The Boating Forum - How accurate are your Loran td to Lat/Lon conversions?

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reel trick
08-11-2004, 11:59 AM
I have aquired several pages of bottom #'s from a commercial fisherman, the bad thing is they are all loran #'s. I have down loaded the POSAID2 conversion program from the USCG site and have been trying to convert the #'s. As you all know, in bottom fishing accuracy is very important so I took the numbers from a wreck that was on the SCDNR site with loran and gps numbers, input the loran numbers into the posaid program and checked to see how close the lat/lon position it gave was to the lat/lon position given by the SCDNR site for the same wreck. There was a difference of over 700 meters!!! Maybe the flaw was using the SCDNR #'s. Has anyone used this program to convert loran numbers they knew were right and checked to see how accurate they were??? I have several hundred numbers to convert, but do not want to waste my time if the conversions are going to be 700 meters off.


mscontender
08-11-2004, 12:26 PM
I've tried many and not a one has worked well. They will just drive you crazy.

will_wahoo
08-11-2004, 12:33 PM
My understanding is that the conversion from Loran to Lat/Long is very inaccurate and often reaches one quarter mile inaccuracy... just in the conversion... then you have to remember that the GPS is not 100% accurate either. Loran's repeatable accuracy with the same unit and same antennae is "decent" you should try to buy the old unit from the person who gave you the numbers.


Bob VH
08-11-2004, 12:38 PM
What you need to do is get his Loran as well as the numbers. Run to the location and hit save on the GPS.

I am in the same situation. I acquired more Loran positions than I will ever be able to get to. BUT, I do have a working Furuno LC 90 MkII Loran. I have been cherry picking them as I work known areas.

Good luck with it..........

Bob

In Too Deep
08-11-2004, 12:43 PM
My suggestion is to buy a cheap Loran unit, use it to go to the numbers, verify something is there with your FF, mark that spot on your GPS. Loran requires corrections that vary based on where you are and most of these formulas don't take that into account.
Good luck,
Chris

AGULHAS
08-11-2004, 12:49 PM
i just bought the andren conversion software and have been plotting the gps and loran. when i put the same known location on the chart in both formats it is putting them right on top of each other. it seems to work well. you can also plot using both numbers if you have for a location. for $80 i figured i would give it a try. i will let you know how accurate it turns out to be.

Recovery Room
08-11-2004, 01:07 PM
Though I've never used it, my Garmin 2010c has the ability to function using Loran TD format.

scubadown
08-11-2004, 01:51 PM
Adren Software makes a program which converts from TD to LL using calibration data from your area. It has worked pretty well for me but nothing is perfect. With this program you save a couple of point in both the TD and LL formats and the program calculates a converstion based on this data and applies it to all other TD points.

http://www.andren.com/

This program is also great for saving your points, graphing them and gives me the ability to download points directly to/from my GPS .

treedawg
08-11-2004, 01:52 PM
I think my Lowrance will also convert the numbers although I've never tried it either.

TD

200%Dave
08-11-2004, 02:02 PM
http://www.rdc.uscg.gov/rdcpages/soft-posaid2.html

I too have pages of LORAN numbers from commercial fishermen. I have used POSAID2 to convert lat/lon to GPS and compared the results to HOT SPOT maps. HOT SPOT used the conversion program from Andren Software Co. www.andren.com. Consistency between the two is lacking. I can't point to either one and claim that they are on the money. My Garmin hand held has conversion capability. Your note has peaked my curiosity. I'll play with it a bit and see what shakes out.

The Dude
08-11-2004, 03:47 PM
For whatever it's worth, the conversion features on GPS units are pretty much worthless. Tried it out w/ some old loran numbers a few weeks back and the conversion on my Garmin was about a half mile off. Really sucks b/c all of my old bottom numbers are in TDs.

mscontender
08-11-2004, 03:56 PM
I've used the Andren Software and enjoyed some of its charting abilities, but it was worthless for converting bottom spots. I also had a bear of a time finding and buying a loran, cheap or otherwise. I bought a cheapo model that lasted less than three months.

MScontender

Thom
08-11-2004, 04:03 PM
It seems to me this is sort of a non-question anyway. What seems to make sense really doesn't have anything to do with a conversion program or where the inaccuracy comes from (the problem is not the conversion formulas or the GPS, its all the LORAN's fault). The real question is how can you use the numbers you have and have to do the least work to get them into a machine where you can use them.

