Garmin Base Map Problems With Accuracy

[+sundogranch]

While searching for a "very close to the freeway" cache I used the Map Page on my map60 to Zoom in and I saw the cache on the West side of I-5. After hiking a mile or so the Compass pointer indicated the cache on the other side of the freeway- Where I found it after a DNF. Anyone else had problems like that? Pretty serious error that has me wondering how accurate the Garmin maps are.

[+sundogranch]

Well- my map60 clearly showed it on the west side of I-5 At least 120' or so. When I got to that position and swithced to the "Pointer" it was 500 feet other side of freeway. Now I don't trust the Garmin and that is how some airplane Pilots crash and burn- when they dont trust their instrumentation..

[+park2]

The base maps are *very* basic, they're a guideline at best.

The extra mapping products are a different story.

[peter]

Garmin certanly makes no claims to great accuracy of its basemaps (and neither do the other manufacturers). In fact Garmin deliberately tries to make the inaccuracy very clear in at least two ways.

1)When you zoom in below about the 5-mile scale there's an 'Overzoom' indicator that comes on. That means that the inaccuracies inherent in the basemap are likely to become very visible at those scales and you can't trust how your position appears relative to other items shown on the map page. OTOH, with MetroGuide maps loaded the 'Overzoom' indicator only appears at the 200' scale, thus reflecting the greater accuracy of those maps allowing you to zoom in much farther before the inaccuracy will become visible.

2) Garmin provides an 'Accuracy circle' that partly reflects the GPS measurement uncertainty, but is primarily based on the accuracy of the map being displayed. So when I only have the basemap displayed I see this circle having a diameter of about 0.4 miles. OTOH, with the more accurate MetroGuide maps displayed it shrinks to about 150'. So if I am only looking at the basemap and it may show me a tenth of a mile or so to the east of a road. But the 'accuracy circle' with a diameter of 0.4 miles indicates that I could easily be somewhere to the west of the road instead and still be within that circle.

BTW, if you go outside the geographic area covered by the basemap, say to Europe with an American basemap, then the accuracy circle gets larger still - about a 20 mile diameter. This reflects the even coarser and less accurate nature of the basemap for the rest of the world.

 

Trying to display maps for the entire North & South American hemisphere in only an 8MB basemap is bound to require a great deal of simplification and elimination of detail and also minimizing the precision with which points are stored. So pay attention to the clues that Garmin provides about the limits to accuracy in this kind of map and you won't be so surprised or disappointed. And note that when you use the detailed downloaded maps where 8MB may only cover a single medium-sized city then the accuracy that can be provided is much better.

[+sundogranch]

Thanks for the info. I'm getting closer and closer to putting my new map60 up for auction. Cobra with included maps is a much better GPSr for the money.

[+sundogranch]

What good is a map if its inacurate?

[peter]

What good is a map if its inacurate?

No map is ever perfectly accurate, so the real question is whether the level of accuracy provided is sufficient for your needs. My main use of the basemap on my GPS receivers is for portions of extended trips where I'm on main highways enroute to a destination area for which I've loaded the more detailed maps. In that situation it's not of much concern if some features shown on the basemap are off by a tenth of a mile or even a bit more. It can still let me know about how much longer to the next exit with a gas station or restaurant or where the next rest area might be located.

 

But if you're counting on having a detailed map then it is frequently more cost effective to get a bundled package that includes the desired maps directly with the GPS. That's true of Garmin as well as Cobra and other manufacturers.

[+sundogranch]

Yes- having the detailed maps is a plus but I wonder why my Magellan 200 shows the cache on the right side of the freeway along with my Cobra and only the Garmin has Bogus Map Info displayed? That map 60 is a junker as far as I am concerned- It SUCKS when it comes to close cache hunting- and they take your arms and legs when you want maps

[arthurking83]

I'm not a Garmin user (Magellan is my preference), but you may be able to upload a new (more accurate) basemap to the '60'??

 

I know there are third party mapping products that can do this.

 

Magellan have basemaps that are changeable (definitely not condoned by the manufacturers, and possible warranty voiding stuff??)

 

Don't despair, and definitely don't blame the '60' for Garmins inability to supply accurate basemaps!

[peter]

Yes- having the detailed maps is a plus but I wonder why my Magellan 200 shows the cache on the right side of the freeway along with my Cobra and only the Garmin has Bogus Map Info displayed?

I haven't found the Magellan basemaps to be any better overall as far as accuracy goes. Any one particular point on a given basemap may be accurate, but they all store a huge geographic area in a very limited memory. That means they can't contain enough points with sufficient precision to accurately show every turn that each road makes. So in some places the road will happen to be shown in just the right place but then the road makes a slight bend not included on the basemap and the position shown will be off a bit. Higher accuracy maps require more data (and corresponding memory) than is provided in the basemaps.

 

BTW, AFAIK, no one has figured out yet how to overwrite or modify Garmin basemaps, but there are certainly more detailed maps (either commercial or self-made) that can be loaded into the available 24 MB map memory of the GPSMap60.

Edited January 26, 2006 by peter

[+sundogranch]

Here is the cache where I had problems with the Garmin maps;

Lost Hills Treasure (GCH224)

http://www.geocaching.com/seek/cache_detai...36-0a4f2122b0c8

 

Edit- this area of the I-5 is pretty much perfectly striaght for miles and miles...

Edited January 27, 2006 by sundogranch

[Neo_Geo]

That map 60 is a junker as far as I am concerned- It SUCKS when it comes to close cache hunting- and they take your arms and legs when you want maps

I'll give you $15.00 for that P.O.S. Map60. Sounds like I'm being more than generous to hear you tell it.

[+sundogranch]

That map 60 is a junker as far as I am concerned- It SUCKS when it comes to close cache hunting- and they take your arms and legs when you want maps

I'll give you $15.00 for that P.O.S. Map60. Sounds like I'm being more than generous to hear you tell it.

I guess I was expecting more from the map60 than it can deliver( like accuracy for example) I'll keep it for a back-up unit if some kids happen to want to help in the cache hunt- me, I'll stick to my new Cobra1000 (which also has it's flaws) and my old yellow Exp200.

 

I would sell the new map60 for the right price...Thanks.