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#1
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When we look at a GPS error value in a so-equipped GPS, does that value
reflect *both* the vertical (i.e. altitude) and the horizontal component? Can a separate value for each be determined? The reason I ask is that in firing up my Garmin 496 (an early Christmas present, comments to follow in another post ![]() initially register at 100 ft below actual then right itself after a few minutes. All of this happened while the horizontal location did not noticeably move (had it fully-zoom by the way) which led me to believe that the error can be different from each other at least initially. Marco |
#2
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![]() "Marco Leon" wrote in message ups.com... When we look at a GPS error value in a so-equipped GPS, does that value reflect *both* the vertical (i.e. altitude) and the horizontal component? Can a separate value for each be determined? The reason I ask is that in firing up my Garmin 496 (an early Christmas present, comments to follow in another post ![]() initially register at 100 ft below actual then right itself after a few minutes. All of this happened while the horizontal location did not noticeably move (had it fully-zoom by the way) which led me to believe that the error can be different from each other at least initially. Marco The two errors are separate. So if they spec 100 ft +/- it could be 100ft in the horizontal plane and 100ft in the vertical plane. |
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Marco Leon writes:
When we look at a GPS error value in a so-equipped GPS, does that value reflect *both* the vertical (i.e. altitude) and the horizontal component? No. The vertical error is usually much greater than the horizontal error. The displayed error, if there is only one, will generally be the horizontal error. Can a separate value for each be determined? Yes, but it's only practical for the computer to do it. The reason I ask is that in firing up my Garmin 496 (an early Christmas present, comments to follow in another post ![]() initially register at 100 ft below actual then right itself after a few minutes. All of this happened while the horizontal location did not noticeably move (had it fully-zoom by the way) which led me to believe that the error can be different from each other at least initially. Initially and continually. GPS altitudes should be treated as general indications, not as precise altitudes that can be substituted for altimeter readings. -- Transpose mxsmanic and gmail to reach me by e-mail. |
#4
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"Marco Leon" wrote in message
ups.com... When we look at a GPS error value in a so-equipped GPS, does that value reflect *both* the vertical (i.e. altitude) and the horizontal component? Can a separate value for each be determined? The reason I ask is that in firing up my Garmin 496 (an early Christmas present, comments to follow in another post ![]() initially register at 100 ft below actual then right itself after a few minutes. All of this happened while the horizontal location did not noticeably move (had it fully-zoom by the way) which led me to believe that the error can be different from each other at least initially. Marco They have separate error characteristics and separate error indicators - although they are dependent. Typically, if a GPS unit displays any kind of error value, it is for the horizontal component simply because that is what most users will want to see. It is most likely an interpretation of the statistical error in units of meters or feet. The statistical error is called dilution of position and most GPS receivers determine an HDOP, a VDOP, and a couple of others. -- ------------------------------- Travis Lake N3094P PWK |
#5
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"Travis Marlatte" wrote:
"Marco Leon" wrote in message oups.com... When we look at a GPS error value in a so-equipped GPS, does that value reflect *both* the vertical (i.e. altitude) and the horizontal component? Can a separate value for each be determined? The reason I ask is that in firing up my Garmin 496 (an early Christmas present, comments to follow in another post ![]() initially register at 100 ft below actual then right itself after a few minutes. All of this happened while the horizontal location did not noticeably move (had it fully-zoom by the way) which led me to believe that the error can be different from each other at least initially. Marco They have separate error characteristics and separate error indicators - although they are dependent. Typically, if a GPS unit displays any kind of error value, it is for the horizontal component simply because that is what most users will want to see. It is most likely an interpretation of the statistical error in units of meters or feet. The statistical error is called dilution of position and most GPS receivers determine an HDOP, a VDOP, and a couple of others. --------------------- Travis Lake N3094P PWK Actually the DOPs are related to satellite geometry and are unitless. It is only when you multiply the DOP by the signal/user equipment error that you get some sort of error in feet/meters. Ron Lee |
#6
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"Ron Lee" wrote in message
