Garmin Gps Position Error
Contents |
they see the altitude readout at a fixed point vary by many hundreds of feet. This is NORMAL. With most low cost GPS receivers, the horizontal error (without SA now that it is off) is
Gps Garmin
specified to be within about +/- 15 meters (50 feet) 95% of the time. Most how gps works users find this is a conservative specification and that their modern GPS receivers routinely perform better than this worst case specification. But..
What Is Gps
Users should expect that SOMETIMES they may see the error approach the specification limits. AND.. 5% of the time, the error may be "any value" from zero to whatever". Note: Unless you have a CLEAR AND gps wiki UNOBSTRUCTED view of the sky (on your dash or looking out of an airplane window with no externally mounted antenna, or similar obstructed view does not count!) you can count on your error excursions to be much greater than the above numbers. Your GPS
particular value. For instance, If you are told that your GPS position measurement is accurate to 10 meters CEP, this means that there is a 50% probability that your measurement lies INSIDE a circle with a radius of 10 meters.
Gps Satellite
This also means that there is a 50% probability that your measurement lies OUTSIDE the 10 what is gprs meter radius circle! Should you be told that your GPS measurement is accurate to within 25 meters (95% confidence), This means that you can
Indian Gps
be 95% sure that your measurement is somewhere within a 25 meter error circle and there is a 5% chance that the error is LARGER than 25 meters. Relationships between some common measurement notations (from Sam Wormley) sqr(alpha) Probability http://gpsinformation.net/main/altitude.htm Notation ----------------------------------------------------------------------- 1.00 39.4% 1-sigma or standard ellipse 1.18 50.0% Circular Error Probable (CEP) 1.414 63.2% Distance RMS (DRMS) 2.00 86.5% 2 sigma ellipse 2.45 95.0% 95% confidence level 2.818 98.2% 2DRMS 3.00 98.9% 3 sigma ellipse ------------------------------------------------------------------------ What if you want a perfectly precise GPS measurement? From an engineering standpoint, there is no such thing. There is always a "probability of error" in -any- measurement. With present GPS technology and expensive survey grade GPS instruments and by post processing http://gpsinformation.net/main/errors.htm field measurements, it is possible to achieve SUB-CENTIMETER measurement accuracy. This kind of accuracy is not possible with "inexpensive consumer GPS receivers". You can find information about the achievable position and altitude accuracy of a "common consumer GPS receiver" with and without DGPS now that SA is turned OFF with reference to David Wilson's article CLICK HERE. But then: What does EPE mean on my GPS? If EPE reads 10 feet, then I am within 10 feet of the actual lon/lat position readout on the GPS- Right??? DEFINITELY NOT!! EPE is generally an ESTIMATE OF POSITION ERROR and not a GUARANTEE of maximum position error. In fact, Garmin's EPE readout is generally accepted to indicate that there is an EQUAL probability that the error is GREATER or LESS THAN the indicated EPE. This is the 50% CEP value given above. As shown above, to be 95% confident that your measurement is within a circle of a fixed radius, you would have to multiply Garmin's EPE value by two. To be 98.9% sure that your measurement is within a circle of a fixed radius, you would have to multiply Garmin's EPE value by about 2.55. Magellan's EPE numbers appear to be even more optimistic (maybe the 1 sigma value or even lower) while Lowrance seems to be someplace between the RMS and 2 sigma values. In short. Your GPS's EPE readout is just a "figure of merit
Earth I - Proprietary software and EasyGPS Importing GPS Data Into Google Earth II - GPSBabel and GPSVisualizer » Why Are My GPS Positions In http://freegeographytools.com/2007/why-are-my-gps-positions-in-the-wrong-place-in-google-earth The Wrong Place In Google Earth? Published by Leszek Pawlowicz in Google http://gis.stackexchange.com/questions/43617/what-is-the-maximum-theoretical-accuracy-of-gps Earth and GPS. 3Comments If you import data from a GPS unit into Google Earth, you may notice a mismatch between the plotted position of a waypoint or path and the actual position. Here's an example of that from my neighborhood: The flag marker is where Google Earth has plotted what is a GPS waypoint imported as a GPX file, while the drawn line that starts at that marker ends at the actual position where the GPS position was taken. The position plotted by Google Earth is about 30 feet off from the actual position. How come? There are at least two reasons: 1. GPS Error - Even though GPS units give positions to garmin gps position a precision eight decimal places in latitude and longitude, corresponding to an accuracy of about a millimeter or so, the true accuracy with which you can measure a position with GPS is far poorer than that. Consumer GPS unit manufacturers commonly quote an position accuracy figure of plus or minus 15 meters; while modern units can usually achieve better accuracy in real-world use, there is still likely to be some error in the position you measure with GPS. I'll be covering techniques and tools for maximizing the accuracy of your GPS measurements in a future series of posts; suffice it to say that I took pains to get as accurate a measurement of that position with my GPS as possible. 2. The positions in Google Earth don't necessarily coincide with the correct position on Mother Earth. In my neighborhood, an exact geographic location shows up offset about 30 feet in Google Earth, or pretty much what we see in the picture above. So don't expect to use GPS with Google Earth (or Google Maps) for high-precision positioning and mapping - it's just not accurate enough
Badges sign up log in tour help Tour Start here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more about Stack Overflow the company Business Learn more about hiring developers or posting ads with us Geographic Information Systems Questions Tags Users Badges Unanswered Ask Question _ Geographic Information Systems Stack Exchange is a question and answer site for cartographers, geographers and GIS professionals. Join them; it only takes a minute: Sign up Here's how it works: Anybody can ask a question Anybody can answer The best answers are voted up and rise to the top What is the maximum Theoretical accuracy of GPS? up vote 26 down vote favorite 12 I was talking with a potential client, and they requested that we plot some points with GPS, with a maximum (or should that be minimum?) accuracy of 2 m. This is an area with no WAAS, and I was under the impression that even in the best of conditions, a single gps point can be accurate up-to only 15 meters(Horizontal field). Is this correct? What is the maximum theoretical accuracy of GPS without using WAAS or differential GPS? gps accuracy share|improve this question asked Dec 10 '12 at 11:59 Devdatta Tengshe 25.5k1375168 2 What type of receivers will you be using? –R.K. Dec 10 '12 at 13:05 2 It is receiver dependent of course, see what a low end gps provides in this thread gis.stackexchange.com/questions/12011/… –user681 Dec 10 '12 at 13:27 I'm not interested in any specific instrument. I'm concerned with the Theoretical Accuracy in Typical Environmental conditions. –Devdatta Tengshe Dec 10 '12 at 13:42 @DanPatterson I think that particular instrument and readings are using WAAS, and are not pure Satellite based GPS positioning. –Devdatta Tengshe Dec 10 '12 at 13:45 2 Not WAAS enabled @Devdatta, 7 year old Garmin Etrexs. Sometimes stationarity at a site yields better results for any GPS. The link was just food for thought. –user681 Dec 10 '12 at 14:28 | show 3 more comments 5 Answers 5 active ol