Wednesday, July 4, 2007


Xtide is a fantastic tide prediction program. I have been using it for over 10 years to produce tide calendars for Saipan and Chuuk Lagoon. Harmonic constants are coefficients for a location, that can be plugged into the tide prediction equation to generate predictions as a function of time. Harmonic constants used to be readily available for a few locations in the Philippine Islands for Xtide; however, in recent years apparently political and legal forces have conspired to make such constants less readily available for various places. If one had an old version of Xtide, he might generate tidal predictions---at least baselines, not expected to be accurate---for the PI, but not anymore for new versions. I am attempting to obtain Harmonic constants for sites in the PI.

Harmonic constants are generated from a tidal dataset for a year, from hourly tide data for any site. It should in theory be a relatively straightforward matter to generate constants from tide data if a tide logger or tide gauge is present at a site. Once a set of harmonics is available for a site, if a single month's data is available for a nearby site, it is often possible to generate offsets, or corrections of minutes and heights, for nearby sites: the primary site with existing constants is used as a reference point, with differences added or subtracted for secondary sites.

In short, I have not found tide predictions easily available for the PI.

Here are some links:

Cebu tide predictor (using an older version of Xtide!)
Bohol tide predictor (using another method).

Manila tides are also found on line.

Mobile Geographics : search or browse

Cookie crumbs for Philippines constants:

UK Hydrographic Office (look under Singapore, Japan)
Easy Tide (6 day free use?)

Here's a question: Where is the office of the Hydrographic authority for the Philippines, and are they willing to allow the use of existing constants.

Cebu is the reference station for the region. Offsets for Dumaguete are available. Unverified offsets are 0.8 or 0.9 FEET, and +30 min.
I have also learned that the Coast and Geodetic Survey (C&GS) of NAMRIA is the authority for hydrographic constants. I hope to visit NAMRIA in Manila.

If you wonder about strategic advantages, read about the Battle of Tarawa.

Here is the tidal prediction for bohol for today:

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