Faulty base map in Garmin Legend(?)

[nemethmik]

Today we found "The Other Side of the Rose" cache

at N 32° 51.751 W 117° 12.248 (San Diego)

We used a Garmin Legend with its base map. The cache is marked on the northern side of the railroad on the Garmin's map, but its on the southern side in reality. The coordinates are perfect because the cache was exectly where the coordinates lead us.

What may be the problem?

Is the base map in my Garmin Legend is not accurate?

Would you be so kind to check this location with your GPSR and let us know your finding?

[Bagbug]

I Had a Legend and I always had those kind of problems. I was almost always 100 feet away from where the GPS indicated me on his map.

 

Thats why I sold it and now Im considering a Meridian or a Sportrak.

 

Bag

[Atilla the Pun]

I suspect the basemaps on all the handheld GPS's we play with are not very accurate... I have marked waypoints for about 4 intersections around my home and they bear no resemblance what-so-ever to the map in my Map 330X.

 

AtP

[+Alan2]

Basemaps as far as I can tell are no more accurate then your typical state road map. You can follow direction pretty well but thats about it.

 

For more accurate, you have to use a product like Mapsourse Street and Recreation or Topo although even here you will find that the street don't line up the same between these two either. That's just difference in mapmaking. The Topo appears closer but of course the street names are missing.

 

If your creating waypoit, you can spot them on STreets, switch to topo and then move them according to Topo.

 

Or use a 24000 quad map like NG Topo, spot your waypoints and transfer them to the GPS.

 

Finally, worse then being on the wrong side of a rail track is when you're on the wrong side of a stream with no bridge anywhere in sight!

 

Alan

[+Alan2]

Basemaps as far as I can tell are no more accurate then your typical state road map. You can follow direction pretty well but thats about it.

 

For more accurate, you have to use a product like Mapsourse Street and Recreation or Topo although even here you will find that the street don't line up the same between these two either. That's just difference in mapmaking. The Topo appears closer but of course the street names are missing.

 

If your creating waypoit, you can spot them on STreets, switch to topo and then move them according to Topo.

 

Or use a 24000 quad map like NG Topo, spot your waypoints and transfer them to the GPS.

 

Finally, worse then being on the wrong side of a rail track is when you're on the wrong side of a stream with no bridge anywhere in sight!

 

Alan

[jfitzpat]

FWIW, the basemaps in Garmin and Magellan units are largely based on the 'Tiger Line Data' from the US Census. Because the data was originally take from many printed sources, and because it is stored as geometric vectors (lines, etc.), it has an inherent margin for error.

 

You'll see the same thing if you take a MapQuest map, etc. (which is also based on the same data) and compare it to an aerial photo of the actual streets.

 

-jjf

[+Alan2]

Are we talking about the same thing? I thought base maps are the ones that come in the unit without Mapsource being loaded with Street and Recreation. Yes? If so, they're like state road maps. ARen't the Street maps the Tiger ones?

 

Alan

[peter]

Yes, the original poster was just using the buit-in basemap. Since that map covers N&S America in a limited amount of memory the data is reduced in several ways, some of which reduce accuracy, including:

1) only highways and major roads are shown

2) bends in roads, railroads, streams, etc. are presented as line segments with only a few points

3) the coordinates of each point are stored with fewer digits to save memory.

 

Garmin tries to make the user aware of the accuracy limitation by displaying "Overzoom" under the scale when zoomed in below the 5 mile scale. (Note that at that scale setting the waypoint appears to be right on the RR and the discrepancy can only be seen when zooming in much closer.)

 

The MapSource maps are much more accurate than the basemap, particularly Topo and MetroGuide. I checked MG and it shows the waypoint correctly as south of the tracks.

[+Prime Suspect]

A GPSr has a limited amount of overall memory. All roads, highways, railroads, etc. are actually stored in a connect-the-dots manner. More roads can be stored by keeping slightly less accurate data (by essentially increasing the gaps between the "dots"). There's even less incentive to keep really accurate data for railroads.

 

That said, I've ridden my bike along the Katy Trail here in Dallas (a rail-to-trail conversion) with my Vista mounted on the handlebars. The loaded map (not the basemap, though) showed the old Katy railroad line, and it put me dead-center on it the whole way.

 

[pdumas]

I agree with peter, the down loaded maps are more accurate than the base map.

[+Hawk-eye]

If you want to think about the base map as mentioned .... using a limited amount of memory ... it's like setting up a route with a limited amount of waypoints ... the more waypoints ... the more the route follows the path actually taken ... fewer waypoints ... the route starts shaving corners ... so it becomes more of a relative view of the path you're taking than the actual path.

 

The mapsource maps are much more accurate ... but use more memory.

 

Anyway ... just another way to think about it ... but the same as stated above in Peter's post.

 

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Co-founder of the "NC/VA GEO-HOG ASSOCIATION"

... when you absolutely have to find it first!