How acurate is the base map

[jeepmaxx]

I am just getting started in the geocaching game. I purchased an etrex vista gps unit and have been trying to learn how to work it. My neighborhood backs up to a rail-road track. The tracks show up on my basemap but when I walk around the hood, it says that I cross the tracks. I have the waas activated and show acuracy to within 17~23 feet and I know that I am at least 100 feet, most likly more, from crossing the tracks. Could the basemap data be off, or am I doing something wrong? If I was get get mapsource topo would it be more acurate?

[Couch_Potato]

I'd say that the basemap is quite likely in error. Around my town it doesn't even have all the major roads in the right place. Even the ones that are 'right' sometimes straighten out curves and bends so that tracks I make while driving can zig-zag across the basemap road.

 

I would hope that the Mapsource maps were more acceratue. But since I don't have Mapsource I can't say for certain. You can check to see what Mapsource has at Garmin's site. For my area Topo seems slightly more accurate, but the map data is old. Newer roads are not there.

 

I'm not lost!

I just don't know where I am.

[Point2Point]

Ya, the Vista base map is not super-accurate, it basically gives you a rough estimate of where things are. Roads and streets are a little off and things like rivers and lakes aren't perfect when it comes to their shore lines. And forget about secondary roads and streets, they aren't there.

 

Mapsource Metroguide does a LOT better, but you will still find errors here and there. In my local area there are roads drawn where there aren't any, streets mislabled, and even city markers with the wrong name. They even spelled Pittsburgh wrong in the map region indicator.

 

So, I guess getting maps accurate down to the last detail is nearly impossible, but overall Metroguide does a decent job.

[Couch_Potato]

quote:

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Originally posted by Point2Point:

They even spelled Pittsburgh wrong in the map region indicator.

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Did they spell it as it's pronounced (at least locally)? Picksburgh

 

Or maybe they just call it "the 'burgh."

 

I'm not lost!

I just don't know where I am.

[Couch_Potato]

quote:

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Originally posted by Point2Point:

They even spelled Pittsburgh wrong in the map region indicator.

---

 

Did they spell it as it's pronounced (at least locally)? Picksburgh

 

Or maybe they just call it "the 'burgh."

 

I'm not lost!

I just don't know where I am.

[Point2Point]

nah, they just left the 'H' off the end. It's a common mistake, but a typical example of the many errors you will find in Metroguide.

[Kerry.]

About as accurate as the base data that it was derived from, which can be variable, very variable. In general GPS is much better than the methods probably used originally to collect the base map data.

 

The other thing is maps are generally designed to be used at a particular scale so attempting to zoom in on maps not intended to be zoomed in is not all that useful.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Kerry.]

About as accurate as the base data that it was derived from, which can be variable, very variable. In general GPS is much better than the methods probably used originally to collect the base map data.

 

The other thing is maps are generally designed to be used at a particular scale so attempting to zoom in on maps not intended to be zoomed in is not all that useful.

 

Cheers, Kerry.

 

I never get lost everybody keeps telling me where to go

[Couch_Potato]

quote:

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Originally posted by Point2Point:

nah, they just left the 'H' off the end. It's a common mistake, but a typical example of the many errors you will find in Metroguide.

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Ah, the common mistake. I've heard that Pittsburgh is the only 'burgh in the US to have that final 'h' attached. I'm not sure it's true though.

 

Not just Metroguide has these little errors. The tiny little village that my parents live in is named Gastown. But on every version of electronic map I've ever seen (including Mapsource) the 'n' is dropped, making it Gastow. That makes it pretty obvious to me that all of these comanies are getting their info from the same source and that source's errors are never corrected.

 

I'm not lost!

I just don't know where I am.

[+apersson850]

Pay attention to the "overzoom" warning below the map scale ruler. If you zoom in further than the maps is intended to, you'll be able to make conclusions about the position of objects where the accuracy of the placement of that object on the map isn't matching your scale.

 

Just look at an ordinary paper map, covering a larger area. A main road may be 700 meters wide, if you take the width of the line on the map and multiply by the scale. When making a vectorised map, you have to place that road somewhere. Unless they do have better data available, the road could end up just about anywhere within that 700 meters.

 

Anders

[Couch_Potato]

quote:

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Originally posted by Anders:

Just look at an ordinary paper map, covering a larger area. A main road may be 700 meters wide, if you take the width of the line on the map and multiply by the scale.

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Wow, that must be a 100 lane highway or something.

 

Is that really what that 'overzoom' means? I never really paid attention to it other than to notice that if the map scale is below 5 miles I've overzoomed. It's never been a concern for me.

 

Hmm, reading that users's manual thingy that came with my Legend ( what a thought, read the manual ) it states: "It is possible to zoom in so far that the resolution of the map data is exceeded and the appearance of the map will no longer be accurate." That's overzooming. Learn something every day.

 

I'm not lost!

I just don't know where I am.

[Couch_Potato]

quote:

---

Originally posted by Anders:

Just look at an ordinary paper map, covering a larger area. A main road may be 700 meters wide, if you take the width of the line on the map and multiply by the scale.

---

 

Wow, that must be a 100 lane highway or something.

 

Is that really what that 'overzoom' means? I never really paid attention to it other than to notice that if the map scale is below 5 miles I've overzoomed. It's never been a concern for me.

 

Hmm, reading that users's manual thingy that came with my Legend ( what a thought, read the manual ) it states: "It is possible to zoom in so far that the resolution of the map data is exceeded and the appearance of the map will no longer be accurate." That's overzooming. Learn something every day.

 

I'm not lost!

I just don't know where I am.

[AlanShutko]

Overzooming can do weird things... I haven't experienced it so much on my GPS V screen, but on mapsource I can zoom in to a degree that road segments disappear. The road may appear to dead-end or make a sudden weird angle to another road. Zoom out and all is well....