[GRASS-user] Manually Defining Location

[Bryan Keith]

On Fri, May 20, 2011 at 17:12, Rich Shepard <rshepard at appl-ecosys.com> wrote:
> On Fri, 20 May 2011, Bryan Keith wrote:
>
>> I'm not convinced that's a valid wkt description of the coordinate system
>> you want.
>
> Bryan,
>
>  Let me back up and ask the question from a different point.
>
>  Here's the WKT from one of the maps:
>
> PROJCS["Project",
> GEOGCS["GCS_North_American_1927",
> DATUM["D_North_American_1927",
> SPHEROID["Clarke_1866",6378206.4,294.9786982]],
> PRIMEM["Greenwich",0.0],
> UNIT["Degree",0.0174532925199433]],
> PROJECTION["Transverse_Mercator"],
> PARAMETER["False_Easting",500000.0],
> PARAMETER["False_Northing",-2000000.0],
> PARAMETER["Central_Meridian",-115.583333],
> PARAMETER["Scale_Factor",0.9999],
> PARAMETER["Latitude_Of_Origin",34.75],
> UNIT["Foot_US",0.3048006096012192]]
>
>  From this I created this proj4 description:
>
> +proj=tmerc +datum=nad27 +ellipse=clrk66 +a=6378206.4 +b=294.9786982
> +x_0=500000.0 +y_0=-2000000.0 +k_0=0.9999 +lat_0=34.75 +lon_0=-115.583333
> +no_defs +units=us-ft
>
> Is this an accurate description to use in creating the location? Does the
> order matter?

Well, let's see how this differs from when I tried to do the same thing:

+proj=tmerc +lat_0=34.75 +lon_0=-115.583333 +k=0.9999
+x_0=152400.3048006096 +y_0=-609601.21920243837 +ellps=clrk66
+datum=NAD27 +units=us-ft +no_defs

I think a and b are defined by the ellipsoid.  Are the a and b you
specified the same as what clrk66 uses?  I don't know.  If you specify
both the ellipsoid and a and b, what takes precedence?  I don't know.
Everything else looks the same except for x and y, the false easting
and false northing.  In proj.4 those are always specified in meters
while in the wkt definition, I think those numbers are in the
projection units.  That's why I changed the numbers

>>> 0.3048006096012192 * 500000
152400.30480060959
>>> 0.3048006096012192 * -2000000
-609601.21920243837
>>>

If you use your proj.4 string, does your data end up in the correct location?

To answer your other question, I don't think the order matters.  These
proj.4 questions are outside my area of expertise.

Bryan

>
> Rich
>
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