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 (harmonic constants) for each and every location for which you want to predict tides. XTide reads this data from harmonics files that you must download along with the distribution.
A complete list of locations in the available harmonics files is at http://www.cs.umbc.edu/~davidf/harmlist.html, kept there because it causes me to exceed my disk quota in my regular account. Check to make sure that your location does not appear anywhere in it 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 have two options:
Subordinate stations are not "real" tide stations -- their tides are figured by applying corrections to a reference station, i.e., one for which we have good harmonics data. However, XTide 2 happily lets you enter subordinate stations into the database under their own names and forget the ugly truth.
Subordinate stations are added to offsets.xml (available from http://www.universe.digex.net/~dave/files/). Someday I will have a valid DTD for it and you will be able to edit it in Word Perfect in a user-friendly way. For now, you must follow the examples.
There are many different flavors of offsets for subordinate stations. At this time, XTide supports all known flavors. Below are some examples of what you get versus what you need to enter. I would, of course, like you to send me the data when you are finished so that I can distribute it to others.
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"> <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 33m. 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 Current" pedigree="NOS data via Ed Wallner" latitude="37.8105" longitude="-122.5022" timezone=":America/Los_Angeles" reference="Golden Gate Bridge, California Current"> <offsets> <max> <timeadd value="-0:48"/> <levelmultiply value="0.1"/> </max><min> <timeadd value="-1:40"/> <levelmultiply value="0.1"/> </min> <!-- Slack offsets are only of the timeadd variety. --> <floodbegins value="-2:38"/> <ebbbegins value="-1:12"/> </offsets> </subordinatestation>
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.cs.umbc.edu/~davidf/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, and you can just omit timezone to get this behavior.
Please see the note in the next section about the need to unambiguously specify the name of the reference station.
National Ocean Service tide station at Bar Harbor, Maine, 1997-06-24.
© 1997 David Flater.
If you are ambitious and have access to regular water level readings for your locale over the course of a year or so, you can derive the harmonic constants yourself. Unfortunately, I do not yet distribute a program for deriving harmonic constants. See the news page for more information about this.