What to do if your location isn't listed

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 this data from harmonics files.  Information on obtaining harmonics files is at http://www.flaterco.com/xtide/files.html.

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.  This data and the set of locations it supports can be found at http://www.flaterco.com/xtide/files.html.  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.  This data and the much larger set of locations it supports can be found at http://harmonics.unh.edu/xtide/files.html.

Check to make sure that your location does not appear anywhere in these harmonics files by any alias.  It is possible that the data set is available, but due to lack of known coordinates it does not show up in the location chooser unless you select List All.

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 this 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

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 above.  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 junks up the database.

As far as I know, all published reference stations for the U.S. are already supported.  If one has been missed, you might be able to obtain it from NOAA for between $10 and $40.  A tolerable facsimile might be available for free from http://co-ops.nos.noaa.gov/data_retrieve.shtml?input_code=100201001har.  If so, just let David Flater and/or Bob Kenney know it's there.

In countries other than the U.S., you might have a really hard time getting the resident tide-predicting authority to release data.  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 be greedy, and want to retain a lucrative monopoly on tide predictions.  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 this 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.

Corrections

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.

You should be able to get offsets with relative ease from a local boating magazine, chartbook, yacht club, or marine authority.  Another source is NOAA's web page at http://co-ops.nos.noaa.gov/tp4days.html.  The file at http://www.flaterco.com/xtide/lots_of_offsets.txt contains similar data, but has already been imported.

If you find suitable offsets, you can add them to harmonics.tcd using the tideEditor program in the tcd-utils package (available from http://www.flaterco.com/xtide/files.html).  Alternately, you can add your offsets to offsets.xml (available from http://harmonics.unh.edu/xtide/files.html) and then regenerate harmonics.tcd using the build_tide_db program in the tcd-utils package.  Please refer to the README file in tcd-utils for more information on using build_tide_db.

General notes and warnings

• There are many different flavors of offsets for subordinate stations.  At this time, XTide supports all commonly appearing flavors except for the Admiralty one that has different height differences depending on the time of month.  The following rare and freakish sorts are not supported:  those that use different offsets depending on whether the flood at the reference station crossed some threshold; those that rely on more than one reference station; those that use different offsets for higher high or low water versus lower high or low water; currents that use a regular tide station as reference, or vice-versa.
• In most cases you won't be given the latitude and longitude with the offsets for a subordinate station.  A 1-meg list of many NOS tide stations with coordinates is available at http://www.flaterco.com/xtide/nos_station_list.txt.  If it's not in there and you can't find it anywhere else, just estimate the coordinates as best you can using an atlas.
• The timezone attribute is only used to choose the time zone in which to render output for the location.  In the majority of cases this will be the same as for the reference station.  However, if your reference station is in a different time zone, you may need to alter the time offsets to REMOVE compensation for the time zone difference.  In XTide, offsets are independent of the time zone.
• If you don't get slack offsets (floodbegins, ebbbegins) for a current station, OMIT those fields!  When slack offsets are omitted, XTide will interpolate a reasonable value.  But if you specify zero, you get zero -- even if that's unreasonable given the skew of max and min.

Adding corrections using tideEditor

First set the environment variable HFILE_PATH to point at the TCD file that you want to modify.  (This would be a good time to make a back-up copy.)

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 and Offsets, initially showing General.  General has the following fields.

• Record Number.  Ignore this.
• Reference Station.  Already filled in to what you selected.
• Station Name.  Enter the name of the new subordinate station.
• Latitude.  Already filled in to the location you clicked, but you can change it.  Unlike XTide, which uses degrees out to four decimals, tideEditor expresses the latitude in degrees, minutes, and seconds.
• Longitude.  Already filled in to the location you clicked, but you can change it.  Unlike XTide, which uses degrees out to four decimals, tideEditor expresses the longitude in degrees, minutes, and seconds.
• Source.  Enter a description of where you got the data.
• Time Zone.  Use the pull-down to set the time zone.
• Country.  Optional.  Use the pull-down to set the country.
• Pedigree.  Pedigree is a numerical measure of the authoritativeness of the data.  "Hearsay" is always a safe choice.
• Restriction.  Everybody except DoD personnel can ignore this.
• Comments.  Optional.

Offsets has the following fields.

