&.vars=longitude|latitude|sst&.colorBar=Rainbow|C|Linear|0|32|
  Or, if you change the fileType in the URL from .png to a data fileType (e.g., .htmlTable),
  you can download the data that was mapped:
  http://upwell.pfeg.noaa.gov/erddap/griddap/erdBAssta5day.htmlTable?sst[(2007-10-21)][0][(-75):50:(75)][(180):50:(360)]&.draw=surface&.vars=longitude|latitude|sst&.colorBar=Rainbow|C|Linear|0|32|

Other Information

Data Model - Each griddap dataset can be represented as:
- An ordered list of one or more 1-dimensional axis variables.
  Each axis variable has data of one specific type.
  If the data source has a dimension with a size but no values, ERDDAP uses the values
  0, 1, 2, ...
  The supported types are int8, uint16, int16, int32, int64, float32, and float64.
  Missing values are not allowed.
  The values MUST be sorted in either ascending (recommended) or descending order.
  Unsorted values are not allowed because [(start):(stop)] requests must
  translate into a contiguous range of indices.
  Tied values are not allowed because requests for a single [(value)] must
  translate unambiguously to one index.
  Each axis variable has a name composed of a letter (A-Z, a-z) and then 0 or more
  characters (A-Z, a-z, 0-9, _).
  Each axis variable has metadata which is a set of Key=Value pairs.
- A set of one or more n-dimensional data variables.
  All data variables use all of the axis variables, in order, as their dimensions.
  Each data variable has data of one specific type.
  The supported types are (int8, uint16, int16, int32, int64, float32, float64, and
  String of any length).
  Missing values are allowed.
  Each data variable has a name composed of a letter (A-Z, a-z) and then 0
  or more characters (A-Z, a-z, 0-9, _).
  Each data variable has metadata which is a set of Key=Value pairs.
- The dataset has Global metadata which is a set of Key=Value pairs.
- Note about metadata: each variable's metadata and the global metadata is a set of 0
  or more Key=Value pairs.
  Each Key is a String consisting of a letter (A-Z, a-z) and then 0 or more other
  characters (A-Z, a-z, 0-9, '_').
  Each Value is either one or more numbers (of one Java type), or one or more Strings
  of any length (using \n as the separator).
Special Variables
- In griddap, a longitude axis variable (if present) always has the name "longitude"
  and the units "degrees_east".
- In griddap, a latitude axis variable (if present) always has the name, "latitude"
  and the units "degrees_north".
- In griddap, an altitude axis variable (if present) always has the name "altitude"
  and the units "m" above sea level.
  Locations below sea level have negative altitude values.
- In griddap, a time axis variable (if present) always has the name "time"
  and the units "seconds since 1970-01-01T00:00:00Z".
  If you request data and specify a start and/or stop value for the time axis,
  you can specify the time as a number (in seconds since 1970-01-01T00:00:00Z)
  or as a String value (e.g., "2002-12-25T07:00:00Z" in the GMT/Zulu time zone).
- Because the longitude, latitude, altitude, and time axis variables are specifically
  recognized, ERDDAP is aware of the spatiotemporal features of each dataset.
  This is useful when making images with maps or time-series, and when saving data
  in geo-referenced file types (e.g., .esriAscii and .kml).
Incompatibilities
- File Types - Some results file types have restrictions.
  For example, .kml is only appropriate for results with a range of longitude and
  latitude values.
  If a given request is incompatible with the requested file type, griddap throws an error.
Requesting Compressed Files
ERDDAP doesn't offer results stored in compressed (e.g., .zip or .gzip) files.
Instead, ERDDAP looks for accept-encoding in the HTTP GET request header sent
by the client. If a supported compression type ("gzip", "x-gzip", or "deflate") is found
in the accept-encoding list, ERDDAP includes "content-encoding" in the HTTP response
header and compresses the data as it transmits it.
It is up to the client program to look for "content-encoding" and decompress the data.
Browsers and OPeNDAP clients do this by default. They request compressed data and
decompress the returned data automatically.
Other clients (e.g., Java programs) have to do this explicitly.

ERDDAP, Version 1.42
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