Re: [thredds] Simple fix => much smaller TDS WMS GetCapabilities size (for model output)

• To: thredds@xxxxxxxxxxxxxxxx
• Subject: Re: [thredds] Simple fix => much smaller TDS WMS GetCapabilities size (for model output)
• From: John Caron <caron@xxxxxxxxxxxxxxxx>
• Date: Wed, 22 Dec 2010 11:22:15 -0700

On 12/22/2010 11:14 AM, Doug Lindholm wrote:

I'd like to add to that. At what point is it sufficient to fill gaps (assuming uniform time steps) with fill values? In some cases, determining the precise time samples available is an expensive task. I'd rather get a single time range quickly then deal with fill values (such as NaNs which often require no special handling).

Doug

On 12/22/10 11:01 AM, Bob Simons wrote:

Some other issues to consider:

* Where is the dividing line between a few missing values (where it's
"okay" to say the values are evenly spaced) and too many missing values
(where it isn't okay)?  Some number?  Some percentage?  Does it matter
if the missing values are adjacent or scattered?

* getCapabilities purpose is to tell the client what is available. It is
probably fine for a human to read that the data is evenly spaced (even
if it isn't perfectly); humans are sometimes forgiving.  But will there
be problems if a computer program client expects (perfectly) evenly
spaced values and requests data for those values?

On 12/22/2010 9:47 AM, Ethan Davis wrote:

Hi Rich,

I've added this feature request to our list. Jon Blower might have some
thoughts on this as well.

One thing that I wonder about is client support. In particular, does
Godiva2 support this? Again, a question for Jon.

Ethan

On 12/22/2010 6:00 AM, Rich Signell wrote:

For getCapabilities requests, it would be great if the TDS would
express the available times using the WMS multiple time interval
syntax if it is more efficient than listing each time value
separately.

This can result in huge (100 or more) savings in the WMS
getCapabilities size when dealing with model output, which is usually
equally spaced, but perhaps with a few gaps.   Instead of listing
every available time step in ISO format, as is done currently

In WMS 1.1.1, Annex C.3 states that multiple intervals are allowed in
the "Extent" element.
In WMS 1.3.0, Annex C.2 states that multiple intervals are allowed in
the "Dimension" element.

Both list the sample format: "min1/max1/res1,min2/max2/res2,..."
(thanks to Kyle Wilcox for digging out this info)

For example, we have a dataset

http://testbedapps.sura.org/thredds/clean.html?dataset=estuarine_hypoxia/ch3d/agg

that contains hourly output over 21 years (183984 time records) for 11
different variables.  There are only 4 gaps longer than 1 hour.

If you access the WMS getCapabilities document for this dataset, be
prepared to wait for a while, because it's 51Mb!!

The problem is that each time value is listed in ISO ASCII:
<Dimension name="time" units="ISO8601" multipleValues="true"
current="true" default="2006-01-01T00:00:00.000Z">

1985-01-01T01:00:00.000Z,1985-01-01T02:00:00.000Z,1985-01-01T03:00:00.000Z,1985-01-01T04:00:00.000Z,1985-01-01T05:00:00.000Z,1985-01-01T06:00:00.000Z,1985-01-01T07:00:00.000Z,1985-01-01T08:00:00.000Z,1985-01-01T09:00:00.000Z,1985-01-01T10:00:00.000Z,

...

which goes on for 5MB of ASCII values and then this whole mess is
repeated for each variable "layer".

Instead, the entire time record could be simply expressed using 5
intervals of the form:

"1985-01-01T01:00:00.000Z/1988-12-31T00:00:00.000Z/PT3600S",
"1989-01-01T01:00:00.000Z/1992-12-31T00:00:00.000Z/PT3600S",
"1993-01-01T01:00:00.000Z/2000-12-31T00:00:00.000Z/PT3600S",
"2001-01-01T01:00:00.000Z/2004-12-31T00:00:00.000Z/PT3600S",
"2005-01-01T01:00:00.000Z/2006-01-01T00:00:00.000Z/PT3600S",


Existing way (every time step written out): 51MB

New way (specifying intervals): 100Kb (500 times smaller!!!!!)

This would greatly reduce the file sizes on Motherlode, our IOOS
testbed server, and every other TDS (or WMS, actually) with aggregated
model output.

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• Follow-Ups:
  • Re: [thredds] Simple fix => much smaller TDS WMS GetCapabilities size (for model output)
    • From: Kyle Wilcox

• References:
  • [thredds] Simple fix => much smaller TDS WMS GetCapabilities size (for model output)
    • From: Rich Signell
  • Re: [thredds] Simple fix => much smaller TDS WMS GetCapabilities size (for model output)
    • From: Ethan Davis
  • Re: [thredds] Simple fix => much smaller TDS WMS GetCapabilities size (for model output)
    • From: Bob Simons
  • Re: [thredds] Simple fix => much smaller TDS WMS GetCapabilities size (for model output)
    • From: Doug Lindholm