Help for cTide 1.1

cTide Copyright 2003 & Disclaimers
How Does This Work?
The Display
The Quick List
Choose Location ...
Preferences ...
Normal Graph
Scrolling Graph
Clock Mode
Station Types
Getting New Data
My Location Is Not Listed
Reference Publications

last update: 05.mar.03

Standard Disclaimers

cTide 1.1
A Harmonic Tide Clock & Tide Predictor
Copyright 2003 David Buchholz, Airtaxi Software

Based on WXTide32 Copyright 1998 Michael Hopper, and XTide 1.6.2 Copyright 1997 David Flater

Not for Navigation!

cTide provides tide and current predictions for about 5,900 locations around the world. Data can be displayed in a variety of graphical formats. Predictions can be performed on any date between 1970 and 2038. This program uses the harmonic method to display tide levels either as a tide clock (showing the current level) or as a graph. All of the data and constants are read in from the harmonics file.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. Please read the GNU General Public License for more details.

Please send any comments, suggestions or issues with bad behavior (by the program please) to me a

Check at for the latest information and possibly even revisions of this program, as I do intend to keep it up to date with my own device. The Michael Hopper program this was based on can be found at and the original XTide program of David Flater, good reading on tide predictions, links to other ports of xtide (and possibly even updated harmonics & offsets files) can be found at For more information about this see Getting New Data.

This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This program and the output it produces have NOT been verified by any official authorizing agency so the determination of suitability as to purpose or use is left to the user. This means that you should not rely solely on the tide predictions from this program but should use any and all available sources before making any decisions.

The tide prediction algorithm used in this program was developed with United States Government funding, so no proprietary rights can be attached to it. For more information, refer to the following publications.

How Does This Work?

To quote David Flater, "The algorithm that XTide uses to predict tides is the one used by the National Ocean Service in the U.S. It is significantly more accurate than the simple tide clocks that can be bought in novelty stores. However, it takes more to predict tides accurately than just a spiffy algorithm -- you also need some special data for each and every location for which you want to predict tides. XTide reads this data from harmonics files..." cTide is based on XTide 1.6.2 so it has the same spiffy gee-whiz algorithm. To update this program with newer data files you need to get your hands dirty. How to do this is not a drag and drop operation. If you are prepared to hoist and bale read up on Getting New Data Files.

The Display

The main screen shows the predicted tide or current in a format that can be presented in any of three graphics modes: Normal Graph, Scrolling Graph and Clock Modes. All modes show the water level, whether the water is rising or falling, and additional information such as the present and predicted tide levels (or speeds) and time for high and low tides, sunrise and sunset times, and moon phase changes. The color of the water gives a visual indication of the direction of tide. The water is blue if it is flooding, green if it is ebbing. The sunrise and sunset times are shown by changes in the background colors; daylight hours are a light yellow and nighttime hours are gray.

Popup Spot Info can be obtained by pressing and holding a spot on your display with your pen/stylus. A window will appear at the top the screen that will display specific information about the predicted tide or current at the point where your stylus is held.

Quick List

The Quick List can be turned on and off from the Options Menu. It shows or hides the "Location" pick list at the top of the screen. The Location pick list stores up to your 5 last unique tidal stations. Pick one and you will re-load that location.

Choose Location ...

This Options Menu item displays the Choose Location dialog. This dialog is split into 3 major vertical areas. The top allows you to choose a new location by selecting the location and choosing OK, or by double tapping on the location. The middle section will display information about the selected location. It will show longitude and latitude, the Type of Station and distance from your currently loaded station. The bottom portion of the dialog allows you to choose Station Types, sort order, and states, countries, and geographic regions.

Preferences ...

The cTide preferences are set from the Options Menu. The following tabs are available under preferences:

Display Current Time: lets you display the current time, or not, in the text based information at the top of the display.
Show Current Level: lets you display the current level, or not, in the text based information at the top of the display.
Display Mean Tide Level: this draws yellow line where the mean tide level is.
Mean Lower Low Water: this draws a lighter yellow line where the mean lower low water level is.
Display Moon Phases: this turns on an off the display of major phases of the moon.
Do Not Fill Tide Graphs: this truns off the filling in under the area of the tide graph using the falling or rising colors (green & blue respectively).

