Recent GPS inaccuracy (in the Caribbean)?

[Michael Davis]

Has anyone else been encountering GPS positioning to be off by about 100 feet the past several months

We had that very problem going into Hollywood Cove in the Bunsby Group, south of Brooks Penn. last summer. I know we were in the middle of the channel but the plotter showed us running over a couple of small islands. I know we weren't where it showed or we would have stopped suddenly.

 

[Tom J]

What do local magnetic anamolies have do with GPS errors?

This is off-topic, but is in response to a comment I made about our dependency on electronics; GPS, autopilot, etc.
So, to be more on-topic, as NotCook pointed out, the GPS is not always accurate. For that reason, we always have a hard copy of the chart displayed on our plotter, on hand in the cockpit. Even when navigating our home waters, the chart is there. On some difficult passages, such as Woods Hole in Massachusetts, the Admiral reads out the buoy numbers from the chart and points out the turns coming up, while I verify our position on the plotter.
Back in the day, it was not uncommon for the GPS to be off by 100', but as the OP said, we have come to expect accuracy of 6 to 10 feet. This type of accuracy is vital to AIS, so he is right to be concerned.

 

[heritage]

I think a lot of the perceived gps inaccuracies are actually due to the maps loaded in the chartplotter. For instance,,, if your plotter is loaded with raster charts (digital copies of noaa based paper charts), as you zoom in to go through a narrow channel the markers will get very large and fuzzy. The location of the channel will be difficult to see. This problem will be worse if the original paper chart covers a large geographical area. Actually this fuzzing up of the channel markers is a good thing, because it tells you that the accuracy of this raster chart was never intended for precise navigation.

On the other hand, if your chartplotter is loaded with vector charts and you zoom in to see the channel in detail, the channel markers will look very distinct and give you the impression that their location is very accurate. While in fact, their estimated locations were picked off the same noaa charts that were not designed for close in, precise navigation.

If the channel has been charted with small area, detailed charts, and your chart plotter uses them when you zoom in, then both vector or raster charts should work well.

 

[justsomeguy]

I think a lot of the perceived gps inaccuracies are actually due to the maps loaded in the chartplotter.

Have noticed this too. Especially in Mexico.

 

[NotCook]

Following up, I’ve only noticed this here in the Caribbean, and only at specific locations. In my opinion, it isn’t the GPS - it’s the charts. Some of the details, like an opening or a rock, are just off a little sometimes with respect to the position on the planet. And the chart plotters are just depicting what the paper charts have - but either raster scan or vector - they all basically come from the same chart datum source. Depending on the chart plotter, you may be using either or both types of source.

 

[Meriachee]

Have noticed this too. Especially in Mexico.

Who authorized marker installation in Mexico, and who is paying for it?

 

[capta]

I have been using my GPS to get in and out of some pretty tight places, like the Tobago Cays north entrance and I have not noticed it here in the Grenadines, but as we are off on charter tomorrow, I will look more closely.
I'll check back in a week.

 

[JimInPB]

Most GPS units are good within 6', even when the circle of uncertainty indicates several times that amount.

Several things can cause reduced accuracy. Some things can cause a fixed offset from previous performance.

If there is active military action in your area, then Selective Availability will often be activated & accuracy will be reduced. Poor weather, a poor view of the sky, & local electromagnetic wave radiation can reduce accuracy. If someone monkeys with the settings in the GPS & changes the reference datum, that can give you a fixed offset from previous performance. If someone turns off WAAS, that can mess with your accuracy. Sometimes updates mess with your settings, without you knowing about it.

Check if your new error is consistent. Check if other GPS units near you are having the same accuracy issues. I normally carry a handheld GPS as both a back up & a double check, anytime I am going offshore.

 

[justsomeguy]

Who authorized marker installation in Mexico, and who is paying for it?

Prolly the cartels. Some can be very civic-minded.

 

[isaksp00]

Jiminpb, I am pretty sure SA (selective availability) can't be turned on for just a local area, I think it is a form of encrypted degradation of the downlinks that applies to all the satellites in the worldwide constellation. And I am also pretty sure the Defense Dept has not been activating it on and off - since Clinton (I think it was his Presidency) allowed civilians to get the same 6 or so foot accuracy as the military, it has been off.
As an aside, I think the newer blocks of GPS vehicles have or will have something that provides even greater accuracy to the military, but I'm not sure of that.