• Level Units.  Select feet or meters for tides, knots or knots^2 for currents.  knots^2 occur only rarely, so if you are unsure, choose knots.
• Direction Units.  Optional.  If you use directions, choose degrees true or degrees.
• Minimum Time Add.  The time adjustment for low tide / max ebb.  It is expressed as an integer that is hours times 100 plus minutes, so for -0:20 (negative 0 hours, 20 minutes) you would write -20, and for 1:40 (positive 1 hour, 40 minutes) you would write 140.  If you don't have this, leave it blank.
• Minimum Level Add.  A value, in the units identified by Level Units, that is added to the tide level or current velocity predicted at low tide or max ebb.  If you don't have this, leave it blank.
• Minimum Level Multiply.  A multiplier for the tide level or current velocity predicted at low tide or max ebb.  If you don't have this, leave it blank.
• Minimum Average Level.  Optional.  Not used by XTide.
• Minimum Direction.  Optional.  If desired, specify the direction of max ebb in degrees or degrees true as indicated by Direction Units.
• Maximum Time Add, Level Add, Level Multiply, Average Level, and Direction are analogous, but correspond to high tide / max flood.
• Flood Begins.  Another kind of "Time Add" used only by currents to adjust the time of the slack preceding a flood.  If you don't have this, leave it blank.  If it got initialized to zero, make it blank.
• Ebb Begins.  Analogous to Flood Begins.

When finished, click OK.  When you quit tideEditor, your new station will be saved in the updated harmonics.tcd.

Notations used to describe corrections will vary:

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.

Adding corrections in XML format

Following are examples to illustrate the XML format for offsets.

If you get:

Head Harbor, Isle au Haut    -0:20   (Portland)

Then you enter:

<subordinatestation name="Head Harbor, Isle au Haut, Maine"
  pedigree="From NOAA http://www.opsd.nos.noaa.gov/tp4days.html"
  latitude="44.021666667"
  longitude="-68.62"
  timezone=":America/New_York"
  reference="Portland, Maine"
  note="This is just an example of how to add a note.">
<simpleoffsets> <timeadd value="-0:20"/> </simpleoffsets>
</subordinatestation>

If you get:

          Time meridian, 150` E           on Ponape Harbor

Marcus Island                           -0 19   -0 19    (*0.65+0.3)

Then you enter:

<subordinatestation name="Marcus Island (Minami Tori Shima)"
  pedigree="From Alan Eugene Davis"
  latitude="24.2667"
  longitude="154.0000"
  timezone=":Pacific/Guam"
  reference="Ponape Harbor">
<simpleoffsets>

  <timeadd value="-0:19"/>
  <levelmultiply value="0.65"/>
  <leveladd value="0.3" units="feet"/>
</simpleoffsets>
</subordinatestation>

If you get:

Reagan National Airport   +0 16  -0 02  *1.07  *1.06  Washington DC

Then you enter:

<subordinatestation name="Reagan National Airport, Washington, D.C."
  pedigree="From NOAA http://www.opsd.nos.noaa.gov/tp4days.html"
  latitude="38.8520867"
  longitude="-77.0377119"
  timezone=":America/New_York"
  reference="Washington, D.C.">
<offsets>

  <max>
    <timeadd value="+0:16"/>
    <levelmultiply value="1.07"/>
  </max><min>
    <timeadd value="-0:02"/>
    <levelmultiply value="1.06"/>

  </min>
</offsets>
</subordinatestation>

If you get:

          Time meridian, 180` E           on Kwajalein Atoll

Ailinglapalap Atoll                     +0 08   +0 07    +0.4    +0.3

Then you enter:

<subordinatestation name="Ailinglapalap Atoll, Marshall Islands"
  pedigree="From Alan Eugene Davis"
  latitude="7.5"
  longitude="168"
  timezone=":Pacific/Kwajalein"
  reference="Kwajalein Atoll">
<offsets>
  <max>
    <timeadd value="+0:08"/>

    <leveladd value="0.4" units="feet"/>
  </max><min>
    <timeadd value="+0:07"/>
    <leveladd value="0.3" units="feet"/>
  </min>
</offsets>

</subordinatestation>

If you get:

For Oakland Inner Harbor Reach, depth 33 ft. below datum, 37d 47.67'N 122d
17.15'W the time differences are:

Min.     Flood    Min.     Ebb     Speed       Average Speed & Direction
before            before           ratios      Min  Fld      Min  Ebb
Flood             Ebb        
h  m     h  m     h  m     h  m    Flood Ebb
 
-2 38    -0 48    -1 12    -1 40   0.1   0.1   - -  0.3 082  - -  0.2 255  

Then you enter:

<subordinatestation name="Oakland Inner Harbor Reach, 33 ft. below datum Current"
  pedigree="NOS data via Ed Wallner"
  latitude="37.7945"
  longitude="-122.28583333"
  timezone=":America/Los_Angeles"
  reference="San Francisco Bay Entrance (Golden Gate), California Current">
<offsets>
  <max>
    <timeadd value="-0:48"/>
    <levelmultiply value="0.1"/>
    <avglevel value="0.3" units="knots"/>

    <direction value="082" units="degrees true"/>
  </max><min>
    <timeadd value="-1:40"/>
    <levelmultiply value="0.1"/>
    <avglevel value="0.2" units="knots"/>
    <direction value="255" units="degrees true"/>

  </min>
  <!-- Slack offsets are only of the timeadd variety. -->
  <floodbegins value="-2:38"/>
  <ebbbegins value="-1:12"/>
</offsets>
</subordinatestation>