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I have a Hummingbird 937C GPS/Bottom Machine on my boat and it appears that it may be a "little off". While doing some work with some other folks out on the water last week, I ended up being off the mark of the sites we were going to, although each boat was given the same lat/longs. The other boats were using handheld Trimble units that were capable of getting to sub-meter accuracy. I didn't expect that I would be right on the mark, but to be hundreds of feet away has given me some concern. Couple this with the fact that a buddy of mine gave me a few numbers to use for grouper fishing and I have yet to find the structure he advised was there and it makes me think that my machine is off. The GPS came with my boat (used) and I assume that it is a 2006 model as the boat is a 2006 model. I looked on the web to see if there are any downloads needed to "correct" the machine, but havent found any. Any ideas, suggestions, etc. that you all can provide to help me resolve this problem would be greatly appreciated. Thanks.

See if one of your buddies will load you a Garmin hand-held unit and compare the reading on the water. Or pull up next to one of you buds on the water to see if you are getting the same reading within a few feet.
As far as GPS goes, Garmin is the way to go.

I think you have the GPS set up wrong compare the lat and lon with another GPS and see if the dots are in same place

Try finding a spot and marking it as a waypoint or MOB "Man Over Board"..then drive off a mile or two and then go back to the spot...you should be right on top of it...if not, I suspect the problem isn't the gps, it's the lat and lon numbers you entered... converting lat and lon (decimal degrees) to gps coordinates (Degrees + minutes/ seconds) is seldom right on

Juan: I suspect that you are correct on the error of entering the numbers. I've got a handheld Garmin that I will take out on the water the next time I go (hopefully Friday ) and see what happens. If ya'll think of anything else, please don't hesitate to throw it on the thread. Thanks for the replies.

the "datum format" can cause a difference/offset in locations

You mentioned he was using a hand held Trimble, this is a commercial unit capabale of sub meter accuracy, your unit is older and probably not high sensitivity GPS reciever found on most marine GPS units today still not capable of sub meter accuracy real time positioning! Other errors mentioned are certainly viable, but most marine internal antennae unit GPS are no where near this accurate even with WAAS or DGPS they are 2 to 5 meter errors depending on signal strength/error. A good external antenna will help, but you can NOT compare a Garmin with a Trimble!

Also something to think about. Trimble makes commercial grade GPS units for surveying and GIS mapping purposes. Normally, a recreational grade GPS unit (Garmin, Magellan, or anything you pay less than about $5,000 for), are factory set to WGS 84 coordinate system (a world wide coordinate system). Surveyors and mappers generally use a local coordinate system, such a NAD 87 (a collection of coordiante systems made up of smaller regions that cover the entire U.S.). In NAD 87, the state of Florida is actually made up of 3 different regions (with some overlap) that do not necessarily correspond to each other exactly.

If they were using a different coordinate system, you could be a little off. If they were using NAD 87, it's not really designed to be used offshore, and wouldn't correspond to WGS 84 very well at all. Inland, they are supposedly within centimeters of one-another. For most purposes, I'd reccomend always using WGS 84.

I had almost the same issue... a friend of mine gave me 3 years worth of waypoints that went anywhere from steinahatchi to carrabelle. I already had many waypoints added to my gps from either creating them or finding them on the web.....I used GPS BABEL to convert all his Garmin waypoints to my lowrance format...after I installed the waypoints onto my unit and booted it up, I noticed on the map that there were 2 waypoints created for the spots that are near Carrabelle such as the Yamaha and rose city reefs.and they were all off by the same amount, about 250 yards to the south east.....after doing a little homework I found that I had selected the wrong datum format when I saved the new file.....went and deleted the file I installed, corrected my datum mistake, reinstalled the waypoints and bam!!!!! all of his know reef waypoints were located exactly where the ones I had installed.....using his #'s in june, I was able to park right over the areas he gave me and find whatever it was that he had marked, be it relief or structure....

I will add that I agree with the above post with regards to the external GPS antennas......my lowrance 520c is equiped with an external antenna...

