GPS Accuracy

[Scott MacCready]

I'm sitting here fooling around with my GPSmap 176C. The base map that comes with it includes N.America, the northern half of South America, and the North Atlantic. I was trying to locate the approaches to Bermuda using coordinates I found on the official Bermuda Tourism website. When I plug in their coordinates (32 degrees 23 min N lat & 64 degrees 38 minutes W log), it loctates me far off tot the NE of the islands. Wanna tell me why?

Question #2
If using gps to navigate to Bermuda from here in NC, would you just have one waypoint across the ocean or would you set several waypoints to attempt to follow a great circle route? I know for short day trips things like this don't matter. Just wondering how using gps effects planning compared to paper charts.

[Neil Gordon]

When I plug in their coordinates (32 degrees 23 min N lat & 64 degrees 38 minutes W log), it loctates me far off tot the NE of the islands. Wanna tell me why?

Question #2
If using gps to navigate to Bermuda from here in NC, would you just have one waypoint across the ocean or would you set several waypoints to attempt to follow a great circle route?

1 - Using the tourism website for navigation is something like using a placemat instead of a chart. Separate from that, you do have to watch that the right datum is used. Islands which are properly charted relative to themselves might not be exactly in the ocean where the GPS suggests.

2 - It's more complicated than that because there are prevailing winds and currents to factor in. You have to cross the Gulf Stream and how and where you do that matters. From NC to Bermuda, there wouldn't be much to gain from a great circle route.

[Scott MacCready]

Hi Neil,
Thanks for the reply.
The website I was looking athttp://www.rccbermuda.bm/portal/server. ... nerMode=2& gives the location of their inlets for tourists coming by boat. Be sure, I'd make sure it's accurate before shoving off but, without getting into politics, I'm sure an official government site shouldn't too far off.

As far as question #2. I guess I was asking if the gps takes into account the curvature of the earth like I assume charts do? I'm still sitting here playing with the gps and a paper chart. I plotted the route in a straight line on the gps. Now, recording those positions on the paper chart, I see they do not form a straight line but a curve. Guess those people at garmin are pretty smart.

[Efrost]

Thee can be dozens of reasons for your GPS coordinates to be different from a tourist map.

First what data is your GPS on? What datum is the map you are pulling the coordinates from? What datum is the map you are carting on. There are over 100 datums worldwide. If yo are not familar with a datum, just think of it as the place they map maker starts from.

Most GPS rcvrs can plot to most of the datums. From the factory they are usually set to WGS-84, the standard in the US and in most of the major chart producing countries. However, that doesn't mean you won't get brand new from the store (or NOAA) charts that are on a different datum. Thats because of time and money in up dating.

The legend on your charts should tell you what datum they are on.

As to your second question. It depends upon the map projection whether you will get a great circle or a rhumb line or something else. If you use mercator (one of the most common) the shortest distance between two points will draw out as a great circle.

Any good seamanship book or other GIS (mapping) book should be able to explain all the basics. Understanding your charts, what they tell you, and how they are made are important as you broaden horizons and sail into "unkown", to you at least, seas.

[Bill Cochrane]

and as part of advice for sailing to bermda, it lists those coords for five fathom hole; given that they are to the nearest minute, they're pretty much spot on. The location is to the NE, just inside the sea bouy and just north of a rhumb line from the sea bouy to town cut. For a single waypoint it's pretty close.

The base map may not be deatiled enough for that level of accuracy, you may need the charts for Bermuda. You certainly need them if you're sailing there.

You wouldn't sail directly to that waypoint because of the reefs surrounding the islands...you need to go around Bermuda and approach from the Northeast.

Some GPS units have a setting to select whether a course line follows a rhumb line (for short distances) or a great circle route (for longer ones). Dunno if yours does that or not. When all else fails, RTFM.

[Dan & Pat]

Scott,

I have the Garmin 176 , the black-and-white model though, not Color . I found that the low resolution base mapping that's built into memory with it, leaves much to be desired, so I purchased a Blue-Chart chip for my area. It covers from Jacksonville to Key Largo and it is a marvelous improvement. You just open the little door on the bottom of the unit, pop in the chip and its ready to use. At around $130 to $150 apiece, they're not cheap, but change the way you look at the details immensely. Its like a paper NAV chart on the screen, showing buoys, reefs, landmarks etc, and even the compass rose.

For example, the entrance to Port Everglades in Fort Lauderdale has 5 separate markers, 3 buoys and two pilings, and all of them show up on the chart within a variance of less than 10' for the pilings. I'd say that's pretty accurate. I'm an avid fisherman and I have a bunch of favorite reefs that I like to bottom fish on. These reefs I have marked on my old GPS (Garmin 12) show up on the new one with amazing accuracy as well.

My advice is to check e-bay or overstock.com or even planetgps.com to see if you can't get a good price on the Blue Chart upgrade chips. You'll be amazed at the difference.

Dan

[Scott MacCready]

Thanks for the above info. I do have a few of the more detailed chips, for the ICW and also DownEast Maine. I don't have one for Bermuda though. I'm not planning on a Bermuda trip but someday hope to head offshore to the Caribbean and knowledge of Bermuda would be helpful "just in case".