How accurate are Latitude and Longitude values in Google Maps?

Locations are quite "off" when used with a GPS receiver on actual ground. GPS receiver (Garmin) specifications are within 15m accuracy. So which is more accurate, Google Map co-ordinates, or my GPS receiver?How accurate are Latitude and Longitude values in Google Maps?
Latitude and Longitude co-ordinates are extremely accurate, but are dependent on how precise the figure you use. The Global Positioning System uses this co-ordinate system as it is the only precise system available. The numeric information that a user sees while using a GPS will differ slightly from actual co-ordinates as the information after the number of degrees (in both latitude and longitude) is arranged and calculated differently. Remember a GPS co-ordinate, regardless of system, uses 50掳 north (latitude) and 4掳 west (longitude) for Plymouth England. The differences come out when the remainder is expressed in terms the computers can use.



A full set of precise co-ordinates for Plymouth, England look like: 50掳 22' 17.03" north and 4掳 08' 32.75" west. Looking at Latitude, when 22 minutes (which is based on a scale of 60) is changed to a digital decimal based on 100 it becomes .36666667. Now 17 seconds needs to be changed as well, but the 17 is based on a scale of 3600 (60X60), and now becomes .004722222, so in order to change this figure to a base of 100, we would write 4.72" (or 5 seconds) for easy use we write it as 50掳 37' 5". Now before we go any further we know that 5 is not 4.72 and 37 is not 36.67, this is the key to the differences. Using a way to display co-ordinates that a person can easily see and use, is quite different than the almost instant calculation computers can use when calculating digits. And we did not even use the .03" which would be 60X60X60, which would probably pinpoint a freckle on your nose. A computer is capable of this precise procedure, hence the dawn of Global Positioning. For human use, a map and approximate co-ordinates are fine, a tourist does not need to find a freckle, but it is possible when computers use the Geographic co-ordinate system combined with a device (GPS) which can interpret numbers a human can't.



Hope I have been able to explain it to you





SD
The numerical management answer above is all very well for working with the received data but takes no accoount of errors in the signal as received.

The mathematics is based on the travelling-time differences in the signals from several satellites that your GPS unit may be a able to receive signals from in a particular location.

It is assumed that the signals are travelling through a standard atmosphere which has specified pressure density gradients, and compensations are made for the various angles at which the signals arrive, and in some units, for the temperature and measured air pressure for which a thermometer and barometer has to be built into the unit or manual inputs are made from external measurements.

Radio waves are electromagnetic waves just as light is, and when the density of the medium they travel through changes the speed of the wave changes.

Light only has a constant velocity in a vaccuum.

Light passing through a lens is slowed more by denser glass and by the thicker parts of the curved lens elements than the thinner parts. Slowing the light by having smoothly changing thicknesses of glass, that is, by having smoothly polished curved lens surfaces, is what makes the light bend and form an image through the lens.

In the atmosphere the effect causes stars to twinkle amongst other causes, and mirages to form by the bending of light through thicker and thinner layers of air, and the Sun to appear to be setting when it has already set. That effect, called astronomical refraction, is allowed for in precise tables of sunrise and sunset times, and the actual altitude of stars also varies from their observed altitude and must be corrected for from tables calculated for it or these days by a convenient computer algorithm as on here

http://www.google.com/search?q=observed+鈥?/a> . . . .

The number of factors involved can be seen here...quite a lot.

http://aa.usno.navy.mil/data/docs/celnav鈥?/a> . . . . . . . .

You can be 120 miles out easily by ignoring it, a fact known well before the twentieth century.

http://books.google.com/books?id=8InOAAA鈥?/a>



All that of course applies equally to very high frequency radio waves, which behave very much like light.

Low frequency radio waves behave differently than very high frequency ones as far as radio reception is concerned, which is why FM broadcasting on VHF is a short range signal but MW and LW signals can go round the world easily.

Long range VHF is possible during solar storms. South African TV has been picked up in England.

