PDOP or GDOP

I would like to know in general which one is typically chosen to
indicate the quality of DOP and GPS accuracy, PDOP or GDOP?

 

http://www.Colorado.EDU/geography/gcraft/notes/gps/gps_f.html


PDOP = Position Dilution of Precision (3-D), sometimes the Spherical DOP.
HDOP = Horizontal Dilution of Precision (Latitude, Longitude).
VDOP = Vertical Dilution of Precision (Height).
TDOP = Time Dilution of Precision (Time).

I use PDOP and HDOP

 

Amd it is probably worth mentioning the GDOP is a term that refers to all of
these--they are all geometry only estimates of relative precison.. So the
choice is never between PDOP and GDOP ) as in the orignal post question) as
PDOP is "one of the kinds of GDOP". (The webpage calls the above
"components" of GDOP but they are not components in the usual sense.)

 

Avoid NOJOP = Dilution of the economy (Money).

 

You can think of DOP along each coordinate axis, so that in a North-
East-Down reference frame there is a North DOP, an East DOP as well as
the vertical DOP.
HDOP = sqrt( northDop^2 + eastDOP^2)
PDOP = sqrt(HDOP^2 + VDOP^2)
Finally, GDOP = sqrt(PDOP^2 + TDOP^2)
So, use PDOP if you are interested in your receiver's position
dilution of precision, use GDOP if you are interested in accounting
for the Time dilution of precision as well. In my experience, most
people use PDOP or TDOP, but rarely GDOP.

 

You also need to be aware that it is possible to have a GPS unit
report a good DOP figure, small error ellipse values and still be some
10-20 km in error on its position. I have seen this with units
positioned around 50 deg N and 45 deg S where the sky view is severely
limited. In one case the receiver was on a windowsill in a tall
building with other buildings across the street. It just worked out
that with the signal reflections and satellite obscuration present,
the location algorithms got a 'good' solution for the position that
just happened to be wrong.

DOP != GPS position accuracy.

 

Estimated Position Error (EPE) and Error Sources

EPE (1-sigma) = HDOP * UERE (1-sigma) (1)

Multiplying the HDOP * UERE * 2 gives EPE (2drms) and is commonly taken as the 95% limit
for the magnitude of the horizontal error. The probability of horizontal error is within
an ellipse of radius 2drms ranges between 0.95 and 0.98 depending on the ratio of the
ellipse semi-axes. User Equivalent Range Error (UERE) is computed in the tables lower on
this page.

EPE (2drms) = 2 * HDOP * SQRT [URE^2 + UEE^2] (2)

HDOP (Horizontal Geometric Dilution of Precision), GDOP, PDOP and VDOP are determined by
the geometry of the current satellites visible above the receiver's mask angle with
respect to user receiver's antenna. DOPs can be degraded (made larger) by signal
obstruction due to terrain, foliage, building, vehicle structure, etc.

URE (User Range Error) is an estimate of "Signals in Space" errors, i.e., ephemeris data,
satellite clocks, ionospheric delay and tropospheric delay. These errors can be greatly
reduced by differential and multiple frequency techniques. Differential correction sources
include user provided reference stations, community base stations, governmental beacon
transmissions, FM sub-carrier transmissions and geosynchronous satellite transmissions.

UEE (User Equipment Errors) includes receiver noise, multipath, antenna orientation,
EMI/RFI. Receiver and antenna design can greatly reduce UEE error sources--usually at
substantial cost.

Position error can range from tens of meters (recreational) to a few millimeters (survey)
depending on equipment, signals and usage. Professional mapping and survey equipment often
includes user-settable minimum thresholds for SNR, mask angle, DOP, number of SVs used, etc.

 

In the cases that I was alluding to (IIRC) the reported position error
estimate from the GPS (I would rather not identify the brand as this
is irrelevant, save that it was from one of the industry major
players) was less than 10 metres and the DOP value was well within the
normal range for a good fix so it was not possible to declare the fix
invalid on either of these grounds. The reported position was stable
for a considerable period but between 10 and 20 km in error when
compared to the true position.

Ian

 

You can think of DOP along each coordinate axis, so that in a North-
East-Down reference frame there is a North DOP, an East DOP as well as
the vertical DOP.
HDOP = sqrt( northDop^2 + eastDOP^2)
PDOP = sqrt(HDOP^2 + VDOP^2)
Finally, GDOP = sqrt(PDOP^2 + TDOP^2)

I have not seen the last equation and it has a problem in that exactly in
what units of what measurement is it a Dilution of Precision? Most people
use GDOP as a general term for geometric dilution of precision of which the
other DOP's are types of.

 

DOP has no units. This confuses people sometimes, as you can become
accustomed to associating DOP with your navigation accuracy 1-for-1.
With that assumption, some might think DOP would have the units of
meters. Navigation accuracy actually takes into account the
pseudorange errors as well, which is where the length units come in.
See the Position Errors section here: http://blogs.agi.com/navigationAccuracy/?page_id=6
Ted

 

"Most people use GDOP as a general term for geometric dilution of
precision of which the other DOP's are types of. " I am actually looking
for DOP so I think GDOP will be my choice.

Thanks for all your replies.

 

No confusion here. Understood it has not units but it is meant to be a
dilution of precision due to satellite geometry and only so on a statistical
basis. GDOP is the general term for HDOP, PDOP, and TDOP but to consider
GDOP as a RMS of PDOP and TDOP yeilds a really useless quantity because it
does not measure the dilution of precision of any physical quantity.

 

Absolutely agree, which is why it is rarely used. Most of the GPS
references define it this way however.

 

Probably though worth noting that the SPS Specification defines DOP. HDOP,
PDOP, and TDOP but makes no reference to the acronym GDOP at all.