Re: [Gpsbabel-misc] Question about UTM coordinates

[Greg T. <gd...@ir...>]

Scout <Scout@GPSgames.org> writes:

This is not really a gpsbabel issue, but probably of some interest to
most list members, so:

> I have some data of this form:
> Northing	Easting
> 7032815.975	2500879.162
> 7032846.600	2500860.699
> 7032856.677	2500843.205
> 7032881.251	2500842.079
> 7032904.872	2500825.768
> 7032896.334	2500807.050
> 7032916.177	2500795.347
> 7032921.525	2500777.533
> 7032953.630	2500751.679
> 7032960.084	2500736.130
> ...
>
> I also have a control file:
> PROJCS["NAD_1983_StatePlane_Texas_North_Central_FIPS_4202_Feet",
> GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
> PROJECTION["Lambert_Conformal_Conic"],
> PARAMETER["False_Easting",1968500.0],
> PARAMETER["False_Northing",6561666.666666666],
> PARAMETER["Central_Meridian",-98.5],
> PARAMETER["Standard_Parallel_1",32.13333333333333],
> PARAMETER["Standard_Parallel_2",33.96666666666667],
> PARAMETER["Latitude_Of_Origin",31.66666666666667],
> UNIT["Foot_US",0.3048006096012192]]
>
> This seems to be UTM data in a form that I am not familiar with.
> Instead of a familiar grid, there seems to be some arbitrary origin
> given. My question: How do I convert this data to WGS84 lat/lon?
> Any guidance would be appreciated.

Your data is not in UTM.  It's in Texas North Central State Plane, which
is Lamber Conformal Conic (IIRC typical for areas that are more
east-west than north-south).

Get 'proj', which is a library&program for coordinate conversion.  Look
in the epsg parameter file for your projection.

# NAD83 / Texas North Central (ftUS)
<2276> +proj=lcc +lat_1=33.96666666666667 +lat_2=32.13333333333333 +lat_0=31.66666666666667 +lon_0=-98.5 +x_0=600000 +y_0=2000000.0001016 +ellps=GRS80 +datum=NAD83 +units=us-ft +no_defs  <>
# NAD83(HARN) / Texas North Central (ftUS)
<2917> +proj=lcc +lat_1=33.96666666666667 +lat_2=32.13333333333333 +lat_0=31.66666666666667 +lon_0=-98.5 +x_0=600000 +y_0=2000000.0001016 +ellps=GRS80 +units=us-ft +no_defs  <>
# NAD83(NSRS2007) / Texas North Central (ftUS)
<3670> +proj=lcc +lat_1=33.96666666666667 +lat_2=32.13333333333333 +lat_0=31.66666666666667 +lon_0=-98.5 +x_0=600000 +y_0=2000000.0001016 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=us-ft +no_defs  <>

The differences here are subtle and complicated, but if you don't care
about 1m I would suggest ignoring them.  If you do care about 1m then
you have a lot of studying to do, because those issues are far more
difficult than UTM vs state plane :-) But, proj gives the same answer
for all 3.

$ for p in 2276 2917 3670; do echo 2500879.162 7032815.975 | cs2cs -f '%.15f' +init=/usr/pkg/share/proj/epsg:$p  +to +proj=latlong +datum=WGS84; done
-96.764316884936477     32.949618267402684 0.000000000000000
-96.764316884936477     32.949618267402684 0.000000000000000
-96.764316884936477     32.949618267402684 0.000000000000000