Expensive FM tuners like my Kenwood 6005 and 8005 have a multipath output and using it means you can detect signals reflected from buliding etc arriving slightly later than the direct signal, introducing phase distortions and spoiling the quality of sound, and you move the aerial until no multipath distortion is detected.



Now the upshott of that is that there is no point manipulating figures to twelve places of decimals if the signal is no more accurate than six places of decimals and it also means there is there is no such thing as a particular accuracy of a particular GPS unit.

The posh approach is to give confidence limits to your results at the time you get them.

95% confident the result is accurate to within 10 meters

70% confident the result is accurate to within 5 meters

10% confident the result is accurate to within 1 meter

and so on

The limits vary with weather, (density and depth of cloud cover, rain etc ), your particular location compared to the positions of the satellites observable, your altitude above sea level, the stabilty of the circuitry in your own GPS unit,(whether it has automatic compensation for temperature and pressure differences for the signals, temperature drift compensation for the circuits etc), the software algorithms used to reduce the time data to positional data, and probably more I've never even heard of.

Scroll down to navigation error sources on here

http://www.agi.com/downloads/support/pro鈥?/a> . . . . . .

You can only know the truth of your unit's accuracy on any day by having a standard to compare your readings to and that standard can only be by dead reckoning ....except for one line on Earth for longitude and one place on Earth for latitude.

At Greenwich is the meridian line. It is known to within a foot for many places due north and south of Greenwich.

Everywhere else on Earth has to be measured to find out where it is in relation to that line....that is, what longitude it has.

In Equador is the equator mark. That's where it was measured by Charles de la Condomine and his team, now known poor soul not for his explorations and the huge scientifc haul he got through but for condoms. He sent the first samples of rubber to Paris from Equador after completing the measurement of where the equator is and the length of one degree of it.

http://uk.answers.yahoo.com/question/ind鈥?/a> . . . . . . .

On Ordance survey maps you will find trig points marked. They are very accurately measured and the corordinates can be read from the map on the lat/long scales.

Even they are now surpassed by the best GPS sytems and control data for them is no longer charged for.

http://benchmarks.ordnancesurvey.co.uk/



Here is a calculator and a method for converting lat/long differences to feet or meters

http://bytes.com/topic/visual-basic-net/鈥?/a> . . . . . .

http://bytes.com/forum/thread368097.html . . . . . .

Find a place to measure on Google Earth, measure it with your GPS, and do the conversions.

Measuring my garden at the four corners using Google Earth showed a total inccauracy of less than 0.5" of arc from the four measurements, getting to within twenty inches in forty three feet which means any pair of corner points must have been identified to better than 0.13 seconds of arc in accuracy for the distances to have tied up so well with reality.

I make astronomical telescopes and measure variable stars and double star position angles etc. Getting 0.1" accuracy in a telescopic observation using very good equipment is a dream. We do a dozen measurements and average them to get near that.That is pretty good.

http://www.maps-gps-info.com/gps-accurac鈥?/a> . . . . . . .How accurate are Latitude and Longitude values in Google Maps?
Excellent question.



I recommend you do a little test yourself to find out!



1. Visit this site to find a land survey marker near you: http://www.waymarking.com/default.aspx?f鈥?/a> that has a high-precision value for it's position. This would be a position computed by a surveyor or geodesist.

2. With your GPS set to WGS 84 datum, go there and let your receiver spend a while.

4. Find the survey mark on Google Earth.

5. Compare the GPS and Google Earth values to the published values.



Make sure you're getting the best possible observations of a location by having an unobstructed sky, and letting the receiver lie still for at least 20 minutes. Avoid blocking the sky with your body. If you can download the tracks to your computer, you should be able to see slight changes in the position over time.



If you don't like the idea of doing that, here's something that might help. In a surveying class, we used Google Earth to compare values by entering the high-precision position for a survey marker near my school. When Google Earth settled on the spot, it was about a meter from the actual location, which was easily visible in the photograph. Have fun.How accurate are Latitude and Longitude values in Google Maps?
pretty acurate