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Author Topic: free garmin download accuracy?  (Read 6539 times)

jetjockey

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free garmin download accuracy?
« on: February 01, 2010, 09:57:42 AM »
I just downloaded a couple of the free maps onto my Garmin Astro dog tracking system.  However, the map doesn't seem to be very accurate.  It shows me about 1/2 mile from wher I actually am.  I downloaded the Georgia map.  Anyone else have this problem?   Im also not 100% sure I didn't get a virus from the download.  I downloaded the maps on Sat and got a nasty virus last night.  Anyone else have any problems with the free maps?  Thanks

Jet

Boyd

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Re: free garmin download accuracy?
« Reply #1 on: February 01, 2010, 09:59:58 AM »
I don't see how anybody can generalize about "the free maps" since they have been contributed by many different people....

jetjockey

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Re: free garmin download accuracy?
« Reply #2 on: February 01, 2010, 10:07:12 AM »
Is there certain maps that are better then others?   Sorry for my lack of knowledge, someone just sent me a link to this site so Im not exactly sure how the maps work.

Boyd

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Re: free garmin download accuracy?
« Reply #3 on: February 01, 2010, 10:31:40 AM »
Different authors have created these maps themselves, so depending on their experience and what they hoped to accomplish with the map there can be big differences. If you ask for help with a specific map, the author or somebody else knowledgeable will probably respond. I have never used the Georgia map myself.

maps4gps

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Re: free garmin download accuracy?
« Reply #4 on: February 01, 2010, 11:46:28 AM »
The accuracy of any map depends on the accuracy of the source.  We use data that is available for free from various Federal and State agencies.  Contrary to the situation in Canada, a topo map has to be built with data from various agencies and usually from data at various scales.

Off by 1/2 mile is considerably greater than anyone has mentioned about any mapset here.  What are you comparing it with; road, hydro, point feature?  Locally I noticed a road making a turn-back over a ridge that was misplaced by 1/4 mile; however, this was in the 2007 Census data and the county had not been realigned.  All counties had been realigned in the 2008 Census release.  Most mapsets available here use that data; the exceptions being Ibycus uses 2007 and my US Planimetric series uses 2008.  

Usually if something is off by more than twice the accuracy of the GPSr, it is because of an issue in the source data or the unit was set to an incorrect data.  I have no idea what to expect with an Astro dog tracking system.  There have been over 1,400 downloads of the Georgia Topo without a issue be posted; however, you may have just been lucky  :)  and there is always a first time that a virsus couild be placed on the website.   Thanks for the info.

'Better' - sure.  But you, or anyone else needs to define what you mean by better.  Many of the feature types are available from more than one source - different map authors will choose a different source.  Different users will have different priorities for the feature types and detail they would like on a map.  As Boyd pointed out, you need to be more specific.  Which auto, TV, etc., etc. is 'better' ? ?

« Last Edit: February 01, 2010, 01:24:13 PM by maps4gps »

jetjockey

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Re: free garmin download accuracy?
« Reply #5 on: February 01, 2010, 11:57:34 AM »
Maps

Thanks for the reply.  Its off by about 1/2 mile based on the roads around my house.  It shows me about 3 blocks away in the woods when it should be showing right on the road.  Im gonna re-check some more when I get home to see what happens.  Thats the reason I ask.  Probably wouldn't be a huge deal if I was just tracking myself, but its a little more important when my GPS shows me where my dog is standing.  Its not the end of the world, I was just curious since Ive never used free maps before.......     When I was refering to "better" I was refering to the accuracy of topp maps.  I want the most accurate topo maps I can get.  It the free maps are as good as the Garmin maps, Im going to use them.  But if the garmin maps are more accurate as far as topo and road locations, then it would be worth it to spend the money and buy them.  However, from what Im learning the Garmin maps might not be any better.  Im going to compare the free topo with the Garmin South East maps this weekend.  I have a friend who has the Garmin maps and the exact same gps.

Jet

maps4gps

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Re: free garmin download accuracy?
« Reply #6 on: February 01, 2010, 12:15:50 PM »
Could be that although Census has all the counties realigned, they did not get the area you are in correct. 

Except for the road/street data, which Garmin purchases from NavTec, I would expect they use the same free Government sources we have access too; but then they do not document there sources so maybe not.  Most of us make the contours from USGS NED 10m gridded data - most of these data were created by USGS from their 1:24k topos; however not all the NED data was created by the same process nor is it of equal quality.  USGS is slowly correcting this - and also adding 3m resolution elevation data from LIDAR sources. 

Please let us know the results of you comparison.

Boyd

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Re: free garmin download accuracy?
« Reply #7 on: February 01, 2010, 12:29:56 PM »
The Garmin Topo US 100k maps are not terribly accurate either. I've never seen anything official, but most people assume the roads on these maps also came from TIGER data. The more expensive Garmin 24k topo's get their road data from Navteq.

The location of my own home and road are off significantly on Garmin's Topo 100k, but only by a couple hundred feet and not a half mile. But my point is, the 100k Garmin topo's are not going to win many awards for accuracy.  :)

Cuchillo

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Re: free garmin download accuracy?
« Reply #8 on: February 01, 2010, 01:10:40 PM »
But my point is, the 100k Garmin topo's are not going to win many awards for accuracy.  :)
You can say that again. I've had instances where I was on a road/trail and the map said otherwise. Much less than 1/2 mile, but more than two or three times my GPS's stated accuracy. It has never caused any problems, just a little head scratching.  ???  ;)

Indrid Cold

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Re: free garmin download accuracy?
« Reply #9 on: February 01, 2010, 01:13:53 PM »
I downloaded the Georgia map.  Anyone else have this problem?   Im also not 100% sure I didn't get a virus from the download.  I downloaded the maps on Sat and got a nasty virus last night.  Anyone else have any problems with the free maps?
You can be 100% sure you didn't get a virus from the GA TOPO map from this site. I just ran the d/l through Nortons and it show as fine.

maps4gps

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Re: free garmin download accuracy?
« Reply #10 on: February 01, 2010, 01:27:55 PM »
I have read that the thinest line which can be used on a printed map is 1/100 inch.
On a 100k scale map, that represents about 80ft.
I have not found a statement on how close to the exact location a line can be placed.

Boyd

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Re: free garmin download accuracy?
« Reply #11 on: February 01, 2010, 01:40:38 PM »
My understanding is also that Garmin's map format only supports a maximum of 24 bit coordinates, and that limits the accuracy of any map to +/- 5 meters.

maps4gps

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Re: free garmin download accuracy?
« Reply #12 on: February 01, 2010, 02:56:18 PM »
Is my math that bad?  :)

On a sphere, 360 degrees divided by 2^24 is about 7.8 feet at the equator.  This decreases by the cosine of the latitude to zero at the poles.
Unless Garmin needed an extra bit in the north-south direction, 180 degrees divided by 2^24 is about 3.9 feet.
At 60 degrees north or south the two would be about equal (about 3.9 feet).
I have never been able to figure out how the numbers in the cgpsmapper manual can be obtained.

 

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