Map or Accuracy

[Guest kellytklp]

I need some expertise. Today as I was driving to downtown St Louis I noticed that as I zoomed in with MAP76 The arrow showing my car on the map display was off the road by some distance. As I checked the accuracy of the unit, it showed that I was within 20 foot accuracy. The displacement was considerably more than the accuracy.

 

Curious!

 

My questions is, what is more reliable:

 

1. The moving map display

2. The accuracy indication on the GPS unit

 

Thanks,

Terry

[Guest Prime Suspect]

If the 76 is like the Vista, you may have encountered overzoom error. Zoom in and see if you get an "overzoom" message somewhere on the dislay. This means the magnification exceeds the acuracy of the map.

 

Does the 76 have the "lock on road" feature?

 

Other possibilities, the road's been changed since the map was made, or the map's just wrong. Remember, pretty much all map databases are obsolete the day they're created.

[Guest PneumaticDeath]

I've seen several instances where entire towns have been shifted by 50-60 ft. I suspect that when the towns were surveyed, it was all in reference to a single point, and the location of that point is wrong in some document.

 

-- Mitch

[Guest Gliderguy]

I vote that the basemaps are off a little. I have downloaded some tracks onto USA Photomaps and have had really good accurate plots.

 

USA Photomaps is available for free at

 

www.jdmcox.com

 

Many thanks to Rubbertoe's post latching me on to this one!!!

[Guest Hawk-eye]

Just a bit of info ... if the Map76 is like the Vista ...the "lock on Road" feature doesn't work with the base map ... or the topo map ...just with the metro series ... straight poop from the boys at Garmin ... and not to mention they are a bit out of date ... but worst ... general accuracy stinks in many respects ... and really inconsistent. I've found on the metro series ... roads missing ... that were in place 10 years before the series was made! You'll find sections of the maps that, if you can imagine trying to line up sections of a printed map and being off by about 1/8 to 14 inch ... that's what it looks like. I've found that in over five locations on their topo maps just in NC. Just find a zoom level that makes it close but that you can use and go for it. It's not just Garmin's maps ... Just for example ... Street Atlas 8.0 ... the mapping program is nice ... but you have to use it with a grain of salt ... addresses can show up three blocks off and on the wrong side of the street. Routing ... well that can be a bit bizzare ...

 

Just keep looking ... one of these days ... someone is going to get it right and make people happy and make a fortune.

 

Base maps ... my opinion ... are those little freebies that a company throws in to make it look like you have something ... but I think they fall in the catagory of "better than nothing".

 

[This message has been edited by Hawk-eye (edited 18 August 2001).]

[Guest Hawk-eye]

Just a bit of info ... if the Map76 is like the Vista ...the "lock on Road" feature doesn't work with the base map ... or the topo map ...just with the metro series ... straight poop from the boys at Garmin ... and not to mention they are a bit out of date ... but worst ... general accuracy stinks in many respects ... and really inconsistent. I've found on the metro series ... roads missing ... that were in place 10 years before the series was made! You'll find sections of the maps that, if you can imagine trying to line up sections of a printed map and being off by about 1/8 to 14 inch ... that's what it looks like. I've found that in over five locations on their topo maps just in NC. Just find a zoom level that makes it close but that you can use and go for it. It's not just Garmin's maps ... Just for example ... Street Atlas 8.0 ... the mapping program is nice ... but you have to use it with a grain of salt ... addresses can show up three blocks off and on the wrong side of the street. Routing ... well that can be a bit bizzare ...

 

Just keep looking ... one of these days ... someone is going to get it right and make people happy and make a fortune.

 

Base maps ... my opinion ... are those little freebies that a company throws in to make it look like you have something ... but I think they fall in the catagory of "better than nothing".

 

[This message has been edited by Hawk-eye (edited 18 August 2001).]

[Guest geomapper]

I'm a relatively new GPSR user but I have noticed the map that comes with the Etrex Legend is incorrect at times. Particularly, an intersection I drive through daily in an urban area incorrect on the map but I have observed that when looking at the tracking feature of the GPSR, the tracks are accurate, based on my knowledge of how a correct map of this area should appear. You have to remember, maps in these GPSR's cover the entire USA and there is no economic method for a GPS company to them their version of the map up to date.

[Guest EraSeek]

I don't have a mapping unit and have debated with myself whether to get a 76 or a 76 Map. Not sure which way to go. BUT I wonder, are all these map programs using the same map datum, and do they match up with the one you are using on your unit. Does it switch automatically. I was using a gps coordinates book, you know, long and lats for every little place around here, then discovered that it was using a different datum then is the most common in the gps units. Things and places were way off if I didn't input the cords while in their datum first and then switch.

[Guest RealDcoy]

Most of the inaccuracies are in the basemaps. There is a small river that I cross on my trip West to Gaylord,MI from Traverse City, MI.

 

If I am on the base map, it shows the bridge more than 1/2 MILE away from the real location. When I have the detailed basemap entered from the Mapsource CD it is DEAD ON ACCURATE!

[Guest Kerry]

Basically the technology has outpaced the capability to keep map data up to date.

 

A lot of mapping data is based on pre GPS technology and some are pre pre pre GPS so one should expect there to be some difference until updates can be done. That will generally take some time.

 

Cheers, Kerry.

[Guest JDMC]

If you used USAPhotoMaps(http://jdmcox.com)you'd see your GPS was "right on".

[Guest DaveA]

I am well aware of the innacuracies on the Garmin maps, that is why I bought the Magellan Mapsend topo (their latest product). Suprise! In my hometown the coordinates are about 300 feet off.

 

I asked some questions of more knowledgeable people than I as to why this is and here is the answer I got: Most of the map software out there is licensed from the US Tiger maps as the royalties are cheap in comparison to other sources. The problem with Tiger maps is that they were compiled by having local communities get the data and send it in. Some communities did a good job and others did not. In some cases the communities simply used the wrong datum and in other cases they were just plain sloppy. The end result is that you can drive through town "A" and have your position displayed a block away from where you are and as soon as you enter the next town, you are dead on accurate.

 

While I am a loyal Magellan fan, at the present time the most accurate map software out there (from a GPS vendor) is the new Garmin Mapsource II. This software is based off the USGS maps which are of much higher quality and all of the data was compiled by the same source so it is uniform.

 

Hopefully Magellan follows suit and releases new software to accompany thier new Meridian line that uses the USGS maps.