[GRASS-user] Projection parameters- what is significant?

[Dwight Needels]

Hi all,

My standard location is a Lambert Conformal Conic (2SP) created using  
EPSG 2234. When I receive data from my city GIS system, it arrives (as  
a shape file) in a very similar but non-identical projection. I  
created a new location based on one of the data files, and re-project  
to and from my standard location.

One of the differences between the two locations is that the values in  
lat_1 and lat_2 are swapped.

[1] Does it matter which standard parallel is called lat_1, and which  
is called lat_2? If so, what is the result of swapping them?

The second difference I noticed is that the False Easting and False  
Northing values for my standard location are nice round numbers, but  
those in the city location are slightly off.

EPSG 2234 False Easting, False Northing  [1000000, 500000]
City location False Easting, False Northing  [999999.9999960001,  
499999.999998]

Moreover, the x_0 and y_0 values of the other location are either  
truncated or rounded relative to the EPSG location.

EPSG 2234 x_0, y_0  [304800.6096012192, 152400.3048006096]
City location x_0, y_0  [304800.6096, 152400.3048]

It also turns out that the state GIS data are in exactly the same  
projection as the city data (and thus different from the EPSG standard).

I would have thought that x_0 and y_0 would be calculated from the  
False Easting and False Northing values, and not vice versa. And yet,  
it looks like the odd False Easting and False Northing values might  
have been calculated from the truncated x_0 and y_0 values.

[2] Is there a functional reason for the odd False Easting and False  
Northing values (e.g. a correction put in place after the State Plane  
system was defined), or is this a truncation error?

[3] Given the described differences, is there any reason not to import  
data directly into my standard location using the override flag? For  
example, I have a 0.25 ft resolution raster image that I would like to  
bring into my standard location without any resampling.

[4] Is there an easy way to convert an entire location, with all  
associated files, to a new projection?

Thanks

-Dwight