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Author Topic: More Datum Shift: Translation from WGS84 to Alberta 3TM  (Read 263 times)

Resourceful

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More Datum Shift: Translation from WGS84 to Alberta 3TM
« on: November 16, 2020, 07:25:58 AM »
I have a couple of datasets that lined up perfectly with the PPK solution from my drone - it seems to work best to convert my check point, which is Alberta 3TM and HT2 vertical datum to ellipsoidal elevation and WGS84, this gives me a couple of centimeters of XYZ error.

When I convert my tags and check point to Alberta 3TM, the whole project shifts by around a meter of XYZ.  What would cause this?  If there is no fix within the software / datums, can I manually shift my map to the known point?  A workflow for this would also solve my problem.

Thanks,

Paulo

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #1 on: November 16, 2020, 07:58:05 AM »
Resourceful,

if your CRS definition  includes a (0,0,0,0,0,0,0) TOWGS84 as in:

Code: [Select]
PROJCS["NAD83(CSRS) / Alberta 3TM ref merid 114 W",
    GEOGCS["NAD83(CSRS)",
        DATUM["NAD83_Canadian_Spatial_Reference_System",
            SPHEROID["GRS 1980",6378137,298.257222101,
                AUTHORITY["EPSG","7019"]],
            TOWGS84[0,0,0,0,0,0,0],
            AUTHORITY["EPSG","6140"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4617"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",-114],
    PARAMETER["scale_factor",0.9999],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AXIS["Easting",EAST],
    AXIS["Northing",NORTH],
    AUTHORITY["EPSG","3780"]]

then the error in transforming to or from WGS84 can be up to 2 meters....

I would propose that you use TOWGS84 entry from following parameters published by NRCan:

ITRF2014 to NAD83(CSRS) Transformation parameters                     
Epoch:   2010                  
TX(m)   1.0053   dTX(m/y)   0.00079            
TY(m)   -1.90921   dTY(m/y)   -0.0006            
TZ(m)   -0.54157   dTZ(m/y)   -0.00144            
RX(mas)   -26.78138   dRX(mas/y)   -0.06667            
RY(mas)   0.42027   dRY(mas/y)   0.75744            
RZ(mas)   -10.93206   dRZ(mas/y)   0.05133            
DS(ppb)   0.36891   dDS(ppb/y)   -0.07201            
                     
At Epoch 2010                     
TOWGS84[-1.0053, 1.90921, 0.54157, 0.02678138, -0.00042027, 0.01093206, -0.00036891]

and at Epoch 2020.9 for example
TOWGS84[-1.013911, 1.91575, 0.557266, 0.027508083, -0.008676366, 0.010372563, 0.000415999]

 applying these parameters modified according to date of your survey ,should give  better transformation accuracy...
« Last Edit: November 16, 2020, 01:25:38 PM by Paulo »
Best Regards,

Paul Pelletier

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #2 on: November 16, 2020, 08:42:39 PM »
Thank you for this - I have run into similar issues a few times this season, ground control was the fix for all of it.

These numbers not being adjusted would have been the cause - converting to NAD83 stayed accurate.

I also thought I knew the NRCan site, apparently not good enough!  The only thing I can't seem to find within Metashape is where to input those adjustments, and do I need to change the figures to negatives as well?

Only if you have to transmit that info, of course. 

I'll deep dive into NRCan and find the transformation information for the other projections that threw me off this summer.

Thank you,

Resourceful

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #3 on: November 16, 2020, 11:12:30 PM »
Thanks again Paulo,

Learning has occurred:  The shift numbers you gave moved the model closer, but Alberta 3TM is based on a 2002 epoch.  I am digging into trying to find the numbers for this, I suspect I will get closer.  I found the download with a CSV containing the numbers you gave me, so I will keep digging through NRCan to try to find what I need.

I did discover a .GSB file for Alberta transformation, now to extract datum shift values from it.

Thanks again.

Paulo

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #4 on: November 16, 2020, 11:16:27 PM »
Resourceful,


these 7 parameters can be inputted inside Reference Settings (tool icon) in MetaShape by  using the Datum Transformation Settings tool and entering Custom Transformation parameters....see following

in example I entered values corresponding to Epoch 2002....

Unfortunately, use of GPB shift files for datum transformation is not implemented inside MS. To use shift files for transforming coordinates, you would need to use an external Tool for example TRX from NR Can....
« Last Edit: November 16, 2020, 11:24:59 PM by Paulo »
Best Regards,

Paul Pelletier

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #5 on: November 17, 2020, 01:15:35 AM »
Thanks again Paul,

I did work through the Datum Transformation function in Reference settings, and I got much closer.  Now, it will be getting it into epoch of 2002.  You have given me more than enough education to work this out, but if I'm in a jam I'll run you numbers.

The CSV from NRCan has the 2010 epoch numbers, with a +- change per annum - I will work with that, and build my own spreadsheet to do the math much like you have.  I'll let you know how it goes from here.

Thank you so much, I owe you a virtual (or real) beer.  ;D

Paulo

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #6 on: November 17, 2020, 01:30:12 AM »
No problem!

