On Sat, 16 Aug 2008, Jim Tolbert wrote:
Running back to the desire for DAO packets..... I am assuming that with
standard APRS packets having 2 decimal places and DAO having 3 decimal places
that I will be locating a particular GPS receiver to an area that is an order
of magnitude more precise. I am probably not saying this correctly, but
with the standard APRS packet, I have an area of uncertainty of about 20' x
40' ( in northern Wisconsin, US ( N45,W92)). By moving to a DAO packet, I
will have an area of uncertainty of about 2' x 4'.
If I have a bunch of GPS receivers of the same type, can I assume that the
relative location is pretty good, even if the absolute location may be off a
bit? I am really primarily interested in relative positioning.
It almost sounds like you're talking apples and oranges. Accuracy and
Precision are not synonyms.
DAO gives you another decimal place of precision, but that means
absolutely nothing if the GPS doesn't calculate that digit accurately.
...and as for your actual question re: Absolute or Relative positioning.
Several years ago, I had a need to survey a parcel of land. Rather than
pay the $15k the surveyor wanted, I though of making a 'poor man's DGPS'
by locating one receiver on the section corner post on one boundary of the
property, and placing another one on the lawnmower, and recording the log
of both on separate laptops. I then thought I could analyse the fixed GPS
log, locate the center of the wandering path, and generate a time-stamped
'error vector' file which I could then use to correct the lawn-mower's
data log, and generate a set of points which define the contour surface of
our property.
I proposed this scheme to Tom Clark one day at a Dayton Hamvention, and
he politely lauged at me. According to him, the GPS error vector isn't
constant even for identical GPS engines located close (<1/2 mile) to each
other, and furthermore, the firmware in the inexpensive GPS units I was
proposing to use is optimized for accuracy in lat/long, at the expense of
altitude, so I would never get even close to usable contour data.
--
Rick Green, N8BJX
"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."
-Benjamin Franklin
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