... "Travis Marlatte" wrote: "Marco Leon" wrote in message roups.com... When we look at a GPS error value in a so-equipped GPS, does that value reflect *both* the vertical (i.e. altitude) and the horizontal component? Can a separate value for each be determined? The reason I ask is that in firing up my Garmin 496 (an early Christmas present, comments to follow in another post ![]() initially register at 100 ft below actual then right itself after a few minutes. All of this happened while the horizontal location did not noticeably move (had it fully-zoom by the way) which led me to believe that the error can be different from each other at least initially. Marco They have separate error characteristics and separate error indicators - although they are dependent. Typically, if a GPS unit displays any kind of error value, it is for the horizontal component simply because that is what most users will want to see. It is most likely an interpretation of the statistical error in units of meters or feet. The statistical error is called dilution of position and most GPS receivers determine an HDOP, a VDOP, and a couple of others. --------------------- Travis Lake N3094P PWK Actually the DOPs are related to satellite geometry and are unitless. It is only when you multiply the DOP by the signal/user equipment error that you get some sort of error in feet/meters. Ron Lee The DOPs represent a statistical confidence. They do have a unit but you're right, it's not in units of distance and it is not standardized. Does the 496 display a value for the GPS error? Or, was it a hypothetical question. If a GPS receiver does indicate the accuracy with something like "+/- 5 meters" then it is really expressing a confidence and is more correctly stated as "+/- 5 meters, 95% of the time" Or whatever confidence they think looks the best to their consumer. -- ------------------------------- Travis Lake N3094P PWK |
#7
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Travis Marlatte wrote:
"Ron Lee" wrote in message ... Actually the DOPs are related to satellite geometry and are unitless. It is only when you multiply the DOP by the signal/user equipment error that you get some sort of error in feet/meters. The DOPs represent a statistical confidence. They do have a unit but you're right, it's not in units of distance and it is not standardized. No, Ron is correct that the various DOPs (PDOP, VDOP, HDOP, TDOP, etc.) are unit-less measures based on the satellite geometry. They are well-defined and standardized as specified in Section 3.1.3 of the Navstar User Equipment guide: http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf It's when the DOP, typically the HDOP, is translated into a corresponding EPE (Estimated Position Error) or accuracy value that it is converted into a measure with units of distance and these measures are not standardized across manufacturers or even sometimes between models of a given manufacturer. I agree with some previous comments that the VDOP is generally somewhat larger than HDOP due to the inherent geometry of the situation. You usually have satellites on all sides of your position, but you can only get signals from satellites above you, none below, and that reduces the accuracy of the altitude calculation. An additional consideration in the OP's case is that when the unit first gets enough satellite signals to report a position it may only have enough for a '2D' calculation. With three satellite signals, the unit can only determine the exact time and two space coordinates, so it assumes the altitude based on the last value previously seen and only calculates the lat. and long. coordinates. Then when it gets one or more additional satellite signals it can switch to '3D' mode and calculate the actual altitude as well. |
#8
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Marco,
When we look at a GPS error value in a so-equipped GPS, does that value reflect *both* the vertical (i.e. altitude) and the horizontal component? Can a separate value for each be determined? The error values commonly given a VDOP: Vertical Dilution of Position HDOP: Horizontal Dilution of Position PDOP: 3-D dilution of position, i.e. the above two combined Most GPS receivers show these values. Google and Wikipedia are your friends... -- Thomas Borchert (EDDH) |
#9
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On 13 Dec 2006 11:40:00 -0800, "Marco Leon" wrote:
When we look at a GPS error value in a so-equipped GPS, does that value reflect *both* the vertical (i.e. altitude) and the horizontal component? Can a separate value for each be determined? The reason I ask is that in firing up my Garmin 496 (an early Christmas present, comments to follow in another post ![]() initially register at 100 ft below actual then right itself after a few minutes. All of this happened while the horizontal location did not noticeably move (had it fully-zoom by the way) which led me to believe that the error can be different from each other at least initially. Marco You can try to improve vertical accuracy using calculations from one of the following websites but you need to know if your GPS is already correcting or not! http://sps.unavco.org/geoid/ http://earth-info.nga.mil/GandG/wgs8...0/intptW.html? |
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