Posted by Brendon Baillod on September 01, 1999 at 23:56:42:
In Reply to: LORAN to GPS Coordinates.... posted by Carl R. Falk on August 28, 1999 at 08:47:11:
: Hi..Does anyone know how to convert LORAN coordinates to GPS? I'd like to mark Lake Michigan wreck sites into my GPS, but the wreck books only specify LORAN. Thanx.....
Carl,
It is possible to convert LORAN to GPS and visa versa by either hand calculating based on a mathematical algorythm or by using most modern GPS or LORAN machines. Nearly all support the conversion in their features. The conversion is somewhat difficult to do manually but is quite easy for a programmed machine. However, the converted numbers will be almost useless for finding wrecks. This is a lesson that many Great Lakes divers have learned the hard way. I published a comprehensive article about this a few years ago and I placed in online at:
http://www.execpc.com/~bbaillod/navi.html.
The main reason that the numbers are not useful once converted is that true LORAN TD grids are not standard at all points and are highly distorted. Because LORAN is produced by land based radio transmitters, there is a nonstandard error, known as a propagation error which is introduced by the radio waves being slowed or distorted as they encounter land masses. In the ocean this is not significant, but in the Great Lakes, it is VERY significant. The LORAN C overlay grids that are printed on some NOAA charts are theoretical lines based on mathematical averaging. These theoretical LORAN grids are generated by the same algorythm that most machines use to convert LORAN to GPS and will correspond exactly to any converted numbers you might be able to produce using calculations or machine conversions. However, they will not take into account the propagation error. The propagation error cannot be compensated for because it is based on land mass encumberance between any two given points.
Because the propagation error is relatively small at most points in the Great Lakes (under 1 mile), theoretical LORAN C overlay lines are of some value and are printed on some NOAA Charts, and converted numbers are subseqeuntly accurate enough for rough navigational purposes. However, they are not at all accurate enough for finding wrecks and are not permitted to be used for commercial navigation. If you try to find a wreck with converted numbers, I guarantee that you will be at least 100 yards off and probably closer to 1/2 mile off, depending on where you are looking.
The only way to get GPS for a wreck that you have only LORAN for is to find the site using a LORAN unit, then take readings with a GPS. The alternative is to go to the rough area using the converted numbers and try searching the bottom with a sonar. This can take a very long time and is seldom worth the effort.
I hope the above is helpful. If you would like further info. on this, please consult my online article.