1 minute tide data

NOAA/COOPS has a website for displaying 1 minute tide data from their NWLON stations for use in tsunami detection.  The interface is Google and Yahoo, and is really quite nice.  Only problem, access to the data is limited to 4 days at a time.

I was interested in getting 1 minute data for a couple stations for the summer of 2009 to see how it would compare to hydro survey boat GPS heights, thinking that perhaps the 1 minute data might pick up some of the higher frequency water level changes that appear (correctly or not) in the survey vessel GPS heights.

Wrote a python script with urlib and urlib2 to pull any amount of data in 4 day increments, which saves me a lot of button clicking.  The hardest part was picking up on a unique tmpname variable that is assigned on the first webpage with the graph.  Regardless of what begin and end dates you might post back to the server, the tmpname variable actually controls what time period you will see on the following page when you click through to view the tabular data.

Set up the script to pull the data while I snoozed.  Only, there wasn't any data from the summer.  In fact, there was only a couple of weeks of data available for these stations at all.  Checked a few other stations and they also seemed to be fairly spotty on coverage.

I'm not sure if that's a NOAA data retention policy, or if they've had problems collecting or storing this 1 minute data.  Anyways, it was worth it to learn a bit about urlib and urlib2.

UPDATE: got a very prompt response from NOAA folks that the 1 minute data is available from their OpenDAP website, in chunks of 30 days.  And the data appears complete on this website.

Disappearing HWI and LWI

The high water inequality (HWI) and low water inequality (LWI) as defined by NOAA are the time in hours after the transit of the moon at the Greenwich Meridian that the high or low tide occurs at the tide station.  It's easier to grasp the reason for computing HWI and LWI if you first think in terms of the local meridian.  Relative to the local meridian, HWI would show (in most places) that high tide follows closely after the transit of the moon, which makes intuitive sense.

So if you go looking for HWI and LWI on the datum of a NOAA tide station, why do some have them and some not?  As far as I understand it, that's due to the tide being diurnal or semidiurnal at that location.  HWI and LWI are not computed for diurnal stations (which can be determined by looking at their "form number" which is a ratio of the primary semidiurnal and diurnal consituents...see The Secret of the Tide).  But for some stations HWI and LWI seems to come and go in the monthly means, for example, Unalaska, AK.  This is a mixed, diurnal station, so I'd guess that HWI and LWI are only computed for months when the tide is more mixed, then diurnal.

I'd be interested to know how that is determined...

Tides, GPS and VDATUM...looking back...

Some people may look at VDATUM and see the demise of tides.  I think the transition from traditional tides to VDATUM is an interesting opportunity because they provide redundant methods of vertical height measurment on the water.  Each method informs the other, and neither is perfect.  Where VDATUM doesn't exist yet (Alaska), using GPS water level heights can aid in the creation of a traditional tidal zoning scheme or allow you to roll your own custom VDATUM.

Some of the projects JOA has been involved with relating to GPS tides/VDATUM.

2009
VDATUM evaluation using GPS heights from hydrographic survey vessels and Seabird SBE26 underwater tide gauges. Offshore of San Francisco, CA.
Unimak Pass, Aleutian Island in Alaska.  Supporting a hydrographic survey for nautical charting using GPS heights from hydrographic vessels to develop better tidal zoning in complex area where Pacific and Bering Sea meet.
VDATUM  evaluation using GPS buoy and radar tide gauge in Chesapeake Bay.

2008
Upper Cook Inlet, Alaska.  Supporting a hydrographic survey for nautical charting using GPS heights from hydrographic vessels,  Seabird SBE26 underwater tide gauges and traditional shore based tide gauges to develop better tidal zoning (10m tide range).  Developed ellipsoid/MLLW separation grid which was used to process soundings to final MLLW chart depth (roll your own VDATUM...).

2007
Akutan Pass, Aleutian Islands in Alaska. Supporting a hydrographic survey for nautical charting.  Started with a traditional tides approach, tide gauges at either end of pass, but couldn't develop adequate zoning using tide data from just a few points.  The Pacific to the south tends to have a much more regular semidiurnal tide, whereas the Bering Sea to the north has much less consistent tides which vary between semidiurnal and diurnal throughout the month.

Began extracting tide data from moving hydrographic survey vessels GPS measurements (corrected for pitch and roll) for the purpose understanding more about the tide over a geographic area, instead of one discrete point.  It was more of a descriptive tool at this point, since we really didn't have the tools to thoroughly correct and analyze the millions of points.  Crunched the data as best we could (in Excel and Access) and then painted a picture of the tide in ArcGIS and with post-it-notes on a large map.

2006
Mitrofania Island, Alaska.  Deployed JOA designed GPS buoy for 3 - 7 days at various locations throughout hydrographic survey area.  Testing concept of tides with GPS buoy.  Excellent tool to measure separation between MSL and geoid corrected ellipsoid surface, but deployment periods were too short to compute accurate datums.

2005
Shumagin Islands, Alaska.  Installed multiple tide gauges, tied their water level heights to the ellipsoid to create a MLLW/ellipsoid surface, so the survey vessels could process their soundings directly to MLLW from the ellipsoid, without tide corrections.