Divers Guide to Electronic Navigation

How to find wrecks using GPS, LORAN and Charts

So you've got "numbers" for a wreck? That's a start, but what kind of numbers are they? LORAN TDs? Are they corrected or uncorrected? How old are they? LAT/LON? What format are they in? Which datum? Were they actually measured at the wrecksite, or did someone extrapolate them from a map? What kind of map were they extrapolated from? How old was the map? These considerations have been the cause of many lost diving hours and are rarely talked about, even by knowledgeable Great Lakes divers. In many ways, these navigational difficulties in finding wrecks have served to keep amateurs and souvenir hunters away, but they can also mean headaches for ethical, but uninformed divers. The following essay is therefor designed to acquaint divers with the potential problems inherent in electronic navigation.

I try only to give out wreck numbers that have been generated onsite. This means that they were measured at the wrecksite using modern electronic navigational equipment. This does not necessarily mean that the sites are easy to relocate. I have had tremendous difficulty locating many wrecks despite having "good numbers" for them. Divers are cautioned that LORAN and LAT/LON coordinates are only as accurate as the electronics used to plot them. GPS plotters can vary dramatically with satellite geometry, overhead obstructions and machine quality, while LORAN plotters may also vary according to signal geometry and location. Thus, the coordinates given may not place you directly over a given wreck. They may not even put you in the neighborhood. If you 're lucky they should put you close enough to see a mooring buoy if one exists. Most wrecks however, don't sport mooring buoys and "dragging" for wrecks can cause irreparable damage. The following page is designed to save both time and wrecks for divers trying to locate them.

Two terms that help to conceptualize the navigational problems in finding wrecks are "accuracy" and "reliability." For our purposes, the accuracy of a navigation device refers to its ability give the correct coordinates for your true location as plotted on a NOAA chart. The reliability of a navigation device refers to its ability to bring you back to a point that it previously plotted. Reliability of both LORAN and GPS in the Great Lakes region is usually exceptional. Either type of unit will generally be able to bring you back to within 75 ft. of a point previously plotted by the same device. The difficulty usually lies in the accuracy of a given device. A good GPS receiver can generally give your actual location to within 75 ft. with good satellite geometry, and no overhead obstructions. However, many LORAN units can be off by 100 to 1000 yards when giving your actual position. This means that if you set your LORAN to express itself in LAT/LON coordinates, and you try to plot the numbers it gives you for your location on a detailed NOAA chart, you're likely to find a significant discrepancy when you look at surrounding landmarks.

This difference in accuracy between LORAN and GPS is due to the different ways the two systems plot location. GPS machines use signals from geosynchronous satellites to plot positions, while LORAN machines measure differences in the time it takes signals from land based transmitters to reach them (TDs). Unfortunately, LORAN signals are slowed when crossing land masses and are subsequently not uniform at all points in a given region. This land mass distortion is particularly pronounced in the Great Lakes and will cause a LORAN unit to vary significantly from the LORAN overlay lines printed on some NOAA charts. Although not an issue in the Great Lakes, LORAN accuracy can also degrade due to poor signal geometry on the fringes of signal coverage areas. Because of these variances, a GPS machine and a LORAN machine sitting in the same spot can sometimes give very different numbers, even when set to express themselves in the same units of measurement. Subsequently, a GPS machine can reliably relocate a spot plotted by another GPS machine, but won't be able to reliably relocate a spot plotted by a LORAN machine. Likewise, a LORAN machine can reliably relocate a spot plotted by another LORAN machine, but won't be able to reliably relocate a spot plotted by a GPS.

Newer LORAN units often have a feature called an "ASF Correction" or Automatic Secondary Factors Correction which attempts to correct for signal distortion from land masses and poor geometry. This feature significantly improves the accuracy of LORAN but adds the further complication of having to guess whether a given LORAN measurement was made with or without the feature. Because of this accuracy problem and because of the advent of GPS, LORAN is slowly falling out of favor for nautical navigation. All coordinates on my website were subsequently measured using a GPS, and as such, should be relocated using a LORAN with ASF or preferably, with a GPS. Numbers given on my website will also plot accurately on a modern NOAA chart. Divers using LORAN without ASF will find a nonstandard error from 100 to 1000 yards. Unfortunately, most TDs floating around out there are non-ASF TDs, and cannot be reliably relocated with a GPS unit even if it is set to express itself in TDs. Further, many people have converted old TDs to LAT/LON using their machines. These "converted" LAT/LON numbers will be just as inaccurate as the TDs they were converted from. For these reasons, when somebody gives you a set of numbers, you must ask what kind of machine generated them and whether LORAN generated numbers are raw or ASF corrected. Do not assume just because you were given numbers as LAT/LONs, that they were measured with a GPS. All modern GPS and LORAN machines can express themselves in different measurement units. If all you have is a set of numbers, don't be surprised if the wreck isn't where your machine says it should be, and be prepared for some trolling.

Another navigational term that divers should be familiar with is that of map "datum." For our purposes, a map's datum refers to the orientation of the coordinate grid used to overlay that map. Surprisingly, not all maps use the same units of measurement or even the same coordinate grid. Thus, 47.14.00'/88.37.90' will find a different spot on a modern NOAA chart than it will on an old USGS 7.5' Series map. The LAT/LON lines are simply in different places. Most modern GPS machines subsequently have a built in datum menu which must be set to the proper datum in order to generate accurate and reliable numbers. The datum which modern NOAA Charts use is the World Geodetic System 1984 datum, or "WGS84." This is the default datum in which all coordinates on this site are expressed. If you have numbers plotted from an old map or chart, (as many shipwreck researchers do) you must convert them to a modern datum.

Divers making use of NOAA charts for locating wrecks should also be aware that NOAA charts express LAT/LON graduations in degrees, minutes and seconds (60ths of minutes), while most GPS and LORAN machines express LAT/LON as degrees, minutes and decimal minutes (100ths of minutes). Subsequently, when your GPS machine gives you a location of 47.14.75'/88.37.90', it must be plotted on a NOAA chart as 47.14'45"/88.37'54". LORAN TDs generated with a GPS will generally correspond to the LORAN TD overlay lines given on some NOAA charts.

Given the above navigational considerations, it is not surprising that a lot of "bad numbers" are floating around out there. Divers are often exasperated after spending hours or even days trying to locate a wreck that someone gave them "numbers" for. In most cases, even good numbers still necessitate a good SONAR unit, especially in deep water. Many divers can attest to dropping down only 50 ft. away from a wreck in 100 ft. of water and not finding it. Thus, when diving an unbuoyed wreck, find it on SONAR first, anchor a safe distance away, then suit up. It's never a good idea to moor a boat of any size to a wreck unless it's buoyed or you've found an existing mooring spot. Many historic wrecks were damaged over the years by boaters creating new moorings on fragile wood or old metal appendages.

The above information won't guarantee that you'll locate a given wreck, but it will significantly increase your chances. If you are using an old LORAN machine, or even a newer one with the ASF turned off, make sure you tell people this when giving out your numbers. Also, if you visit a wreck for which no newer, accurate numbers exist, try to make an accurate reading with a GPS so the wreck won't be lost.


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