It seems to me your choice is clear. Take your GPS and put it into TD format and then just enter the numbers you have. When you flip it over to LL it will do the conversion. The GPS doesn't actually do a conversion from TD's to LL but that is the effect you see. It won't be any more or less accurate than any other method you use, they will only get you so close, but you weren't going to get any closer than that no matter which type of conversion software you used anyway, and doing it this way you only have to do the data entry one time. So the converted numbers get you close and then you use your depthfinder, or whatever you use, to find the exact special spot and resave it with the push of a button or two. That seems far better to me than entering it once into a conversion program and then seeing the results it give and then re-entering it into the GPS, only then to still have to wonder around a bit to find the target and then resave it. See what I mean?

Oh, on that conversion error, its different depending on where you are. One pretty easy way to tell if your conversions are going to be half way accurate or not is to look at a chart of the area where the TD's are located. See if the light purple lines (TD's) intersect each other at nearly 90 degrees. In some parts of the coast they will make neat little square boxes, and those places the conversions tend to be pretty accurate. On the other hand if they look more like little diamonds, with two large and two small angles, well, the conversion isn't going to be very good..

Thom

firstafence
08-11-2004, 10:13 PM
I have the answer. I use a loran to view my present position when I am near the coordinates. My furuno 1850 gps will display converted td's, I can then open a menu that allows me to "edit" the td's displayed on the furuno gps by adding to or subtracting from the displayed td's. Then I can use the gps to navigate to the Loran coordinates. If I move more than one or two miles I must "edit" the displayed td's again to get a proper fix. Of course the key here is I have the Loran receiver to view true td's. The amount of variance from true td's to gps converted LL to td's is extreme. Sometimes 200 foot error, sometimes .4 mile error. Look at a chart with Loran overlay for the N Gulf(where I fish) and then at S East Florida. The distance between the lines is very different. That means one microsecond of distance is not the same in one area as it is in another. That is why I believe no one conversion is accurate to convert a parabolic coordinate system to a linear system. Bottom line is if you really need some N Gulf numbers accurately converted just email them to me and I will be happy to send you the DGPS corresponding LL. Mark :grin: :grin: :grin:

AGULHAS
08-11-2004, 10:47 PM
i think the accuracy of the conversions might be dependant on where you are. i think they may be more accurate in the gulf.

i am using the adren to chart the numbers (i have about 3000 and they are scattered over a lot of miles) and then i will run them over with the loran and mark on the gps when i pick it up on the bottom.

Loranmk
08-12-2004, 06:56 AM
The Coast Guard has a program that is available to the public for converting TD's to Lat/Long haven't used yet, if it is from the CG it should work pretty well link to the program http://www.rdc.uscg.gov/rdcpages/software.html

chris

reel trick
08-12-2004, 08:18 AM
Well, thanks for the response even though it is sad to see that the conversions are pretty much useless. I will have to decide whether to try to install a loran for a while, or sift through the hundreds of numbers and isolate the "clusters" of numbers in a small area, convert them and then use the ff to locate some of the structure.

:( there are some single numbers in deep water for snowies, tiles, and yellowedge that I really wanted to visit one day. :(

troll
08-12-2004, 08:28 PM
The best conversion I have seen so far is the Northstar 951XD. Several AR bouys on my last boat came up right on the money when entered as TDs. I believe the unit uses internal ASF tables that take into account the area you are in when doing the math.

My new boat has a Navnet which allows you to apply an offset for your area. I used numbers that are suppose to be good for SE NC but I have not yet confirmed how well the TDs actually convert.

Kerry
08-12-2004, 11:45 PM
Right place this time.

Same question, same comments as this one

http://thehulltruth.net/forums/thread-view.asp?tid=28339&posts=10#M311743

Cheers, Kerry.

DreamWeaver21
08-13-2004, 09:18 AM
reel trick - 8/12/2004 8:18 AM

Well, thanks for the response even though it is sad to see that the conversions are pretty much useless. I will have to decide whether to try to install a loran for a while, or sift through the hundreds of numbers and isolate the "clusters" of numbers in a small area, convert them and then use the ff to locate some of the structure.

