Garmin GPS map is off by around 1 kilometer !

[hdradaza]

I don't know if there is another area for navigation questions. If so kindly transfer me!

I am a first time boater. Been out in the water only 3 times. I bought a Garmin GPSmap 60csx. Loaded it with the Blue charts for the asia pacific (my area - philippines) which I bought directly from garmin

after the last 2 times out on the water and even being on land I turned on the gps, I uploaded them to the computer. I found out that the map and the actual location (the breadcrumbs of where I have been;tracks) are off by 1 kilometer.

How can I adjust this ?

 

[dmcb]

Re: Garmin GPS map is off by around 1 kilometer !

I think you should contact Garmin. I have never seen a way to adjust but there may be.
Ask the experts.
In one area I boat, all gps, not just Garmin is off. I anchor on the side of a mountain.
But that is just one area about 10 miles long. Get away from there and they are accurate.
Your problem sounds different.
Doug

 

[hdradaza]

Re: Garmin GPS map is off by around 1 kilometer !

I know that the satellites are accurate and the garmin reads the data from the satellites fine. its just the map. it seems like the whole map is to the right of the actual location by 1km. Maybe there is a setting which allows me to calibrate the gps with the map ?

 

[JB]

Re: Garmin GPS map is off by around 1 kilometer !

I had a map in one of my Garmins. . .I think it was the 162. . . that had some roads and intersections off by as much as 1/2 mile. I whined to them and they reminded me about the caution on boot up about not using it for navigation. The errors were in the maps, because when I downloaded an updated map the errors disappeared. That was the internal map, not Mapsource or Blue Chart.

I suggest that you approach Garmin about updates with corrections. One thing you might do is plot both the Lat/Lon coordinates that your receiver shows against where it puts your icon on the displayed map. Plot both on a paper chart.

 

[Nandy]

Re: Garmin GPS map is off by around 1 kilometer !

Call garmin, there ARE a few settings in the gps that can cause this problem. The maps are not always accurate and most of the maps are from goverment povided gis. I built a few maps myself after complaining about garmin maps. My maps end up with the same inconsistencies as the garmin. I did fix them, but this was a very small map of a small lake. I cant imagine what an enterprise will be to debug all topo maps of the usa, much less the ones from the philipines.
anyway, call garmin...

 

[Silvertip]

Re: Garmin GPS map is off by around 1 kilometer !

I have and regularly use an old Garman GPS12Map. The base map is spot on for highway intersections. The base map is not very accurate however for major rivers, including the one I run regularly. The sugar trail tends to follow the right shoreline about rather then the actual river channel in which I travel. I also feel you have a map problem, not a GPS problem. Perhaps the map supplier has a way to adjust for the error. But then, 1km seems like a fairly large error. As was suggested, go to a wide open area with a known visual point of reference and see if the map is off at that location. If not, then it's not likely a map problem or a GPS problem but probably a reception issue. Therefore, if you were to adjust for the known bad area, you probably make the known good areas wrong.

 

[dingbat]

Re: Garmin GPS map is off by around 1 kilometer !

Seeing that you?re in the Philippines there is a good chance that you have a mismatch between the unit and the maps you are using.

Go to setup on the GPS and look under map datum and compare the settings on the unit with the map datum shown on the charts.

Map Datums

A datum describes the model that was used to match the location of features on the ground to coordinates and locations on the map. Maps all start with some form of survey. Early maps and surveys were carried out by teams of surveyors on the ground using transits and distance measuring "chains". Surveyors start with a handful of locations in "known" positions and use them to locate other features. These methods did not span continents well. Frequently they also did not cross political borders either. The "known points" and their positions are the information that the map datum is based. As space based surveying came into use, a standardized datum based on the center of the earth was developed.

Every map that shows a geographic coordinate system such as UTM or Latitude and Longitude with any precision will also list the datum used on the map.

The Global Positioning System uses an earth centered datum called the World Geodetic System 1984 or WGS 84. WGS 84 was adopted as a world standard from a datum called the North American Datum of 1983 or NAD 83. For all practical purposes there is no difference between WGS 84 and NAD 83.

Most USGS topographic maps are based on an earlier datum called the North American Datum of 1927 or NAD 27. (Some GPS units subdivide this datum into several datums spread over the continent. In the Continental United States use NAD27 CONUS.)

In the Continental United States the difference between WGS 84 and NAD 27 can be as much as 200 meters.

You should always set your GPS unit's datum to match the datum of the map you are using.

On a USGS topographic map the datum information is in the fine print at the bottom left of the map. The datum will always be NAD 27. There may be information on how many meters to shift a position to convert it to NAD 83. Think of this as the error that will be introduced if you leave your GPS unit set to WGS 84. A dashed cross in the SW and NE corners of the map gives a visual indication of the difference between the two datums.

 

[Mark42]

Re: Garmin GPS map is off by around 1 kilometer !

Ah HA! I knew it! GPS is junk. Thats why I will stay with my trusty sundial, sextant and compass; proven technology.

 

[hdradaza]

Re: Garmin GPS map is off by around 1 kilometer !

dingbat - thanks. I think you might be on to something! Ok so I found the map datum settings on my gps handheld unit. Yes it was on WGS 84. There are many options available ( including SouthEast Asia which would be the philippines). Do I just try all other options (about 50! )?

What should I compare it to? Where is the map datum show in the charts ?

I bought the blue charts directly from garmin so I would think they already considered this datum problem ?

I also tried opening Mapsource on the PC and was able to adjust to different datum settings. It seems with each datum setting there was no change clearly visible on the map. It was still the same.

 

[Nandy]

Re: Garmin GPS map is off by around 1 kilometer !

That is why Im telling you to check with garmin. I think your datum is wrong. They will tell you if it is set up wrong and what else to look into. Garmin is a supperb company and has a great customer support team. Yes, basemaps are not the best, but 1k off is not right no matter what.

 

[AndyL]

Re: Garmin GPS map is off by around 1 kilometer !

Changing the datum should not affect where you are on the electronic chart. What will change is the "numeric" Lat and Long. If your paper chart is based on SE Asia datum and your GPS is set to default WGS84 any numeric Lat / Long you pull off your GPS could not be directly plotted onto a paper cahrt without applying a correction . This correction should be stated somewhere on the chart and should read something like... "Correction to WGS84 datum “ Positions obtained from satellite navigation systems are normally referred to WGS 84 datum; such positions should be moved 0.06 minutes Northward and 0.08 minutes Eastward to agree with this chart."
If you change the datum of your GPS it would automatically apply this correction to the value of Lat and Long displayed, your graphical position on the electronic chart wouldn't change.
If your postion is constantly "out" by the same amount there should be somewhere in the software a "GPS Offset position" that you can apply. This will put a constant offset onto your position however if the chart is in error as you suspect this error would likely make matters worse as the correction may not be constant.
Hope this helps??

Andy

 

[gss036]

Re: Garmin GPS map is off by around 1 kilometer !

Good OLD GPS, I have 2 on my boat, one Garmin w/BlueChart and a Hummingbird Matrix. They are fairly accurate, BUT not as accurate as my LORAN. The GPS will get you in the area but the Loran will put right on the spot every time. I still use it to find penacles for bottom fishing.