[wxqc] Coordinate system

[Gerry Creager]

I had the opportunity to discuss georegistration with the Google 
Maps/Google Earth team at a recent meeting.  Careful here: The answer 
isn't what you expected.

Google has contracts to obtain data from a variety of sources.  They 
make a reasonalble but not a Herculean attempt to verify registration 
and projection are correct.  They rely almost exclusively on the 
metadata associated with the maps they've obtained.

In the GIS world, getting and maintaining good metadata is a real 
problem.  Creating it in the first place is hard (getting it right isn't 
trivial), time consuming, and not nearly as much fun as the graphical 
part of the work.  What happens then, is that some maps have good 
metadata and some don't; those with good metadata match well because 
Google has folks who understand how to take different geodetic datums 
and transform them to NAD83 or WGS84, and reproject from whatever 
projection they came in, to whatever the projection Google presents them 
is supposed to be (I could get it, I'm told, just for asking but they 
don't advertise it).

Just to resolve this question, I suspect the data you're discussing here 
are the result of a datum shift, based on the vector you cite, between 
NAD27 and NAD83/WGS84.  If you obtained the coordinates from a hand-held 
GPS receiver, go into its menu and determine which Datum it's reporting 
in, and set it for either WGS84, or NAD83 (the difference will only 
matter to a physical geodesist who knows the [minor] differences).  If 
you extrapolated these coordinates from a quad sheet, please recheck, 
and make sure A) that the quad you're using has been updates recently 
enough to have both NAD27 and NAD83 graticules printed on it, and B) 
that you selected the NAD83 coordinates and not the NAD27 coordinates.

If there's interest I can explain a bit more about datums, but then I'd 
feel obligated to have an exam at the end of the summer:-)

gerry

Norm Shea wrote:
> Phillip,
> 
> I've finally had an opportunity to look through our aerial imagery.  The
> Topo, USGS Aerial, and Map seem fine, but the Satellite image seems shifted
> approximately 130' to the NNE.  Our site is at 32.6005, -80.1092.
> 
> Norm
> 
> Norm Shea 
> Director, Lakes Management
> Kiawah Island Community Association Inc
> 20 Kestrel Court, Kiawah Island, SC 29455
> Phone 843-768-2315 x 255  Fax 843-768-0298  Mobile 843-708-3608
> Norm.Shea at kiawah-owners.org --- www.kiawah-owners.org
> 
>  
> 
> -----Original Message-----
> From: wxqc-bounces at lists.gladstonefamily.net
> [mailto:wxqc-bounces at lists.gladstonefamily.net] On Behalf Of Philip
> Gladstone
> Sent: Saturday, 19 May 2007 23:36
> To: Discussion of weather data quality issues
> Subject: Re: [wxqc] Coordinate system
> 
> Norm,
> 
> The coordinates are supposed to be WGS84. There certainly used to be are
> area of Houston (as I recall) that was screwed up. If you can point to a
> specific area, then I can take a look at it. It would especially useful if
> you have some ground truth coordinates.....
> 
> Philip
> 
> Norm Shea wrote:
>> What is the coordinate system used by CWOP?  When looking across the 
>> options of graphic locations in the "Registered Location" section, the 
>> various choices appear to be in different coordinate systems.  The 
>> "USGS Aerial" and "Map" images seem pretty accurate but the 
>> "Satellite" and "Landsat" images appear to be off.  I was wondering what
> the "official"
>> coordinate system is, WGS84? NAD83?
>>  
>> Norm
>>
>> *Norm Shea*
>> /Director, Lakes Management/
>> *Kiawah Island Community Association Inc* 20 Kestrel Court, Kiawah 
>> Island, SC 29455 Phone 843-768-2315 x 255  Fax 843-768-0298  Mobile 
>> 843-708-3608 Norm.Shea at kiawah-owners.org 
>> <mailto:Norm.Shea at kiawah-owners.org> --- www.kiawah-owners.org 
>> <http://www.kiawah-owners.org/>
>>
>>  
>>
>>
>> ----------------------------------------------------------------------
>> --
>>
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-- 
Gerry Creager -- gerry.creager at tamu.edu
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