Daniel,

I hope I am understanding you correctly. Whether it is an aerial photo or LiDAR data you or someone else has created and brought into QGIS, right click on the layer name in the Layer Panel and choose Properties. When the Layer Properties window pops up choose Metadata from the side panel. If you scroll down to Properties you will find Dimensions (X & Y size in pixels), number of bands, the Origin point (upper left coordinate anchor), Pixel Size, Data Type, Layer Spatial Reference System, and Layer Extent (farthest N, E, S, W numbers).

The photo was taken by a drone. The area had several panels of known coordinates which allow for georeferencing. In Qgis, I loaded the photo into georeferencer as a raster layer and I introduced the known coordinates inside the photo. Once I have introduced these I ran the georreferencer (with whatever reference system). The addon asks me then for a transformation type (linear, proyective...) and a resampling method (closest neighbour...). I cannot recall, of these two last details, which ones I used, hence my question.

I am not sure. I have never needed to extract the transform type or resampling method. Of course, one should always use copies so that the originals are untouched in case something happens.

Depending on how many ground control points you have captured, you may want to use different transformation algorithms. Choice of transformation algorithm is also dependent on the type and quality of input data and the amount of geometric distortion that you are willing to introduce to the final result.

Currently, the following Transformation types are available:

The Linear algorithm is used to create a world file and is different from the other algorithms, as it does not actually transform the raster. This algorithm likely won’t be sufficient if you are dealing with scanned material.

The Helmert transformation performs simple scaling and rotation transformations.

The Polynomial algorithms 1-3 are among the most widely used algorithms introduced to match source and destination ground control points. The most widely used polynomial algorithm is the second-order polynomial transformation, which allows some curvature. First-order polynomial transformation (affine) preserves colliniarity and allows scaling, translation and rotation only.

The Thin Plate Spline (TPS) algorithm is a more modern georeferencing method, which is able to introduce local deformations in the data. This algorithm is useful when very low quality originals are being georeferenced.

The Projective transformation is a linear rotation and translation of coordinates.

Thank you very much. Very informative

Just a note of appreciation to Pinofor a clear, comprehensive answer. the only thing I would add is that the scale of the images is important in making the choice. Since these are drone shots and therefore presumably relatively large scale (local) I would think the Helmert would work but the TPS is certainly worth trying. In doing this it's often wise to (after making a copy of your image) trying it with two different transformations and then comparing the results. Use the one that best suits your needs.

It is available in the metadata of the transformed/georeferenced output image

Thank you

Hello,

I dont think that the choices you made (target points, transformation type and resampling method) are saved somewhere attached to your raster dataset ... unless you have expressly saved it in the metadata properties. I dont know QGIS environement very well, but I know that it is recommended to save the target points used in the georeferencing step (X image, Y image, X field, Y field, error etc.) and it may be usefull to save transformation algorithm and resampling algo as well.

I am interested to understand the difference between Affine and Helmert transformation. Both seem to have the same purposes i.e., Translation, Scaling and Rotation.

Can someone throw light on their differences ..?

All the information regarding the georeferencer plugin is very well documented on the official site of QGIS. Please find the link below:

https://docs.qgis.org/2.8/en/docs/user_manual/plugins/plugins_georeferencer.html#available-transformation-algorithms

Along with the same some examples with troubleshooting are listed on this stack exchange page:

https://gis.stackexchange.com/questions/45185/qgis-georeferencer-layer-doesnt-appear/337615#337615