:( there are some single numbers in deep water for snowies, tiles, and yellowedge that I really wanted to visit one day. :(

I have much success using the built in conversion on my furuno GP 36. In the upper gulf my loran numbers were always within .05 miles of my GPS conversion. When I got to the conversion point, I would just go to wide cone on my FF and start a search pattern spiraling out from the conversion point. Never took me more than 15-20 minutes to find a number. Mine seems to always be off in the same direction. For what is is worth I have learned that I can input a loran coordinate into my gps, go to the converted number and head NE (40-60 degrees) to find my spot.

JoeBagadonuts
08-13-2004, 06:58 PM
The key to LORAN conversions is the ASF correction (see http://chartmaker.ncd.noaa.gov/mcd/loranc.htm). What I have done in the past with a list of LORAN coordinates is to start with a couple that match spots I have on my GPS (usually common public #'s). I enter the LORAN coordinates in my GPS (Garmin 182) and see how far they are from the known true (GPS) position. Then I start adjusting the ASFs until the LORAN and GPS targets overlie each other. Once I figure out a set of ASFs that gives me a good match, I use that to convert all of the other LORAN coordinates.

Even with this method I have a lot of LORAN marks that don't turn up anything, but these give me something to start with. I've found lots of spots while cruising to/from a LORAN mark that I would never have turned up if I didn't have a reason to go that way in the first place.

lawtyger
10-27-2004, 03:53 PM
For what is is worth I have learned that I can input a loran coordinate into my gps, go to the converted number and head NE (40-60 degrees) to find my spot..

I, like many here came across a list of comecial fishing spots. And I, like everyone here has read the threads on the inaccuracy in conversion. However, I'm hoping to take the approach you mentioned above. The commercial fishing pages I got had some numbers that corresponded with known spots with Lat. and Long. I let the machine convert the T.D.'s for these several known spots and it always placed the converted T.D. to Lat. / Long. number in approximately the same location southwest of the actual known number. I don't know if it will work but I'm hoping to take this in account when starting my search pattern.

DreamWeaver21
10-27-2004, 07:02 PM
lawtyger - 10/27/2004 3:53 PM

For what is is worth I have learned that I can input a loran coordinate into my gps, go to the converted number and head NE (40-60 degrees) to find my spot..

I, like many here came across a list of comecial fishing spots. And I, like everyone here has read the threads on the inaccuracy in conversion. However, I'm hoping to take the approach you mentioned above. The commercial fishing pages I got had some numbers that corresponded with known spots with Lat. and Long. I let the machine convert the T.D.'s for these several known spots and it always placed the converted T.D. to Lat. / Long. number in approximately the same location southwest of the actual known number. I don't know if it will work but I'm hoping to take this in account when starting my search pattern.

Wow, drug up and old thread here. BTW I also got some of my numbers from a commercial fisherman. A quality bottom machine helps too. Good luck!

grouperdawg
10-27-2004, 11:59 PM
The best thing to do is go buy an old loran on ebay and install it on your boat. No mathmatical formula can get you acceptable accuracy because of the secondary factors which influence the loran signal.

There are many factors that influence sending a signal from a tower over land/ocean, the good news is they are constant. Loran is very accurate and repeatable despite these variations.

The best software will input your area and use formulas to adjust for some of these local variations. However, accuracy is still going to be pretty bad unless it is a really big area you are trying to find. If it is a small wreck, you may get lucky but more than likely you would spend all day in a grid pattern trying to find it.

Some of the newer GPS's also will use your loran chain input data to adjust for some of the variation but again, still not very accurate.

You are much better off purchasing and installing a loran. You can still buy the furuno new but it will cost you $$$. If you buy a loran, make sure it includes the antenna coupler. There is one company left that makes a generic coupler which may or may not work on a particular unit. Many local marine electronics shops can test the loran for you, but most units can not be worked on. Other than the furuno mk series, I think you can still get parts for some of the si-tex models but I can't remember off hand.

lawtyger
10-28-2004, 07:30 AM
Thanks for the advice. As you can tell from digging up this old thread, I've been reading a lot of previous posts which essentially boil down to what you said 95% of the time. The only accurate way is to get al Loran.

In my situation, I think I'm going to just convert the numbers using my Lowrance 335cdf knowing full well that the vast majority of the time, the conversion will not be close at all. I've got enough other spots of my own that should I be fishing at one of my spots and then notice that one of the converted numbers seemed to be close, I may check it out. Plus, the amount of time / effort it would take getting, installing, and running out to all these spots is simply not worth it in my case.

Oh well, it seemed a little to easy to just end up receiving pages of spots and that all I had to do was enter them into my GPS.



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