I've got an external antenna mounted on the console. I'm pretty sure that given its age, 4 years, it is not too bad of a set up, but it may need some tweaking, both on the set up and with the "operator"..... By no means do I think my unit should zero in like a Trimble, but the distance noted was concerning. I suspect that the major difference between the folks with the Trimbles and my unit was the difference in format/upload of the lats/longs provided. The sites we were looking at were all nearshore, i.e. some were only 300 yards off the hill, while some were 3-4 miles off the hill. I'll try and ground truth it some more with a handheld vs. the unit. I've got a buddy who has offered to go out and groundtruth some sites with his boat/unit as well. Since he provided me the numbers/sites, it would be a good start. Thanks for all of the replies.

By the way. If it does turn out to be a problem with coordinate systems, it's pretty easy to get around. You can go into the settings of your GPS and set it to the same coordinate system your buddies are using. You can then enter the points, and switch your unit back to WGS 84 (or whichever system you are using, being that it was used, it may be your unit that is in the "wrong" coordinate system). Your unit should convert all of the points automatically. It's a little quicker and easier than converting the points one at a time, and then entering them.

Also, all coordinates systems have several different formats in which they can be entered. Most GPS units are factory set to DDM [Degrees, Decimal Minutes (##°##.##)], and most coordinates you find online are too. It's easy to confuse these with DMS [Degrees, Minutes, Seconds (##°##'##")] or DD [Decimal Degrees (##.####°), especially when the correct symbology is not used (°,',"). When doing your ground truths, if you were to find that all of the points are off the same number of feet in the same direction, your problem is most likely the coordinate system. If you were to find that some points are closer to where they should be, while others are farther off, it's most likely something to do with the format in which the coordinates were entered.

Scoop Sea, you have been given a lot of good information. There is a lot about GPS that the average user either doesn't know or doesn't understand. Lets start with coordinate format. Most "recreational" GPS units come factory set for DD mm.mmm format. If you go through the setup for your Humminbird there will be a choice of DD mm.mmm , DD mm ss, or possibly DD.ddddd. You have to be aware of which format your machine is set to, and enter the coordinates accordingly. Or, if you are given coordinates in any of the three formats, set your machine to that format and enter the coordinates, then set your machine back to the format you usually use. the machine will do the math. While we're discussing coordinate format, I'll relate that to accuracy.Most "recreational" GPS units have an accuracy of +/- 3 meters (roughly 10 feet). This is because of the precision of the unit. Most people use the factory setting of DD mm.mmm, or a precision of one thousanth of a minute of angle. If you do the math, based on the circumference of the map datum, you'll find that one thousandth of a minute of angle equates to roughly 10 feet. Survey quality units, such as the Trimble units, usually have a DD mm ss.sss format. That's one thousanth of a second of angle of precision. That equates to 0.17 of a foot accuracy. Check to see what format you were given.

If you go through the setup on your unit, you will find there are probably 50 different Datum to choose from. red_yakker hit on this. If your buddies were using a Trimble unit they were probably using datum based on nautical charts. If so, the charts are based on NAD 29 datum. That alone would account for your several hundreds of feet difference.

There is one other possibility. Ranaman mentioned WAAS and DGPS. GPS was developed for the military. The signal the sattelites transmit is scrambled. Without DGPS or WAAS you can sit in one place and watch your coordinates change the equivalent of 2000 feet. The first means of overcoming this for civilian use was DGPS. DGPS is a radio signal that corrects the sattelite signal real time. The shortcoming for civilians is if you aren't within radio range of the DGPS signal you don't get the correction. Some years back the government put two WAAS satellites in place. These satellites do the same thing as DGPS. Look in your unit's setup and verify that WAAS is enabled. If not, you'll think your GPS has lost all of it's sense. Just a point of interest, DGPS and WAAS are maintained for security reasons. In the event of a national emergency, such as a possible attack by a foreign country, the government can shut the publicly broadcast DGPS and WAAS signals. The GPS signal you receive will be scrambled and you can throw accurate navigation out the window.

Hope this helps.

"In the event of a national emergency, such as a possible attack by a foreign country, the government can shut the publicly broadcast DGPS and WAAS signals. The GPS signal you receive will be scrambled and you can throw accurate navigation out the window. "

The reason you should go out one day by compass alone, and learn how to use it to get home.

Dubble

Is the Wass turned off?