6

This query is concerning transformations in QGIS. I have done some checks to find out how transformations are applied in QGIS, but the results indicate that the EPOCH is not applied. ie. GDA2020 is aligned with ITRF2014 at epoch2020, the icsm agreed parameters for the transform from ITRF2014 to gda2020 has yearly rate of change of rotational components.

If I am not able enter the desired epoch so that QGIS can apply the correct yearly rate of change, then how do I upload and use a transformation file that gives fixed values for the transformation at the desired Epoch (eg 2023.5)?

A benchmark measured 29/8/2022 has the following lat lon

GDA2020 -33.3971677056 150.2414593000

ITRF14 -33.3971663917 150.2414598444

If the transformations work correctly in QGIS, then these two points, when applied with the correct CRS, should be at the same spot plus or minus mm. But this does not occur.

Improve this question
2
  • Welcome to GIS SE. Thank you for taking the Tour. Unfortunately, this Question does not conform to the One question per Question policy in the Tour. Two-fold queries are likely to be closed as lacking focus.
    – Vince
    Commented Jan 24, 2023 at 2:23
  • 2
    Thanks for the comment. It is one problem. with two suggested solutions, if they exist. As it was unclear to you I have edited it to make it understandable
    – Graham
    Commented Jan 24, 2023 at 3:02

1 Answer 1

Reset to default
6

Make sure you have QGIS compiled against a GDAL/OGR version greater than 3.4:

QGIS About dialog

We can perform the vector transformation with the Convert Format process (you can look it up in the Toolbox even as ogr2ogr, which is the application that runs in the background). If you wanted to reproject a raster in the same way, the procedure is similar but with the Warp (reproject) process.

The dynamic system is EPSG:9000, ITRF2014, which is the target system (additional parameter -t_srs EPSG:9000), so you must specify the epoch in the target system with the additional parameter -t_coord_epoch 2022.66.

Convert Format (ogr2ogr) dialog

Documentation of allowed parameters: https://gdal.org/programs/ogr2ogr.html

The resulting layer will have the coordinates transformed and referred to the 2022.66 epoch.

Layer properties of dynamic CRS

Improve this answer
4
  • Thank you Gabriel. That seemed to work. I am not sure why you edited out the last part, reverting back to get the coordinates. I am trying to go the other way ITRF2014 to GDA2020, but I will work out the parameters. Thanks again for the help.
    – Graham
    Commented Jan 24, 2023 at 22:04
  • Because I changed the CRS of the layer to see the coordinates in the map, but the coordinates are referred to epoch 2022.66, so do not change the layer CRS. Coordinates can be queried with the node tool or with expressions. You can get the inverse too, just need to assign the epoch to the dynamic CRS, so -s_srs EPSG:9000 -s_coord_epoch 2022.66 -t_srs EPSG:7844 must work.
    – Gabriel De Luca
    Commented Jan 24, 2023 at 22:38
  • Identify tool shows coordinates in the map CRS and on-the-fly reprojection doesn't seem to handle different epochs currently. But the layer handle the dynamic CRS, and we can export and reproject them to reference frame epoch without problem. Just avoid mixing epochs in the map for now.
    – Gabriel De Luca
    Commented Jan 24, 2023 at 22:49
  • 1
    Thanks again for the info. I clearly need a lot more practice with the GDAL functions. This solves my issue which is to process GPS ITRF output and convert to the plate static gda2020 in QGIS rather than using additional conversion software. Thanks again
    – Graham
    Commented Jan 24, 2023 at 23:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.