Loran conversion to GPS

[Halfhitch]

Recently I had some interaction with a fellow Florida dweller because of the recent hurricane. We got along well and he became aware that I fished. He offered that his father- in-law was a life-long fisherman and diver and had passed away a few years ago. The old gentleman told his daughter to guard his "spots" closely. The guy I was dealing with said he would like me to have them because he doesn't fish and didn't want the info lost. So, I have these numbers which amount to about 50 spots, and they are in the area I fish off the West coast of Florida. The snag comes in that they were developed in the '80s and in the LORAN format. The spots are listed as a set of two, seven digit numbers. I am not familiar with handling LORAN numbers and wonder if anyone here has converted any to GPS locations. I have heard there can be an error factor that can be significant at times. My Garmin plotter has the ability to convert LORAN to a GPS location. Has anyone tried the conversion of such numbers? What kind of success did you have?

 

[Hookup1]

You can put your Garmin unit in LORAN mode, enter the numbers, switch the unit back to GPS and use the numbers. This will give you as much accuracy as you can hope for. The "snags" are useful but you will have to work the area carefully with your sonar. Once you find them mark with a new GPS waypoint.

A little background on LORAN FYI. For a given LORAN unit (the originating unit) LORAN is highly REPEATABLE (meaning it will take you back there every time). But LORAN is not ACCURATE (meaning that using those same numbers to on another unit with not get you back accurately to the same spot).

Those two LORAN numbers you see are (actually were since LORAN is no longer operational) are radio towers. Typically there were two or three that covered a large area. The LORAN units triangulated and used the transmission delay (speed of radio waves) to calculate a position. Other factors like the equipment tolerances, environmental conditions and radio wave propagation anomalies in areas affected the accuracy.

If you need help converting PM me. I can do them on Garmin HomePort and send them back to you. I did something like this for someone else in your area. Actually - it was you that I converted a .usr file for!

By the way - NOAA updated their Wrecks and Obstructions database to GPS years ago. They actually sent divers and side scan equipment out to ground truth and update position and added notes.

 

[Halfhitch]

Yes, I remember you doing that for me. I had old Raymarine format files that you converted to GPX. Thanks for explaining the workings of LORAN. I knew it was radio based and now I understand why sharing info with a different machine will not replicate exactly. I guess I can try a couple of them and see if I can find anything. I was surprised at the 7 digit numbers. Is that the norm?

 

[Hookup1]

Yes - those long numbers are normal. You need to find an old chart for the area and see what LORAN chains were used. You need the correct chains when setting up LORAN instead of GPS on Garmin. You will know that you have it correct when you see where the waypoint shows up on your plotter.

Maybe I can tell from a couple of sample numbers.

 

[wspitler]

LORAN C is a two dimensional hyperbolic radio based time difference system. It is highly repeatable as mentioned but not necessarily related accurately to Latitude and longitude. The numbers you see are time differences. For each area there is a master station and several slave stations. The master station sends a signal which is picked up by the receiver and the slave. the slave then sends out a signal at a known time difference. You’ll need to know the master and the slave designation. The lines on the chart appear straight but are actually hyperbolic. There are typically three or four different lines and finding ones that cross perpendicular provides the most accuracy. If you have a Loran C chart of your area it will be easy to figure out the slave and master designations. If not there may be historic information online.

 

[SkunkBoat]

If you have Garmin sidevu sonar it will come in handy trying find the actual snag. I have tracked down some wrecks from published GPS conversions of old Loran numbers. They can be as much as a mile off (I think that one was a bad conversion by someone) and they are never at the conversion spot. Go to your waypoint and do circles outward until you see a snag or wreck out to the side on the sonar. You might never find it without the sidevu because you would have to pass directly over it with Traditional sonar.


I give them a different symbol from verified spots.
I have found that for many of mine they were actually to the northeast of the waypoint so I tend to start looking in that direction now.
It can take a years but I try to check/verify the "possibles" when I'm near one.

 

[Halfhitch]

Yes - those long numbers are normal. You need to find an old chart for the area and see what LORAN chains were used. You need the correct chains when setting up LORAN instead of GPS on Garmin. You will know that you have it correct when you see where the waypoint shows up on your plotter.

Maybe I can tell from a couple of sample numbers.

The first two spots are....2728250 8252966.........his notes call for 44 foot depth. that factor would help to figure out if we are in the neighborhood.
next spot............2715232 8247905.........60 feet of water

 

[Hookup1]

The first two spots are....2728250 8252966.........his notes call for 44 foot depth. that factor would help to figure out if we are in the neighborhood.
next spot............2715232 8247905.........60 feet of water

Your numbers are not LORAN numbers. They are Lat/Lon.

27 28.250 82 52.966

27 15.232 82 47.905

 

[Halfhitch]

Thats what I thought and the guy insisted they were LORAN. Thats why the long string of numbers. They must be decimal GPS and the guy just left the decimal out, correct?

 

[SkunkBoat]

probably not decimal DDD.DDDDD probably DD MM.MMM as in N 27 deg 28.250' just off Venice Fl

I was going to mention in previous post that a lot of conversions are messed up because people mix formats of Lat/Lon. or they screwed it up in Excel trying to convert....

 

[Hookup1]

Thats what I thought and the guy insisted they were LORAN. Thats why the long string of numbers. They must be decimal GPS and the guy just left the decimal out, correct?

LORAN numbers would be a little shorter maybe something like 27282 80529 but these numbers are definitely GPS most likely DD MM.MMM.

The old timers had "snag books" that they kept to prevent their nets from getting hung up. Are these numbers printed out or are you looking at pages from a handwritten notebook?

Good numbers are important to have when fishing in the Gulf. If theses are the numbers the old timer used and they were in fact GPS numbers they should be reasonably accurate. Depends on the GPS unit he used when fishing. You should be able to ground truth them easialy.

 

[Halfhitch]

Yes, they are hand written notes. The guy even recorded who accompanied him on each trip. He was an avid diver also and the guy that gave these to me said the old guy actually dove on most of them to check them out. His notes reveal some interesting things. He was short on words but still helpful.

Soon as the red tide heals a little, I'm going to check out a couple of them before I spend a lot of time loading them into my machine.

 

[Hookup1]

Just out of curiosity how old are these numbers/notes? GPS selective availability has been turned off since May 2000 which increased accuracy substantially (about 30’ radius). And the fact that he dove most of them this could be a gold mine of spots. But storm will move some of them around.

Format likely DD MM.MMM. Look carefully at the MM before the decimal. Should be in the 00 to 59 range and after the decimal 000 to 999. This precision will get you to 6’ square.

If it were me I would create waypoints for all 50 and put a letter at the start of the name. I like Z because they will be at the bottom of my waypoint list. You can also use a diffent symbol so you can visually see them on your chart when you are near them. Once you check therm out you can edit them to show verified or not found.

 

[Halfhitch]

That's a good idea about using Z to segregate them on my list. I will do that.

All the number do have 00 to 59 in the minutes and 3 digits for the seconds, which he didn't put into the number, but I think maybe just because he knew what they were.

He also made notes about what he saw like "golf balls", "engine block", "swiss cheese", "3 large cracks". One note that is repeated several times that I'm not sure of. He mentions "Winter Blacks". I think he is referring to a species of fish, but I can't think of what species he would be talking about. Have you heard that nickname?

 

[Hookup1]

Maybe black grouper