shpproj.txt

Source code, and some other odds and ends can be downloaded from http://shapelib.maptools.org/dl.

shpproj



using the projection tools  proj and invproj
we have implemented a shapefile projection utility

shpproj shp_file new_shp ( -i=in_proj_file | -i="in_params" | -i=geographic ) ( -o=out_info_file | -o="out_params" | -o=geographic ) 

INPUT
  Input can come from one of three sources.  A projection parameter file, 
  directly through parameters or geographic. If the shapefile has an associated
  prj file, name the same as the shapefile but ending in ".prj" it will be used
  by default ignoring all other parameters.  If input is omitted it defaults to
  geographic, unless the default prj file exists..

OUTPUT
  Output can come from one of three sources.  A projection parameter file,
  directly through parameters or geographic.  If output is omitted it defaults
  to geographic.

PROJECTION PARAMETER FILE
  This file MUST end with the extension ".prj".  It has the form of one
  projection parameter per line.  Parameters can be in any order. 

PROJECTION PARAMETERS
  Are the same as used by proj and invproj.
  use      proj -lP   to see available projections
	   proj -lu   to see available units
	   proj -le   to see available ellipsoid


USAGE

the following example projects file rowtest to row3, moving data from Stateplane NAD83  zone 1002 to utm zone 16 in meters
 
shpproj rowtest row -i="init=nad83:1002 units=us-ft" -o="proj=utm zone=16 units=m"

shpproj rowtest row3 -o="proj=utm zone=18 units=m" -i="zone=16 proj=utm units=us-ft"

shpproj rowtest row3 -o="proj=utm zone=18 units=m" 

shpproj rowtest row3 -i=myfile.prj -o=geographic
shpproj rowtest row3 -is=myfile.prj


Datums
Stateplane data can be accessed by the init function
	init=nad83:1003
which is of the form 
             datum:FIPSzone

To move from NAD27 to NAD83 or otherwise the shpproj utility moves to 
geographic coordinates and the back to the desired datum.  proj can handle 
direct datum to datum conversions but it is umimplemented here.

for a complete listing of FIPSZones by datum read nad/nad.lst in the PROJ4
library


CAUTION

It is possible to get very poor results for projection values when using 
coordinates VERY far away from the intended scope of the projection.

An example of a poor projection choice would be to move from nad83:1002
(Georgia West) for data in California into utm.  The resulting data will be
much less accurate than if it had started as geographic.