Label Tide Heights: enables the display of the numeric values of the tide height on the left side of the screen.
Label Hours: enables the display of the numeric values of time in hours along the bottom of the screen.
UTC Time Format: enables the display of all time values in coordinated universal time format (UTC).
2400 Hour Time Format: enables the display of all time values in 2400 hour tome format.
Omit Timezone: disables display of 3-letter timezone abbreviation in text based information.
Use Short Times and Dates: a more compact format for the text based information.
Custom Mark Level: this allows you to enter a custom mark that will be drawn in red at the height you type in this field.

Display Units: this chooses which units to display for the tide heights. As is will use the units found in the data file for that location. Choosing feet or meters will force conversion to the selected units.
Auto Convert Knot^2 to Knots: some current locations may be based on knots squared as the measure of current speed. This setting will cause conversion to knots in the display.
Stretch by x: will cause the graph to be scaled horizontally by the amount you have entered in this field.

Normal Graph

The Normal Mode presents a graph showing the state of the tide centered about the current time. The point in the center, marked by a '+' sign, shows the tide level and trend and will, at a glance, show whether the tide is rising or falling and when the next low and high tide will be. Like the Clock mode, the display is updated once per minute.

Scrolling Graph

The Graph Mode presents a display similar to the Normal Mode with several important differences. The tide graph can be started at any time in the prediction year range instead of always being centered on the current time. Also, all tide events and mark level crossings are annotated, not just the next high and low tide, and depth lines are drawn across the screen. The final difference is that the graph is not self updating every minute. Once the display is drawn, it will not change.

There are 2 ways to use the scrolling graph. One is to click on and drag the scroll button at the bottom of the screen. A small popup window will then appear to show you what date you will see once you stop holding the button. This tends to be somewhat imprecise due to the narrow width of the screen. For this reason the second way to pick a date is to click on the "Go To" button in the lower right hand corner.

The "Go To" button has 2 states. If you are showing the current (now) tide, clicking on the button will bring up the date picker-oid. You can then use this to wander to the specific date you want to see a prediction for. If you are not showing the "now" time, clicking on this button will reset the display to the "now" prediction for the current location.

Clock Mode

The Clock Mode shows the current water level and the predicted times of the next high and low tides and optionally (via the display current time preference), the current time.

Station Types

  • Reference stations (T,C)
  • This option, when checked, will include stations for which a full harmonic data set is available. These stations will be the most accurate. When unchecked, only subordinate stations will be shown.

  • Subordinate stations (t,c)
  • When checked, the station list will include stations for which time and/or level offsets are applied to a given reference station to compute the tidal values for the selected station. These are also known as offset stations are they are calculated in relation to an existing reference station. When unchecked, only reference stations will be shown.

  • Tide level stations (T,t)
  • When checked, tidal stations will be shown in the station list. When unchecked, only current stations will be shown.

  • Current stations (C,c)
  • When checked, current stations will be shown in the station list. When unchecked, only tidal level stations will be shown. Currently there are no subordinate (offset) loctions based on current locations.

    Getting New Data

    As was mentioned in How Does This Work?, to update this program with newer data files you need to get your hands dirty. How to do this is not a drag and drop operation. If you are prepared to get your hands dirty read on. The original XTide program of David Flater, lots of excellent information on the prediction methods and links to new data files can be found at
    The harmonics files are maintained by Bob Kenney and can be found at his website:
    If you do download new data files, you will need to find the source files of cTide which are located in the file You should be able to find this file on your own PC under: C:\Program Files\Microsoft ActiveSync\cTide. At least this is where it lives by default on my win-pc. Read up on the readme.txt for harm2idx included in there. Then make sure to re-run the harm2idx program to create a more compact harmonics file and a corresponding new index file, otherwise you will not be able to load the new stations from them. You will then need to use ActiveSync to copy the new "harmonic" part files and "harmonic.idx" to your device.

    My Location Is Not Listed

    In a word, bummer. In "How Does This Work?" you can infer how the special data does not fall from the sky. At the time of release, this program includes the most recent data available. A more comprehensive discussion of this can be found at
    Note that cTide does not directly support the XML format for offsets as described at David's site. For this you need to use the tools that come with cTide. See Getting New Data.

    Reference Publications

    Manual of Harmonic Analysis and Prediction of Tides. Special Publication No. 98, Revised (1940) Edition (reprinted 1958 with corrections; reprinted again 1994). United States Government Printing Office, 1994.

    Computer Applications to Tides in the National Ocean Survey. Supplement to Manual of Harmonic Analysis and Prediction of Tides (Special Publication No. 98). National Ocean Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, January 1982.

    These publications can be obtained from the NOAA.

    For a more brief overview of the history of tidal analysis and prediction the NOAA has a nice set of pages at