Now decorrelate the GPGLL sentences from the 4-hr test by decimating to
30- or 60-second intervals. Decorrelation really does help.
Average, then use a least-squares method to determine residual error.
I don't use choke-rings or ground planes when doing kinematic work. We
do make sure the antenna is aligned to N when we do it, though, as
pattern will affect results.
Finnegan's Finagiling Factor (also called "bias" in literature) has
saved my butt a bunch of times in the field. I do a version of that in
post-processing, too, but the whole procedure is best described in a
journal article or over a beer. Or both.
Richard Polivka, N6NKO wrote:
Alright, here are the details of the equipment:
- About 7' of 1.25" PVC pipe with a hole through it for a rubber band to
hold onto GPS unit
- US GlobalSat BT-359 Bluetooth GPS receiver - chipset: SiRF STAR-III/LP
- Cingular 8525 smartphone
- "Turbo GPS" software Ver 2.00 alpha 8
Did testing in NMEA mode. SiRF mode has the longitude really messed up.
I think I will hit up a couple of BM's and see how bad the data is.
Maybe it may be recoverable by applying Finnegan's Fudge Factor.
Trying to resolve a garbage can using just L1 and a consumer grade GPS
will just give you garbage when you have only ddmm.mmmm to use. I went
in knowing what the maximum resolution is: one count latitude = 0.2222
meters; one count longitude = 0.1630 meters. I was willing to spot the
rig +/- five counts moving so I would say that I made my self-imposed
limit. I thought that it would be worse.
Using a 2m range pole with a surgical sharp pick at the end will do your
feet some damage. You probably have a outrageously overpriced choke ring
antenna on top and that in itself is a real headknocker in its own
right. I did a test with this setup here at home one day over four
hours. I pulled out the NMEA $GPGLL sentences that were at PDOP= 0.6 and
mapped them out. The resulting x-y graph presented an almost circular
pattern of a diameter of 8 counts. Averaging the readings put the
average right in the middle of the chart. I am quite happy with this
mashup. Not professional grade but it will do the jobs that I need it to
do.
Have a good week. Now to pull up a property survey to figure out an
issue for someone.
Best Regards,
Richard, N6NKO
Gerry Creager wrote:
Richard Polivka, N6NKO wrote:
Gerry,
Yes, the coffee has kicked in and I had egg-on-face for breakfast.
The map in QGIS looks just like the same as the aforementioned area
when at full extent view. Zoomed in, it works.
The egg comment has to do with a Census screwup... It's Census, it's
gonna happen. Their view of GIS has been "interesting" for some time.
I am glad you got it to work!
Error budget - Well, I am within 5 ft on backtracked trails. Can't
complain for working under a tree canopy.
1.8m isn't bad, at all! Now: Imagine what happens when your typical
error is 2-3 meters, and a picnic table is smaller in at least one
dimension is smaller than the error budget. The geometry is, well,
interesting, and usually looks more like a bow-tie than a rectangle.
Getting several points to define the radius of a trash can gets even
more entertaining.
I had the GPS unit on top of a 7 ft PVC pipe talking to my smartphone
over bluetooth. I just ran out of time to cover the whole place but
the proof of concept pans out.
When I go do this stuff, I use a 2m "range" pole. a fixed height
fiberglas and metal deal designed to poke holes in shoes and give one
a good feel that their antenna is exactly this high above the point in
question. We also have a bulls-eye level on it to allow us to get it
spot-on above the point in question.
Your rig sounds easier to work with.
gerry
Now its off to other work tasks.
73 from 807,
Richard, N6NKO
Gerry Creager wrote:
Richard Polivka, N6NKO wrote:
Gerry,
Good morning!
I realize that wholeheartedly, but to have this big of a
screwup....oh, wait, this is the Federal Government....
Aha! Your coffee kicked in? What hardware are you using for your
campground survey? I can tell you stories about those from several
years ago, when folks were trying to resolve objects (e.g., a picnic
table) that was smaller than their uncorrected L1 error budget...
gerry
Gerry Creager wrote:
Richard Polivka, N6NKO wrote:
I admit that one point does not make a trend but what I found was
eyeopening.
I wanted just Fond du Lac county in Wisconsin to use as a base
map for mapping project that I am working on ( a Boy Scout
campground). I pulled in my files into QGIS and then brought in
"fe_2007_55039_edges". I did not get Fond du Lac County but a
huge overview of north Illinois, Indiana and SE Michigan. OUCH!!!
I even went and downloaded a virgin fileset and got the same
results (see picture).
55039.png
This is not good. I plan on sending on a comment to the Census
Bureau.
So, let the user beware - YMMV.
Unfortunately, this warning has always been necessary with TIGER
files. They're not the best GIS data source. They're just
available and free.
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Gerry Creager -- [EMAIL PROTECTED]
Texas Mesonet -- AATLT, Texas A&M University
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983
Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843
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