As was explained in the introduction, tide predictions for a given location cannot be conjured out of the void—you need to get some special data for each and every location for which you want to predict tides. XTide reads these data from harmonics files. Information on obtaining harmonics files is at http://www.flaterco.com/xtide/files.html#harmonicsfiles.
Currently there are two sources of harmonics files. David Flater maintains a conservative set of data, emphasizing responsiveness to updates and traceability to authoritative sources instead of maximum coverage of locations and historical predictions. These data and the list of locations they support can be found at http://www.flaterco.com/xtide/files.html#harmonicsfiles. Bob Kenney maintains a more liberal set of data, retaining some data that are old and expired in order to support users who continue to get good results with the old data. These data and the much longer list of locations they support can be found at http://harmonics.unh.edu/xtide/files.html. Because they are out of date, use of these legacy data is not recommended.
Check to make sure that your location does not appear anywhere in these harmonics files by any alias. If your location is not already on the list, you need to obtain either a set of harmonic constants or a set of corrections and send this information to David Flater and/or Bob Kenney, as appropriate. You could import these data yourself using the tools available at http://www.flaterco.com/xtide/files.html, but please forward the data anyway so that others may benefit.
Harmonic constants of the first kind, the kind worth having, are created by analysis of regular water level readings taken by automated tide stations like the one pictured here. Harmonic constants of the second kind, the kind not worth having, are created by mangling the first kind to approximate the results of applying corrections. We are only interested in the first kind. XTide can do corrections properly, so mangled data just junk up the database.
harmonics-dwf contains all U.S. stations available from the NOS web site. If it has fallen out of date, email dave@flaterco.com to check whether an update is in progress.
In countries other than the U.S., you might have a really hard time getting the resident tide-predicting authority to release harmonic constants. They may be paternalistic, not wanting to trust non-government people with something as dangerous as tide prediction. They may be fearful that the data will be used to time an invasion by sea. Or they may simply want to retain a monopoly on tide predictions, for whatever rea$on. Regardless, if you do manage to obtain harmonic constants, please obtain and forward a statement from the authority either granting permission for non-commercial use or explaining that such permission is not required.
There is a collection of old harmonic constants for ports world wide that had to be withdrawn because of the permissions issue (for the full sob story, see the FAQ). If you have contact with your local marine authorities and could obtain and forward a statement that use of these tide data is not restricted in your region, Mr. Kenney could reinstate the old data. However, it would be better if they just sent us the newest data.
If you have access to at least a year's worth of regular water level readings for some locale, you can derive the harmonic constants yourself using the Harmgen program available from http://www.flaterco.com/xtide/files.html#experts, or send the time series to dave@flaterco.com (again, with all necessary permissions attached) and harmonic constants will be derived for you, time permitting.
A subordinate station is a tide station whose predictions are obtained by applying corrections to a reference station, i.e., one for which we have good harmonic constants. The words 'corrections,' 'differences,' and 'offsets' are used interchangeably.
harmonics-dwf contains all U.S. stations available from the NOS web site. If it has fallen out of date, email dave@flaterco.com to check whether an update is in progress.
While harmonic constants can be hard to get, you should be able to get offsets with relative ease from a local boating magazine, chartbook, yacht club, or marine authority. If you find suitable offsets, you can add them to harmonics.tcd using the tideEditor program available from http://www.flaterco.com/xtide/files.html#extras. There are two other ways to do it (via build_tide_db in tcd-utils or via Harmbase 2), but tideEditor is most expedient for the non-expert.
First make a backup copy of whatever you are about to modify.
TideEditor version 1.4 takes the name of the file to modify as the command-line argument.
bash-3.1$ tideEditor whatever.tcd
When you start tideEditor, you get a map of the world. Point at the location where you want to add a subordinate station and right click.
You will get a prompt asking "Will the new station be a reference station or a subordinate station?" Choose Subordinate.
You will get a prompt saying "Please select the new reference station." Use the pull-down list to select the reference station and click OK.
You will then get a window with the tabs General, Verbiage and Offsets, initially showing General. On the General tab, the Reference Station, Latitude and Longitude fields will be pre-filled based on your previous actions. The other fields that you MUST fill in are as follows:
All other fields on the General and Verbiage tabs are
optional. Descriptions of the other fields are obtainable using
the question mark tool thingy (
).
The Offsets tab has the following fields.
When finished, click OK. When you quit tideEditor, your new station will be saved in the updated TCD file.
Notations used to describe corrections will vary:
| Notation | Translation |
|---|---|
| −0:20 | Time Add −20 |
| 1 23 | Time Add 123 |
| *1.07 | Level Multiply 1.07 |
| +0.4 | Level Add 0.4 |
| (*0.65+0.3) | Level Multiply 0.65, Level Add 0.3 |
If you were not given separate corrections for max and min, set both the max and min values to whatever you got. For example, if you get
Head Harbor, Isle au Haut -0:20 (Portland)
then you should set both Minimum Tide Add and Maximum Time Add to −20.