[wxqc] Coordinate system

Thu May 31 09:17:39 CDT 2007

Gerry,

My comment was regarding the "Registered Location" image section of the
website and that the one available selection, the Satellite image, seemed
off.  I am fairly confident of our station position.  It was determined
using a Trimble Pro XR with post processing and converted from South
Carolina NAD 83 State Plane  International Feet to WGS 84.

Thanks,
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 Gerry Creager
Sent: Thursday, 31 May 2007 09:47
To: Discussion of weather data quality issues
Subject: Re: [wxqc] Coordinate system

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
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