Chart frequently asked questions (FAQs)
What is a "datum"? What is LAT datum and GDA94 datum?What is the positional difference between WGS84, AGD84 and GDA94?
What is a sounding?
How do I calculate the depth of water using soundings and tide heights?
What is DGPS and GPS?
What datum are boating safety charts on?
What is magnetic variation?
Why don't boating safety charts show all the boat ramps?
Where can I get a laminated boating safety chart?
Are boating safety charts available as electronic versions?
What is a "datum"? What is LAT datum and GDA94 datum?
A datum is a “base level” or “reference value” from which other values can be calculated. A position on a map or chart is referenced to an imaginary plane surface (the horizontal datum) so that all positions taken from the chart are consistent and accurate. The horizontal datum for all boating safety charts is GDA94 (compatible with WGS84). GDA94 stands for Geocentric Datum (of) Australia as established in 1994 and WGS84 stands for World Geodetic System as established in 1984. Elevations (for example depths) on maps and charts are also referred to a vertical datum. The vertical datum for all boating safety charts is LAT (Lowest Astronomical Tide). This is the zero value from which all tides and depths are measured. LAT has been calculated as the lowest possible tide that could occur under astronomical conditions – therefore a negative tide which falls below 0m should be a rare occurrence.
What is the positional difference between WGS84, AGD84 and GDA94?
The difference in position between GDA94 and WGS84 can be ignored for normal GPS use. GDA94 is the “official” horizontal datum for precise surveying in Australia but for general use, WGS84 is compatible with GDA94. However, when plotting positions using a WGS84 (or GDA94) coordinate from a GPS onto a map based on the older datum of AGD84, your position could be out by up to 200 m in a north-east/south-west direction. The size of this shift varies from place to place. On even older maps and charts, there may be a reference to AGD66 datum. This was the “official” survey datum adopted by Australia in 1966 and there is about a 1 to 2 metre difference between AGD66 and AGD84 datum.
What is a sounding?
Soundings on charts show the depth of water available if the tide was at 0.0m (at Lowest Astronomical Tide (LAT) datum). Traditionally, soundings on nautical charts are shown as a larger number with a smaller “subscripted” number. On metric charts, such as the boating safety chart series, the larger value is in whole metres and the subscript number is the tenths of a metre. On older charts, the larger number was in fathoms and the smaller number in feet (6 feet = 1 fathom). Sounding values on charts with a small under-line are known as “dry soundings”. This is where the seabed "dries" or rises above the water level when the tide falls (typically called "sandbanks").
A sounding on a chart shows the depth of water at a 0.0 m tide (at LAT datum), but the tide rarely falls to 0.0 m. The tide values seen in Maritime Safety Queensland’s Official Tide Tables and Boating Safety Guide (and in newspapers, weather reports, and so on) are also calculated to LAT datum. Therefore, to determine how much water is at a certain point on a certain day, add the sounding on the chart to the tide value for that day, time and locality. For example, if the sounding is 2.3 m and the tide is 1.8 m at the Brisbane Bar at the time of high tide, the total water available near that sounding is 4.1 m. The exception to this rule is “dry soundings”. Because dry soundings generally represent sandbanks that "dry" as the tide falls, a dry sounding has to be subtracted from the tide value to get the water depth at that point. For example, if your course took you over a dry sounding of 0.4 m, and the tide was 1.8 m at the time of high tide, the actual water available at that dry sounding at that tide height is 1.4 m.
What is DGPS and GPS?
Around 1984, the USA launched satellites for finding global positions for military purposes. These satellites soon became widely used by the commercial market because of their ability to fix a position anywhere on the earth using equipment called Global Positioning Systems or GPS. GPS positions are generally the result of receiving positional information direct from a number of satellites without any corrections for local variations and are accurate to about 10 m to 20 m. DGPS (Differential GPS) are accurate to about 1 m to 10 m (or better for surveying purposes) because the satellite signal is compared with a known position on the earth and corrections are applied to improve your position. Check with your retailer about availability of DGPS services.
What datum are boating safety charts on?
All current boating safety charts are published on the horizontal datum of GDA94 which is compatible with WGS84 datum. Your GPS or DGPS receiver should be set to WGS84 (or GDA94 if available) and you will be able to plot DGPS or GPS positions direct onto the boating safety chart. Boating safety charts earlier than 2000 are on AGD84 datum and you must set your GPS or DGPS receiver to this datum to plot directly onto these older charts or you must apply a correction. The vertical datum (for soundings) of all boating safety charts is Lowest Astronomical Tide (or LAT). Older charts are on Low Water Datum (LWD). The horizontal and vertical datum of all current charts can be found in the title area on each chart.
What is magnetic variation?
The compass rose on the boating safety chart shows both the direction of true north and magnetic north. True north always points along the meridian of longitude towards the true north pole. Magnetic north points to the current position of the magnetic north pole. The difference between true and magnetic north is the “Magnetic Variation”. The magnetic north pole revolves very slowly around the true north pole. Therefore, the magnetic variation changes slowly on charts, and can go from positive to negative to positive again over a very long period. See the notes in the title area on boating safety charts for applying the variation to courses set by compass.
Why don't boating safety charts show all the boat ramps?
The Queensland government maintains a number of public state boat ramps whilst other boat ramps are managed by local govenrments. In the past, only state boat ramps have been showns on boaing safety charts but where possible, Maritime Safety Queensland will show both state and council boat ramps on future editions of boating safety charts.
Where can I get a laminated boating safety chart?
Boating safety charts are sold through Queensland Transport customer service centres as unlaminated paper charts. However, many boating safety charts sold through private retailers are available as paper or laminated. Make enquiries at a private map or chart dealer, or a local chandlery in your area, to see if they have or can obtain a laminated chart.
Are boating safety charts available as electronic versions?
Maritime Safety Queensland does not directly sell electronic versions of its paper boating safety charts. However, Maritime Safety Queensland has licensed electronic charting companies to use the copyright and content of the paper boating safety charts to compile electronic chart versions or to use the chart data in electronic charting systems. To date, companies who have signed a licence agreement with Maritime Safety Queensland include C-Map, Garmin, Jeppesen (Nobeltec), Navionics and MapTrax. You will need to contact these companies to determine which electronic navigation software or electronic charts are suitable for your purpose.
Last updated 15 February 2008
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