THE POSITIONS ARE STATED IN A NAD27 CONUS FORMAT
~ Captain Paul's response to: ~ Mike Baquet ~
I am sure that the CAPTAIN PAUL’S FISHING EDGE program for Calcasieu Lake will give you many locations to try. Thank you for purchasing the program.
<i>USE NAD 27 CONUS (Continental U.S.) </i>
The easy way out is for you to just set your GPS unit to that setting and enter the reefs in your unit, then change back to your normal default setting. All of the EDGE waypoints, however, are determined using WGS 84 Datum and with you unit set to that DATUM when you do the download, will represent as accurate position as possible.
You can actually stop reading now, but doing so you would not see the rest of my explanation. So, I suggest that you at least read the answer so as to have a better understanding of how your changing the DATUM settings affect the accuracy and the actual position viewed in you GPS unit.
~ ~ ~ ~ ~ ~ ~ ~
The DATUM settings in your GPS unit offer the user the ability to use the receiver in almost any part of the world. That is why there are so many different DATUM choices. I believe that your unit offers over 200 different choices, so in order to fully understand the consequences of entering positions in your unit or giving or publishing incorrect positions you should understand the meaning of DATUM.
NAD 27 is short for North American Datum of 1927. Up until the advent of radar mapping of the earth’s surface by satellite, this was the U.S. map makers best formula to calculate the surface of the earth for North America. It was, as was all other maps using other datum calculations, based, on a misconception that the earth was a near prefect sphere. It is NOT. With satellite images, we have discovered that there is a bulge in the surface of the earth with the more pronounced effect being nearest the equator.
The new procedures and calculations were called NAD 83. The United States has given their newest technology to the rest of the world, but most did not want to use the U.S. designation of NAD, so it was recalculated world wide and named the WORLD GEODETIC SURVEY of 1984, for short WGS 84.
It seems like other countries are willing to use the WGS designation more readily than the North American designation.
There is technically a small difference between NAD 83 and WGS 84, but in our neck of the woods, it is only an inch or two.
This technology was offered by a treaty to any country, with the only condition being that any NEW maps that are made, are based on the WGS 84 datum and that it is so indicated in the legend of the map.
The recalculated WGS 84 datum can apply to a location almost anywhere on the face of the earth. It is the most accurate way to express a position as it is based on the bulge and other earth characteristics.
In making maps, you will find that the map maker will state in the legend what DATUM was used to draw the various features. Newer maps, usually drawn after 1985 normally indicate that they are drawn using WGS 84 datum. But older U.S. maps, and there are hundreds of thousands of them out there, will indicate that they were drawn using NAD 27 DATUM or maybe even one of the other choices available in your GPS unit.
Your home survey plot was probably drawn or at least referred to a Benchmark position that was determined using NAD 27 or earlier DATUM.
All of the U.S. Department of the Interior, Offshore Leased Blocks and the well heads and platforms that are located in these blocks are based on the NAD 27 Datum calculations. Many, many, many legal contracts, leases and purchases have been recorded using the NAD 27 Datum.
It is nearly impossible to re-plot all of these millions of documents expressing the newer Datum. So unfortunately we are stuck with having positions plotted by using several different DATUMS.
Positions that were plotted, by sextants and other survey instruments were only as accurate as the instruments that were used in the plotting. Sextants, even the very best in the hands of a highly-skilled and experienced navigator can only determine position to an accuracy of about 0.25-nautical-mile (440 yds). That degree of accuracy is really beyond the average navigator.
It wasn’t a concern until the advent of the GPS constellation and the highly refined consumer units that can now determine a position of accuracy that is wihin 3 meters, that it came to be a concern.
So to get the MOST out of your GPS unit, enter your positions in the DATUM that was determined to plot the position. If it is the same as your default position, just enter the values in their correct position format. If these positions were determined using a different DATUM, you should convert the stated DATUM to your default DATUM and then enter it into your GPS.
A free, yes FREE government download, think of it as your taxes at work for you, is available from the U.S. ARMY, Corp of Engineers. The conversion program can be found at
OR you can simply change the DATUM setting in your unit to the datum of the published waypoints, enter the positions then change back to your default DATUM. The GPS receiver will do the conversion for you.
Does different DATUM make a difference?
Yes it does! Remember the bulge?
Well it is more pronounced at the equator, and the latitudes closest to the equator, so the calculations at that part of the surface of the earth will be more distorted than in other locations.
The difference between NAD 27 CONUS and WGS 84 in Louisiana Latitudes that are between 28 to 33 degrees North are separated by about 168 feet in a North West to South East direction. But others may be a great deal apart. For instance, the difference between WGS 84 and the British Admiralty of 1935 Datum is about 1500 feet and the difference between WGS 84 and the TOYKO datum is about 4 miles. Remember the Admiralty Datum if you are navigating to the Bahamas, as that is what is used on their older charts.
How important is DATUM to you as a sportsman ?
For an oil platform in the Gulf of Mexico that is seven stories high and 600 feet by 600 feet, it is not all that important. You will see the structure from miles away, but try finding an underwater reef or a honey hole in the lake or your tree stand, or the end of a dead end road in the woods or even the end of the Calcasieu Jetties at night or in bad weather and you will quickly see how important it really is to you.
These various DATUM calculations for marine use usually stated in a LATITUDE and LONGITUDE valued GRID, but they also apply for any map, including those that use the UTM or MGRS grids such as Topographical or Military Grid.
The NOAA charts in this area will have a correction factor stated in the legend, such as, “ If using NAD 27 Datum the position will have to be corrected an average of 0.759” northward and 0.238” westward to agree with this chart.”
Topographical maps usually have a small “tick” in the corner indicating the shift in position when using a different datum for determining a plot.
You will also have to take into consideration how the Grid positions are stated.
For instance The Gauging Station at the junction of Calcasieu Pass and the Calcasieu Ship Channel for instance can be stated
as 29.7780973 ° N ~ 93.3420750° W (DDD.ddddddd )
OR 29° 46’ 41.1” N ~ 93° 20’ 31.47” W (DDD,MM,SS.ss)
Or as 29° 46.686’ N ~ 93° 20.524’ W (DDD,MM.mmm)
IN WGS 84 DATUM.
OR when using NAD 27 DATUM as 29.7778751° N ~ 93.3419213° W (DDD.ddddddd)
OR as 29° 46’ 40.35: N ~ 93° 20.30.91“ W ( DDD, MM, SS.ss)
OR 29° 46.673’ N ~ 93° 20.515’ W (DDD,MM.mmm).
The position is exactly the same in each of these coordinates, They all are for the Gauging station, some may have been rounded off, but each is in a different format or datum.
If you were to try and enter a DDD,MM, SS.s in your unit when the unit was set to display a DDD,MM.mmm position format, you would have to convert one to another before trying to enter the values. Of course the same would apply if it was in decimal degrees or if the reverse was needed.
I mention this because the Louisiana Dept. of Wildlife and Fisheries Inshore Reefs are stated ad DDD,MM,SS and were plotted using NAD 27 Datum.
~ Captain Paul ~ </b>