Local Mine Grid to GDA94-MGA51 Transformation

CarrickCon

Hi GM'ers,

I'm banging my head off the wall here with something that should be straight forward and any and all help would be much appreciated. I'm trying to load the attached dxf (BS4_bench) which is in local mine grid into an existing GM workspace which is set up to GDA94 datum and MGA51 projection (site is in Western Australia). 

When I try to use the attached .txt file to change projection I get a "parsing error" as per screen grab. What am I doing wrong? I have also included in zip file an .xls and .csv of transformation info.

Ballpark the transformation from local mine grid to MGA should involve a 45 deg anti-clockwise rotation and approx 600,000 m shift eastwards and approx 6,618,000 m shift northwards. 

Cheers,
/al

sphillips

Hi Al,

Sorry for the late reply, I haven't been on the forum for a while. You've probably figured this out by now but if not, I've calculated the rotation and offset using the reference points using AutoCAD. 

• Rotate 45 degrees clockwise (around the basepoint 0,0,0 if using AutoCAD)
• x + 515232.1267
• y + 6696002.975

I have attached the xform (see zipped folder) so you can load it via Shift - Selected Layer(s) > Specify Coordinate Transformation > Load from File...



I've tested it on the reference points and the DXF and they line up great.

I often have to work these things out as the Mine Grid to the Projected Coordinate system has been lost or just isn't known. I also regularly have to work out what coordinate system has been used for data when the info isn't known to the Client.

Anyway, this is my workflow for calculating the transformation:

1. Plot 2 or 3 known points in both coordinate system (I draw a line between each of the points to trace a shape. This makes it easier to get your bearing when checking the rotation) 
2. Make a copy of the Mine Grid shape and paste it onto the Projected shape and align to a common point
3. Measure the angle difference between the two and make a note of it
4. Delete the temporary shape
5. Rotate the Mine Grid reference points around the basepoint 0,0,0 (origin) and enter the rotation angle noted earlier. AutoCAD is weird so 45 clockwise is 315 or -45.
6. Now draw a rectangle between two common reference points
7. Measure the width of the rectangle and note the difference in the x (easting)
8. Measure the height of the rectangle and note the difference in the the y (northing)
9. You will now have the rotation, and the x and y offsets.

Good luck my friend

Steve

CarrickCon

Thanks again Steve - you're a saint and a scholar!

I had actually given up in GM and had just resorted to rotating and shifting in AutoCAD before re-saving and then loading into GM. That was a backward step however and is much better to be able to do directly in GM.

Cheers,
Alistair.

sphillips

CarrickCon said:

Thanks again Steve - you're a saint and a scholar!

I had actually given up in GM and had just resorted to rotating and shifting in AutoCAD before re-saving and then loading into GM. That was a backward step however and is much better to be able to do directly in GM.

Cheers,
Alistair.

Hi Alistair,

Glad you found my reply useful. I create Actions in AutoCAD to convert back and forth between Mine Grid and the Projected coordinate system. They are a bit like macros and really simple to create. If you go down this route just make sure all layers are unlocked and not frozen otherwise those objects will not be moved.

I also have a habit of using AutoCAD to convert CAD plans/drawings and TIN meshes. As you know, some CAD drawings are ruined when imported into Global Mapper so best to avoid using Global Mapper in those cases. I only use Global Mapper to convert geospatial data like shapefiles, raster based datasets and anything that has attribute data really. 

Cheers,
Steve

CarrickCon

Hi Steve,

Regarding the coordinate rotation and translation I've been revising some high school coordinate geometry at: http://doubleroot.in/lessons/coordinate-geometry-basics/rotation-of-axes/
and worked examples at: https://doubleroot.in/lessons/coordinate-geometry-basics/translation-rotation-examples/  (closest worked example is Example 1 (ii)).

The key equations are:

x’ = xcosθ + ysinθ

y’ = ycosθ – xsinθ

In my case I have two common points (P1 and P2) that I have the coordinates for in both mine grid and GDA94/MGA51 and I know the rotation angle (θ) is 45 degrees, so can calculate the x and y shift directly. I've attached it as a simple worksheet in case it is helpful for some of your projects (yellow highlighted cells are known values, the others are calculated).

Cheers,

/al

sphillips

CarrickCon said:

Hi Steve,

Regarding the coordinate rotation and translation I've been revising some high school coordinate geometry at: http://doubleroot.in/lessons/coordinate-geometry-basics/rotation-of-axes/
and worked examples at: https://doubleroot.in/lessons/coordinate-geometry-basics/translation-rotation-examples/  (closest worked example is Example 1 (ii)).

The key equations are:

x’ = xcosθ + ysinθ

y’ = ycosθ – xsinθ

In my case I have two common points (P1 and P2) that I have the coordinates for in both mine grid and GDA94/MGA51 and I know the rotation angle (θ) is 45 degrees, so can calculate the x and y shift directly. I've attached it as a simple worksheet in case it is helpful for some of your projects (yellow highlighted cells are known values, the others are calculated).

Cheers,

/al

That's pretty cool! Do you have a method that will convert a list of points from Local to Projected if you input the known x and y offset and rotation? That would save a lot of time if you only want to edit the x and y coordinates in a spreadsheet e.g. borehole collars

sphillips

sphillips said:

CarrickCon said:

Hi Steve,

Regarding the coordinate rotation and translation I've been revising some high school coordinate geometry at: http://doubleroot.in/lessons/coordinate-geometry-basics/rotation-of-axes/
and worked examples at: https://doubleroot.in/lessons/coordinate-geometry-basics/translation-rotation-examples/  (closest worked example is Example 1 (ii)).

The key equations are:

x’ = xcosθ + ysinθ

y’ = ycosθ – xsinθ

In my case I have two common points (P1 and P2) that I have the coordinates for in both mine grid and GDA94/MGA51 and I know the rotation angle (θ) is 45 degrees, so can calculate the x and y shift directly. I've attached it as a simple worksheet in case it is helpful for some of your projects (yellow highlighted cells are known values, the others are calculated).

Cheers,

/al

That's pretty cool! Do you have a method that will convert a list of points from Local to Projected if you input the known x and y offset and rotation? That would save a lot of time if you only want to edit the x and y coordinates in a spreadsheet e.g. borehole collars

Ignore my question! As soon as I posted my comment I realised I just needed to make some adjustments to get it to convert the values. Thanks for the post, this will be very useful!