Roughtly converting DOP to metric error

Hi,

My cave survey freeware connects to a GPS for surface tracking. When I
connect to a Bluetooth GPS, the GGA and GSA NMEA sentences provide
various flavors of DOP values (PDOP, HDOP, VDOP). How can I provide
users with a rough error value in meters (as most GPS units do) from
these unitless numbers?


Luc Le Blanc
http://www.speleo.qc.ca/Auriga

DOP is a multiplicative factor, and is used to increase metric errors
calculated by other means; so to evaluate it's effect you would need to know
the reported metric error and whether or not the DOP was already applied to
it. This is a pretty simplistic answer; I'm sure others will weigh in with
more detailed answers :-)

John

http://users.erols.com/dlwilson/gpshdop.htm


John

speleoluc@gmail.com wrote:

See: http://edu-observatory.org/gps/gps_accuracy.html

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.

I found such info pages googling for a simple answer, but I can't help
but think that when I turn on my GPS 76, I don't have to input various
parameters to get a position error in meters. Can't I do the same
thing from the NMEA data I get from the GPS, or is the GPS basing its
precision estimation on data the DOP value doesn't convey?


Luc Le Blanc