[Rda-users] Station Location coordinate systems and projection

[Steve Schroeder]

Brian and everyone else,

To summarize longer correspondence directly with Brian, I figured out what his problem was.  He uses surface observations from mostly cooperative stations in New Mexico.  However, the documentation about a station file from NCDC was missing and he did not realize that the latitudes and longitudes in the file are degrees and minutes (with the decimal point omitted) instead of degrees and hundredths.  This would cause systematic errors within each box of 1 degree of latitude and longitude, with the largest errors in New Mexico (and anywhere in the Northern/Western Hemisphere) near the NW corner of each degree box.

It is sometimes not obvious whether the locations are degrees and minutes or degrees and hundredths if the decimal point is omitted for computer use.  On a long list of stations, if the last 2 digits of the latitude and longitude are all between 00 and 59, it is safe to assume that the system is degrees and minutes.  However, I have seen mistakes and erroneous conversions even in WMO catalogs.  The most likely catalog error is to continue to list an obsolete location when a country or agency does not notify the catalog manager of a new station location.  This can continue for decades.

His errors were as large as 50 km or so.  Older catalog locations may be based on NAD29 instead of NAD83/WGS84, but in North America, the location difference is rarely more than 100 meters (generally increasing with distance from Kansas).  Around Japan, the difference of older systems from WGS84 can be 700 m, and in South America, the difference can be 500 m.  Specifying a location to the nearest minute allows an error up to about 1 km, if the stated minutes are correct, so using an older geodetic system instead of WGS84 often causes no error to the nearest minute.

Regards,

Steve Schroeder
Dept. of Atmospheric Sciences
Texas A&M University



----- Original Message -----
> From: "Brian Woods, DOH" <Brian.Woods at state.nm.us>
> To: rda-users at mailman.ucar.edu
> Sent: Thursday, November 8, 2012 11:49:27 AM
> Subject: [Rda-users] Station Location coordinate systems and projection
> 
> 
> 
> 
> 
> Greetings. I recently attempted spatial joins of meteorological
> stations in New Mexico to zip and census polygons and found the raw
> longitude and latitude problematic to work with. Even after
> converting to decimal values it appeared station values are
> displaced from NM features in NAD 83 projections. Does anyone know
> the coordinate system used for station data or where that is
> documented and can be found?
> 
> 
> 
> Thank you.
> 
> 
> 
> Brian.
> 
> 
> 
> Brian Woods
> 
> Environmental Public Health Epidemiologist
> 
> Environmental Public Health Tracking
> 
> Environmental Health Epidemiology Bureau
> 
> Epidemiology and Response Division
> 
> New Mexico Department of Health
> 
> 1190 St. Francis Drive, N1309
> 
> 505-827-2868 (Office)
> 
> 505-476-1743 (EHEB Office)
> 
> 505-827-2110 (Fax)
> 
> http://nmhealth.org/eheb/
> 
> https://nmtracking.unm.edu/
> 
> 
> 
> 
> 
> 
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