Clownfish, Philippines. Photo by Stephane Rochon.

A dive site atlas made by divers for divers
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Geographical Positioning System Help

Here you will find some information about GPS coordinates and related topics (datums, waypoint files, etc.). This helps to understand how to best use GPS coordinates.

Why is it important to set dive sites GPS coordinates ?

Dive site GPS coordinates are unvaluable information for divers.
If dive sites are set with their GPS coordinates (also named waypoint), you and other divers will be able to:
  • easily find dive sites with a GPS, even in the middle of the Pacific ocean !
    When it is not possible to find a dive site with landmarks or seamarks, GPS is the only way to find it. Recent GPS includes the WAAS system with a 3m accuracy!

  • automatically plot dive sites on internet maps such as OpenStreetMap (OSM), Google maps or Terraserver, but also on standalone applications such as Google Earth or the NASA Earth wind.
    Each dive site with GPS coordinates includes several links to external map servers. This is a very convenient way to see where the dive site is located on a satellite map, even if this dive site has not yet been set on zone map.

    See this example of the Seven Pilars dive site (Safaga, Egypt).

    Screenshot of Google Maps website

    Screenshot of Terraserver website

  • download from full zone waypoint files, and import these files into your own GPS ! With this feature, you don't have to manually enter each dive site coordinates into your GPS. This is faster, easyer, and avoid typing errors.

Global Positioning System (GPS)

GPS is a satellite-based radio positioning systems that provide 24 hour three-dimensional position, velocity and time information to suitably equipped users anywhere on or near the surface of the Earth (and sometimes off the earth). GPS is a 24 satellite constellation, 20,000 Km above the earth in six orbital planes.

GPS is usefull to provide and local position of dive site.

You can either use Decimal Degrees (DD - 44.6644327°), Degrees, Minutes (DM - 44° 39.86597') or Degrees, Minutes, Seconds (DMS - 44° 39' 52'') system of coordinates.

What is a Datum?

The latitude and longitude of places on a chart or map depend on what mathematical shape is used to represent the Earth when the chart or map is drawn. Different shapes get used for mapping different areas of the world and these are known as ellipsoids. A datum references a particular ellipsoid known as the reference ellipsoid. Different datums can also have different origins and rotations. Both these factors affect the numerical representation of a position.

Datums affect the positions of Latitude/Longitude and local grid systems (UTM, British National Grid etc). Remember that a datum error can alter a position by 50 to 200 meters but it could be less or more for some datums.

So how does this affect me?

If you have two charts of the same area and they have been drawn on different datums, then the latitude and longitude for the same place is likely be different on each chart. This could be the difference between one side of a reef and the other !

If you plot a GPS latitude and longitude on a chart which uses a different datum, you must apply some corrections otherwise you will plot your position in the wrong place. allows you to set GPS coordinates with 200+ datums. Here is a short list of datums:
  • WGS84: World Geodesic System 1984 (WGS84). This datum is now the only datum used by GPS. GPS coordinates with other datum are automatically transformed into this datum. Common standalone or web applications usually provides WGS84 datums.
  • WGS72: World Geodesic System 1972
  • NAD27, NAD83: North American datums.
  • ED50: European Datum 1950.
  • OGB36: Ordnance survey of Great Britain 1936
  • ADINDAN: Set of African datums
  • ...

Datum error examples

Waypoint formats & GPS import

Datum shift example

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Datum shift example
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