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GPS Street Address Accuracy
Richkut
0 Points
Admin Note:
We have an article titled How to Correct a GPS Map Error if you want to go about fixing issues you have seen.
I have read dozens of pages about the pros and cons of the NUVI 765T--all understood. However, in all of my reading I have not ONCE ever read a thread concerning the ACCURACY of this GPS!! Since it does not use WAAS, many people that I talk to claim that it could be rather inaccurate. One colleague told me that his Streetpilot 2XXX series gets him right to his driveway, whereas a new NUVI 7XX? that he purchased for his mother only has accuracy to three houses away! From personal experience, my Garmin iQue3600 sometimes gets me to within a house or two (with WAAS), but sometimes only is accurate to a block away!! Now I don't know what is important to all of you (bells, whistles or more useful things such as lane guidance and traffic reports), but shouldn't a GPS be as accurate as possible?!! While my iQue 3600 sometimes notes an accuracy of 10' or less, in reality this is not true (for it is more like 50', which for most situations is good enough unless I was driving blindly on a bridge overlooking the ocean, LOL!!). So folks, what has been your experience not only with the 765T's CLAIMED accuracy (on the xcreen) but with actual accuracy as seen out of your car window? Thanks in advance, as your hoest answers will help me to pare down my next GPS purchase decisions.
Rich
We have an article titled How to Correct a GPS Map Error if you want to go about fixing issues you have seen.
I have read dozens of pages about the pros and cons of the NUVI 765T--all understood. However, in all of my reading I have not ONCE ever read a thread concerning the ACCURACY of this GPS!! Since it does not use WAAS, many people that I talk to claim that it could be rather inaccurate. One colleague told me that his Streetpilot 2XXX series gets him right to his driveway, whereas a new NUVI 7XX? that he purchased for his mother only has accuracy to three houses away! From personal experience, my Garmin iQue3600 sometimes gets me to within a house or two (with WAAS), but sometimes only is accurate to a block away!! Now I don't know what is important to all of you (bells, whistles or more useful things such as lane guidance and traffic reports), but shouldn't a GPS be as accurate as possible?!! While my iQue 3600 sometimes notes an accuracy of 10' or less, in reality this is not true (for it is more like 50', which for most situations is good enough unless I was driving blindly on a bridge overlooking the ocean, LOL!!). So folks, what has been your experience not only with the 765T's CLAIMED accuracy (on the xcreen) but with actual accuracy as seen out of your car window? Thanks in advance, as your hoest answers will help me to pare down my next GPS purchase decisions.
Rich
Comments
The position your GPS calculates you at... the raw latitude and longitude will be accurate to within 10 meters, 95% of the time. Sounds pretty good, right? And if you were to add WAAS the signal will be accurate to about 5 meters, 95% of the time. Even better, right?
Well... no. See what you are talking about is the accuracy of the map, not the accuracy of the GPS signal. And the accuracy of the map is nowhere near the accuracy of the GPS signal. When we consider where the street was mapped versus where it actually is it isn't uncommon for the map to be off by 100 feet or so.
When it comes to the accuracy of a street address, say "500 Main Street" it can be off even further... sometimes by 1,000 feet and sometimes by even more.
The map will be off to a further amount than the GPS signal will be, so the accuracy of the GPS's calculated position becomes almost irrelevant. 5 meters or 10 meters off will not make any difference in the performance of your street GPS as the weakest link is not the calculated GPS position, but rather than the map.
So now let's talk about maps. For the GPS devices sold in North America, all use maps from one of two companies, NAVTEQ and Tele Atlas. Both will have neighborhoods that are mapped well, and both will have neighborhoods that are poorly mapped or missed completely.
So the conversation about "accuracy" almost becomes a mute point. The signal the GPS will calculate is as accurate as it needs to be, based on the weaker accuracy of the map.
And getting back to WAAS for a second, you might now see why it isn't important at all in a street GPS as the map will be a weaker link. WAAS drains the battery faster and therefore WAAS is disabled in most street based GPS devices since it provides no practical benefit and will just drain the battery faster.
Rich
So on every Garmin device I have, as well as any Magellan as they too use NAVTEQ maps, it shows my driveway about 200 feet away when I'm parked there. On any device that uses Tele Atlas maps (TomTom, Mio, etc) my house will show about 600 feet away from where it should be mapped.
There will be some variance between the devices. Given that one device could be off by roughly 10 meters in one direction and another device could be off by 10 meters in the other direction they could show a difference of about 60 feet from each other, provided they are using the same map vendor and version.
So even if the GPS was "perfect" at calculating my postion (no error)-- it would still show me as 200 feet away from Home, even on the NAVTEQ map as the map itself is where the greater deficiency is.
But to get blunt and to the root of the question, I wouldn't have any hesitation going with the 765T over some of the older STP or iQue models with regards to "accuracy". The 756T will have a newer map and therefore hopefully a more accurate map as well.
Truth is, almost any of the modern units will be "good enough" for street navigation.
Should make this a sticky and change the topic name.
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GoogleMaps/GPS Map O-------------------------------O Correct Location
Rich
On the other hand, our Garmin 755T, using Navteq maps, puts our house right in the middle of the intersection (we have a corner lot) at all zoom levels. However on the route home, Garmin's last turn instruction - both visual and voice - is "turn left on xxxxx street," which is correct, and then announces and shows our home on the left side of the street, which is also correct. So Garmin and/or Navteq may have correctly mapped our home, but choose to display the Home icon in the street. What does that say about GPS accuracy?
It is true that many addresses will be mapped a few houses off or even several hundred feet off. The solution, if you have the time, is to stand in the driveway, or wherever you feel is appropriate, and mark that location as your home or your destination. Of course you have to be there first. :lol: But if it's somewhere you visit often it could be worth doing.
