Let me tell you how I learned about datums.
But first, to all the sharp-eyed Latin scholars out there, yes, I did say "datums" and not "data." For some unknown reason when talking about cartography, the plural of datum is datums.
I was keeping the navigation plot on the morning watch as my ship steamed from one set of islands to another in the Caribbean. Our route plan called for us to arrive off the sea buoy just about daybreak. There was a thick morning mist across the water. As we made our approach using radar, I shifted our plot onto a larger-scale chart. Suddenly our dead-reckoning position appeared to jump a quarter mile on the new navigation chart. I discovered that the entrance buoy was on our starboard bow instead of our port bow. We reversed engines and narrowly escaped running aground.
That's when a red-faced captain explained datums to me in extremely colorful language that could only be described as "salty" and I learned an important lesson.
So what happened? The two navigational charts used different datums and the error between the two added up to more than 500 yards!
Let me explain. The datum defines the frame of reference used to create a navigation chart's coordinate grid.
The latitude and longitude coordinates of charted objects are based on hydrographic surveys. A survey starts with a control point — sometimes referred to by surveyors as the "point of beginning" or POB. Once the control point is established, the surveyors can plot the coordinates of other charted objects relative to the POB. (When I first went to sea, we did this using horizontal sextant angles and a three-arm protractor. Now it is done with highly accurate differential GPS.)
The mathematical models that the cartographer uses to depict the irregular ellipsoidal 3-D shape of the earth's surface on a 2-D surface also determine the local datum of a navigational chart. There are hundreds of local and national datums. They have names such as the North American Datum of 1927 (NAD27), the Australian Geodetic Datum of 1966 (AGD66) and so on.
When sailing in local waters, the navigational charts of the region normally use the same datum, making them consistent with one another. But as I discovered the hard way, errors can occur when shifting charts. That is why when navigators switch from one navigational chart to another, they do not simply re-plot the latitude and longitude coordinates; they re-plot the last fix using the actual compass bearings or radar ranges.
The advent of marine GPS made it possible for the first time to establish a worldwide frame of reference for reconciling local navigational chart datums. GPS positions are computed in the World Geodetic System of 1984 (WGS84) datum. When plotting GPS coordinates on paper navigtional charts, it is imperative to verify the navigational chart's datum. If it is not WGS84, you may need to apply conversion factors. Most modern GPS receivers are able to perform the conversion automatically through a datum-selection function.
Jeppesen Marine bases its electronic charts on a vector database calibrated to WGS84, but some raster charts and vector charts that were created by digitizing paper navigational charts may still be based on the local datum. So it is a good idea to check the datum on your navigational charts, just to be sure.
To learn more about datums, visit the datum home page of the U.S. National Geospatial Intelligence Agency at www.ga.gov.au/geodesy/datums/aboutdatums.jsp
But first, to all the sharp-eyed Latin scholars out there, yes, I did say "datums" and not "data." For some unknown reason when talking about cartography, the plural of datum is datums.
I was keeping the navigation plot on the morning watch as my ship steamed from one set of islands to another in the Caribbean. Our route plan called for us to arrive off the sea buoy just about daybreak. There was a thick morning mist across the water. As we made our approach using radar, I shifted our plot onto a larger-scale chart. Suddenly our dead-reckoning position appeared to jump a quarter mile on the new navigation chart. I discovered that the entrance buoy was on our starboard bow instead of our port bow. We reversed engines and narrowly escaped running aground.
That's when a red-faced captain explained datums to me in extremely colorful language that could only be described as "salty" and I learned an important lesson.
So what happened? The two navigational charts used different datums and the error between the two added up to more than 500 yards!
Let me explain. The datum defines the frame of reference used to create a navigation chart's coordinate grid.
The latitude and longitude coordinates of charted objects are based on hydrographic surveys. A survey starts with a control point — sometimes referred to by surveyors as the "point of beginning" or POB. Once the control point is established, the surveyors can plot the coordinates of other charted objects relative to the POB. (When I first went to sea, we did this using horizontal sextant angles and a three-arm protractor. Now it is done with highly accurate differential GPS.)
The mathematical models that the cartographer uses to depict the irregular ellipsoidal 3-D shape of the earth's surface on a 2-D surface also determine the local datum of a navigational chart. There are hundreds of local and national datums. They have names such as the North American Datum of 1927 (NAD27), the Australian Geodetic Datum of 1966 (AGD66) and so on.
When sailing in local waters, the navigational charts of the region normally use the same datum, making them consistent with one another. But as I discovered the hard way, errors can occur when shifting charts. That is why when navigators switch from one navigational chart to another, they do not simply re-plot the latitude and longitude coordinates; they re-plot the last fix using the actual compass bearings or radar ranges.
The advent of marine GPS made it possible for the first time to establish a worldwide frame of reference for reconciling local navigational chart datums. GPS positions are computed in the World Geodetic System of 1984 (WGS84) datum. When plotting GPS coordinates on paper navigtional charts, it is imperative to verify the navigational chart's datum. If it is not WGS84, you may need to apply conversion factors. Most modern GPS receivers are able to perform the conversion automatically through a datum-selection function.
Jeppesen Marine bases its electronic charts on a vector database calibrated to WGS84, but some raster charts and vector charts that were created by digitizing paper navigational charts may still be based on the local datum. So it is a good idea to check the datum on your navigational charts, just to be sure.
To learn more about datums, visit the datum home page of the U.S. National Geospatial Intelligence Agency at www.ga.gov.au/geodesy/datums/aboutdatums.jsp
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