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Briefly:

How to parametrize .prj WKT file so that I can perform 7 point tranformation (wiki). I know how false_easting and false_northing params work, but how can I adjust scale? I do not mean scale_factor'

That's the problem description:

I have transportation network (vector layer) saved in non-GIS environment (transport modeling software). Network consists of nodes (points) and polylines (road links). It's done mostly from random backgrounds, regardless any projection, coordinates, etc.

I need to set appropriate projection for the network.

I have accesss to .prj files (if I'm in an say WGS84 projection I can switch to any other projection)

So that's what I'm trying:
I try 7 point Helmert Transformation (http://proj.maptools.org/gen_parms.html). I use towgs84 transformation as a WKT param in .prj file, where I assume that rotation matrix is zero (can I do so?) and I calculate only delta_x, and delta_y, and scale param.

However it will not work. This is my .prj , params in TOWGS84 do not affect transformation:

PROJCS["UTM 17 (WGS84) in northern hemisphere.",
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563],
        TOWGS84[0,0,0,0,0,0,100000000000000000000000]],
    PRIMEM["Greenwich",0],
    UNIT["DMSH",0.0174532925199433],
    AXIS["Lat",NORTH],
    AXIS["Long",EAST],

PROJECTION["Transverse_Mercator"],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",0],
PARAMETER["scale_factor",1],
PARAMETER["false_easting",0],
PARAMETER["false_northing",0]]

So I tried to use false_norting and false_easting params, and those work good, and transform my network proprely, BUT:

It will not chcange scale of my network, only position. So how can I rescale my network using .prj file?

Thanks for any hints

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The "scale_factor" parameter explicitly changes scale: have you tried varying it? –  whuber Jul 5 '11 at 22:21
    
I've tried. It doesn't really affect anything. –  Lewy Jul 5 '11 at 22:32
    
from: publib.boulder.ibm.com/infocenter/db2luw/v8/index.jsp?topic=/… : Scale_factor is used generally to reduce the amount of distortion in a map projection. –  Lewy Jul 5 '11 at 22:36
    
The scale factor accomplishes that reduction in distortion by changing the scale! How exactly have you experimented with its value? (Another way to change the coordinates, albeit one most GIS people would avoid, is to change the value of the "unit".) Note that merely changing the values in a .prj file does nothing to your data: they stay the same. It is only when the data are reprojected that the changes in the .prj file make a difference. –  whuber Jul 5 '11 at 23:10
    
The towgs84 tag identifies how to convert the GeoCRS that it's in to WGS84. It will only be applied during a datum transformation. –  mkennedy Jul 5 '11 at 23:33
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1 Answer

up vote 1 down vote accepted

Problem solved: both 'scale_factor' , and UNIT['Meter',%scale_factor] works only if datum changes.

Actually comments brought me to solution.

Anyway: .prj files, Geo Coordinate Systems, proj4js, EPSG etc. are vary weakly documented: no API, no tutorials, no examples, no refernces.

i.e. 1)not any straighforward description of what EPSG database codes are, and which should be chosen.

2)what +proj parameters should I choose to define projection

3)how to create .prj and what are parameters of specific .prj file elements.

awful programming area!

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