Another anomaly when looking at the satellite info screen when comparing two 765t's side by side (my girlfriend's and mine), is which satellites are picked up (they are slightly different -- not only which satellites, but how many). I suspect this has to do to where each unit was turned on and working the last time it found signals (we live around 20 miles apart). Looking at them both right now, my unit shows 7 satellites (plus one blinking), and the other shows 5 satellites -- one just barely (plus two blinking). Accuracy is also slightly different.
Of course, my 765t can also do some strange things. While driving down the freeway, it sometimes "forgets" where I am, or thinks I'm suddenly on an adjacent road, then goes into recalculation mode (it corrects rapidly though). Doesn't happen often, but I took it to mean a combination of map accuracy at that certain area and perhaps temporarily having a minimum of satellites locked on. Where this happens often is at one particular freeway interchange (270 degree loop, where it can recalculate as many as three times before I'm out of the loop).
I just tried google maps now, and it now finds my address.... it doesn't flag the position of the driveway at the road, but it comes pretty close to the location of my actual house which is some 900 feet off in the woods! Obviously Teleatlas is doing their homework. :)
I had guessed that the satellite data (accuracy within so many feet) wasn't without a liberal tolerance. When I see the number of feet, I also assume that is an estimated radius. Anything within 50 feet is pretty darn good in my book considering all the variables. It's only when accuracy gets to be a couple hundred feet or greater do I anticipate my Garmin thinking I may be on an adjacent road :).
What amazed me the most is the accuracy of the speed sign locations. Going from say a 50 to 35mph zone, the unit will change to the new speed limit on-screen within 20 feet after you pass the sign.
I remember reading that the census canvassers have an enhanced software they should be using this go-round. Wonder if that is being logged for government use.
Now actually including that amount of data within the mapset might push the size beyond what we can currently use, but with a detail trade-off (even higher compression) maybe could work. I don't have any idea how much something like that adds to data size.
Google Maps used to do that, but now has plots divided in my neighborhood (and most of my city) so it goes right to my address. I think their servers might have more storage space than my 2610, though. :)
So each stretch of road in the map knows its address range, but not the actual addresses. That would inflate the database to about 30 or 40 times as many "address" records as it has now, needing to flag smaller and smaller segments of streets rather than just segments between intersections.
Interpolation might be made more accurate without recording each and every plot if maybe the map contained the max address of each segment, rather than assuming XX99.
First I am a GIS cooordinator and work almost exclusively with street centreline files...the same type that are used in GPS units, although I am in gov't so not affiliated with Navtek or other commercial mapping companies.
The street files most commonly use a block range for addressing. So each street segment (a segment being defined as one piece of street between two intersections) carries 4 addresses. A 'from' address and a 'to' address for each side of the street. So, to determine all addresses along that segment interpolation usually occurs. For instance, lets say your street segment has an address range from 100 to 200 (on one side) and 101 to 199 on the other side...odds on one side and evens on the other. if you search for 150 the GPS will put you halfway down the street. That may, in realiaty not be exactly where 150 is, but most algorithms search that way. Its really simple math. In reality it could be that all of the houses are crammed at one end of the street and if thats the case the error in interpolation will be greater.
A very good reason ranges are used are that it is that it is much easier to keep up-to-date. If individual points for each address were used it would be a nightmare to update every time a new house was built along a street.
Hope that helps a bit.
Are you involved with updating the TIGER files perhaps? I make my own maps, and it sometimes surprises me how many short line segments are used within a street.
Guess that definition of a segment adds another variable to it if it is curved roads or circles...
always thought they used from one corner to the other corner as the min and max for a segment, but in our case our street is named a circle that is more like a regular block, i.e. a U... we are near the mid point of the bottom of the U...
given that, I assume the min and max is from the 'top' of either side of the U and it guesstimates based on house addresses how far around the circle we are....
How about that - just looked @ google for the first time in a while and it now puts our arrow away from the street, very close to the middle of our back yard :) but at least in the correct address now !
As for your observation on short line segments, there could be a number of reasons for short line segments. (By shorter line segments I assume you mean a segment that is broken between intersections.)
It depends entirely on the data set you are looking at and how it was designed, but they could be broken more often for a nmber of reasons. Possibly a change in road surface, a municipal (or some other boundary) line, change in speed of the road, etc, or often for no apparent reason at all.
So the low odd and even address for that street are at the start point and the high odd and even address for the street are at the end point of that segment.
Given that, where your house is placed along the street depends entirely on the low and high address for that segment and the length of the segment. As I said before, very simple math, which I assume the GPS processor performs on the fly. It shouldn't never vary unless the map changes.
Don't confuse this with where it places YOU when you are looking at the live GPS point...meaning your car. THAT can vary depending on how accurate the GPS receiver is feeling at any particular time. And don't ask me about GPS accuracy, I deal mostly with the mapping end, not GPS technology.
Anyway, going to my address in Google Maps goes to my lot, a corner lot at the high end of the address range for my block, 4035. Any other search system I have to put 4099 to get to the end of the street.
Basically, most mapping databases we have these days don't have actual physical addresses mapped, only the range on any given block. This is the 1500 block, so it has 1500-1598 on one side, and 1501-1599 on the other. Even if the block only had 7 houses on it, you'd have to be at 1580-something to get to the high end of the block.
But what they are doing is far different than what you see in a Garmin or Tom Tom, the data and technology.
To avoid these problems as much as possible we do errands without the camper. All routing is done on the computer and then the route(s) or trip(s) are loaded into the GPS. Questions on turns are viewed in Goggle Earth.