Note that changing transformation parameters from 2010 to 2002 will probably introduce something like just a cm level change in coordinates...
« Last Edit: November 17, 2020, 01:32:25 AM by Paulo »
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Paul Pelletier

Paulo

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #7 on: November 17, 2020, 02:50:36 AM »
Hi, me again :)

it seems that using TRX tool with latest velocity grids does give much better results than just applying a 7 parameter transformation.

I placed a marker with following coordinates in WGS84 or ITRF2014:
                 Long       Lat           Height
point 1   -114.00   50.75   750.0000      

then in Metashape I converted to 'NAD83(CSRS) / Alberta 3TM ref merid 114 W (EPSG::3780)' with transformation parameters set for Epoch 2002 and got:
                 Easting                              Northing                       Height
point 1   1.087308865008536   5623711.126048385   750.4414926576342   

then I used TRX to transform same point from ITRF14 at 2020-10-23 to NAD83/Alb 3TM at   epoch 2002 applying latest velocity grids and got result seen in following screen.

Difference in East, North is some 20 cm.... these GVB grids model  local regional plate shifts between epochs which simple 7 parameter transfo do not. That is because using the Epoch 2002 7 parameter transformation supposes that survey was done at same date(2002-01-01) which would not be the case....   
« Last Edit: November 18, 2020, 03:02:37 AM by Paulo »
Best Regards,

Paul Pelletier

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #8 on: November 18, 2020, 05:17:12 AM »
This is excellent info, thank you Paul! 

Interestingly enough, when I convert the 3TM coordinate in Global Mapper to WGS84, the PPK solution from the drone lands within a couple of centimeters - its the conversion back from WGS84 to 3TM where I find my errors.  I suspect I need to get the correct 7 values into Metashape to get back close to my PPK accuracy on my check point.

I will play with the TRX tool some more, if it somehow gives the values needed for the switch back that would come in handy.

Either way, you have given me some more tools to work and think with.

Thanks again!

Resourceful

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #9 on: November 18, 2020, 06:22:36 AM »
Thanks again Paul,

The TRX tool has this handy link to the transformation numbers.  I'll use these to re-align a job or two to see if they confirm the WGS PPK fix.

This is very much appreciated.

Paulo

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #10 on: November 18, 2020, 08:06:43 AM »
Yes the transformation parameters showed in TRX do conform with the ones calculated at 2020.88 from the published 2010 parameters from NRCan which is logical
Code: [Select]
ITRF2014 (WGS84) to NAD83(CSRS) Transformation parameters
Epoch: 2010                   Epoch: 2020.88
TX(m) 1.0053         dTX(m/y) 0.00079         1.0138952
TY(m) -1.90921 dTY(m/y) -0.0006         -1.915738
TZ(m) -0.54157 dTZ(m/y) -0.00144 -0.5572372
RX(mas) -26.78138 dRX(mas/y) -0.06667 -27.5067496
RY(mas) 0.42027         dRY(mas/y) 0.75744         8.6612172
RZ(mas) -10.93206 dRZ(mas/y) 0.05133         -10.3735896
DS(ppb) 0.36891         dDS(ppb/y) -0.07201 -0.4145588

TOWGS84
Epoch TX(m)         TY(m)         TZ(m)         RX(as)         RY(as)         RZ(as)         DS(ppm)
2010 -1.0053         1.90921         0.54157         0.02678138 -0.00042027 0.01093206 -0.00036891
2020.88 -1.0138952 1.915738 0.5572372 0.02750675 -0.008661217 0.01037359 0.000414559

Supposing you need to  produce your results in NAD83(CSRS) AB 3TM epoch 2002 and you have precise RTK/PPK camera centers determined in ITRF2014 then I would do the following:

1. transform your camera center coordinates from ITRF2014 at date of your survey to NAD83(CSRS) AB 3TM at 2002 using TRX with Epoch Transformation checked;
2. Import the transformed camera centers to MetaShape specifying correct CRS;
3. continue MS processing in your official reference frame  (NAD83(CSRS) AB 3TM epoch 2002) including any control points or check points.

This way I think you would get the  most consistent and precise results...
« Last Edit: November 18, 2020, 10:33:10 AM by Paulo »
Best Regards,

Paul Pelletier

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Re: More Datum Shift: Translation from WGS84 to Alberta 3TM
« Reply #11 on: November 22, 2020, 04:38:09 AM »
Progress on learning how to correct epoch has certainly been made, thank you Paul! 

I still can not get a good check out of my check points though - I am a bit out of sorts on what to try next.  I compared my GNSS against a Trimble, I don't think that's the problem - Maybe the drone's PPK is to blame?  But more often than not it lines up with the WGS conversions... 

The workflow from the manufacturer is to put everything into WGS84 for Metashape processing - the mess comes when I convert or export to a NAD83 derived projection.  Applying TRX published shift isn't bringing everything in unfortunately, I am not sure what I am missing.

I still have workflow issues to sort out apparently - I will post again when I figure it out, until then it's full ground control on all projects.