 

[Gunni]

Following up, I’ve only noticed this here in the Caribbean, and only at specific locations. In my opinion, it isn’t the GPS - it’s the charts. Some of the details, like an opening or a rock, are just off a little sometimes with respect to the position on the planet. And the chart plotters are just depicting what the paper charts have - but either raster scan or vector - they all basically come from the same chart datum source. Depending on the chart plotter, you may be using either or both types of source.

Caribbean nations have little financial resources for updating charting done during the colonial era. That includes the backwaters, bays and coves not used by commercials. In those places you need local knowledge or a clear eye and savvy skills. Some of the charts for Grenada date back to the French explorers, some to the British. Don Street updated a bunch of the Windwards through his collaboration with Imray. You can get his excellent charts for harbor charting. Imray-Iolaire. Recently NV Charting has been doing chart updates. Your GPS is working fine, the charts are just largely outdated, or done with survey methods far less accurate than satellite telemetry. Garmin doesn’t even make a representation that their charts are suitable for marine navigation. In some places they may be, but don’t count on it. What you will notice is that nav markers get chart updated. So if you pick up the markers you can interpolate your GPS location discrepancy.

 

[Tom J]

Caribbean nations have little financial resources for updating charting done during the colonial era. That includes the backwaters, bays and coves not used by commercials. In those places you need local knowledge or a clear eye and savvy skills. Some of the charts for Grenada date back to the French explorers, some to the British. Don Street updated a bunch of the Windwards through his collaboration with Imray. You can get his excellent charts for harbor charting. Imray-Iolaire. Recently NV Charting has been doing chart updates. Your GPS is working fine, the charts are just largely outdated, or done with survey methods far less accurate than satellite telemetry. Garmin doesn’t even make a representation that their charts are suitable for marine navigation. In some places they may be, but don’t count on it. What you will notice is that nav markers get chart updated. So if you pick up the markers you can interpolate your GPS location discrepancy.

I sometimes wonder if there isn't a way for private surveys to be incorporated into "official" charts. I worked on a commercial vessel, and our surveys were very accurate. When we were looking for shipwrecks off the Dominican Republic, we recorded bottom profiles from down looking sonar and side looking sonar. When we did surveys off the Florida coasts, the company that hired us to do the survey would set up a beacon on a rooftop on shore, and calibrate the height of our vessel and its location. This data, combined with GPS and sonar provided a bottom profile accurate within inches.

 

[Gunni]

Navionics has a user feature that allow registered users do just what you say Tom- share their sonar data stored on their charting chip. I download the sonar data as Fish n Chip or somesuch. I can do this despite having no sonar. Never tried doing so for foreign ports, just US waters that I find to be accurate enough. It is pretty raw with something like 1 ft contours, so it can be messy.

 

[NotCook]

Navionics has a user feature that allow registered users do just what you say Tom- share their sonar data stored on their charting chip. I download the sonar data as Fish n Chip or somesuch. I can do this despite having no sonar. Never tried doing so for foreign ports, just US waters that I find to be accurate enough. It is pretty raw with something like 1 ft contours, so it can be messy.

I challenged Navionics about that very point. I questioned how they could rely on the accuracy of private sonar track reporting, when the accuracy of the reported depth was dependent on the accuracy of the individual boat owner’s calibration of thei depth sounder offset, if they even did so. Then there is whether they show actual water depth or what we below their keel. Their response was that they take that into consideration. I find that highly dubious. Would also be necessary to account for tides, swells, and other weather related considerations.

 

[Tom J]

I challenged Navionics about that very point. I questioned how they could rely on the accuracy of private sonar track reporting, when the accuracy of the reported depth was dependent on the accuracy of the individual boat owner’s calibration of thei depth sounder offset, if they even did so. Then there is whether they show actual water depth or what we below their keel. Their response was that they take that into consideration. I find that highly dubious. Would also be necessary to account for tides, swells, and other weather related considerations.

Good point. The source of the data would have to be considered. On the vessel I worked on, the client's shore beacons and calibrated sonars ensured that their data was accurate within inches. The tides, swells, motion of the boat were all recorded. Of course, this is proprietary information the company would want to keep for upcoming contracts, so maybe not much hope in them sharing.