Global Mapper v26.0

How to convert QGIS CRS file to Global Mapper .GM_xform file

CarrickCon
CarrickCon
in Projection Questions

G'day GM'ers!

I've been given the following QGIS Coordinate Reference File (CRS) by a client in the Philippines to transform from their LOCAL cartesian site grid to WGS84-UTM Zone 51 North.

+proj=omerc +lat_0=0 +lonc=123 +alpha=0.00000 +gamma=0.031 +k=0.999878 +x_0=-13672.3471 +y_0=-1353533.4306 +ellps=clrk66 +towgs84=-127.62195,-67.24478,-47.04305,-3.06762,4.90291,1.5779,-1.06002 +units=m +no_defs

This is not my specialty (I'm a simple engineer not a surveyor!), but I'm pretty sure that the first half of the parameters are to transform from PRS92-UTM Zone 4 to LOCAL and involves translation, rotation and scaling as follows (input to GM Coord Transformation Setup dialogue box under Shift command - as per attached .GM_xform file):

Transform File from PRS92 PTM Zone 4 to LOCAL Grid.zip
image.png

I also recognize the second half of the parameters as the published 7-parameter Bursa/Wolfe Transformation from PRS92-PTM Zone 4 to WGS 84-UTM Zone 51 North.

So the QGIS CRS file seems to transform from Local to PRS92-PTM Zone 4 to WGS 84-UTM Zone 51 North all in one go. My problem is that I'm not sure how to correctly input this to GM?

I've tried transforming common points between Local and WGS 84-UTM Zone 51 using the attached .GM_xform file with no joy.

I've attached a supplied file of the common points to Local, WGS84 and PRS92.

Any and all help appreciated.

Cheers,

/al

Ground Control Points Local Grid, WGS84 & PRS92.xlsx


Comments

  • sphillips
    sphillips

    Hi Al,

    I've had a look for you, but I'm no expert in this area either. The closest I got the Mine Grid points to the AOI, was using a custom PRJ (below) with what I assume are the modifications to PRS92 Philippines Zone 4 EPSG:3124 (False Easting & Northing and Scale). My best effort places the points around 750m east. I can only assume the next step is to apply a 7 parameter transformation using the values in the CRS.

    PROJCS["Custom_PRS",
GEOGCS["GCS_PHILIPPINE_REFERENCE_SYSTEM_1992",
DATUM["D_Philippine_Reference_System_1992",
SPHEROID["Clarke_1866",6378206.4,294.9786982]],
PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],
PROJECTION["Transverse_Mercator"],
PARAMETER["scale_factor",0.999878],
PARAMETER["central_meridian",123],
PARAMETER["latitude_of_origin",0],
PARAMETER["false_easting",-13672.3471],
PARAMETER["false_northing",-1353533.4306],UNIT["Meter",1]]

    I've use ChatGP/Copilot and found some info that may help:

    The CRS parameters you provided are for transforming from a local cartesian site grid to WGS84-UTM Zone 51 North.

    Key components of the CRS are:

    1. Projection Method: The CRS uses the Oblique Mercator (omerc) projection method. This projection is suitable for areas with irregular shapes or significant distortion.
    2. Central Latitude and Longitude: The central latitude is set to 0°, and the central longitude is 123°. These values define the center of the projection.
    3. Scale Factor (k): The scale factor is 0.999878, which accounts for local scale variations.
    4. False Easting and Northing: The false easting (x_0) is -13672.3471 meters, and the false northing (y_0) is -1353533.4306 meters. These values shift the origin of the coordinate system.
    5. Ellipsoid: The CRS uses the Clarke 1866 ellipsoid (ellps=clrk66).
    6. Datum Transformation (towgs84): The transformation parameters are provided in the form of seven values. I these parameters adjust for differences between the local datum (PRS92) and the global WGS84 datum. They involve translation, rotation, and scaling.
    dx: -127.62195 meters
dy: -67.24478 meters
dz: -47.04305 meters
rx: -3.06762 arcseconds
ry: 4.90291 arcseconds
rz: 1.5779 arcseconds
ds: -1.06002 parts per million

    In summary, this CRS allows you to transform coordinates from the local site grid to the global WGS84-UTM Zone 51 North, accounting for both local and global variations. 

    I have tried making a custom Datum and looked into using SHIFT but neither improve it any further. Maybe someone else can shed light on this?

  • CarrickCon
    CarrickCon

    Thanks for your input Steve.

    I was getting the same ~750m east transformation as you and had resorted to the good old shift/rotate/scale about common points in AutoCAD that we have discussed on this forum previously. However it would be good to be able to re-project directly within GM.

    I'm actually here on site in the Philippines at the moment so will dig a bit deeper in case I'm missing a vital piece of information.

    Cheers,

    /al

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