[Xastir] Tiger 2007 shapefile issue...a beaut

[Gerry Creager]

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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>>>>
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-- 
Gerry Creager -- gerry.creager at tamu.edu
Texas Mesonet -- AATLT, Texas A&M University	
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