Purpose was to digitize geological strike-dip symbols from a georefrenced map. These symbols represent orientation of inclined planar surfaces at a field location. Strike is the azimuth of a horizontal line in the inclined plane such that the inclination (dip) slopes downward to the right. Variations from place to place provide an indication of folding or warping of somewhat parallel surfaces and provide an understanding of changes of the local geological structure.
The following is the procedure I developed.
1. Create a line shapefile layer with the project CRS with fields "Strike" and "Dip".
2. Start point of each line is at intersection of the short dip tick with the long strike line of the symbol being digitized.
3. End points are at the ends of the strike symbols being digitized such that dip is to the right. Right click completes the operation; manually enter an id and the dip value.
4. Run expression to update the field "Strike": degrees( azimuth( point_n( $geometry ,1) , point_n( $geometry ,2 ) ) ) .
5. While the Line layer is active, Use Processing Toolbox/ Vector Geometry/ Extract Specific Verticies option node 0 to create a point layer with the Strike and Dip data. The angle field values this operation produces should match the calculated strikes.
6. Now you can plot your svg symbol for strike-dip using a rotation over-ride set to "Strike" and in a similar manner create labels of dip suitably rotated to be at the end of the dip tick.