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Asked: 2026-05-23T17:46:07+00:00

Briefly: How to parametrize .prj WKT file so that I can perform 7 point

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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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1 Answer

  1. Editorial Team

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

    Actually comments at the same problem at gis.stackexchange.com/ here 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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