You're correct that this UoM is called decimal degrees, and partially correct that the value varies depending where you are on earth.
One degree of latitude is always about the same width. One degree of longitude varies depending on what latitude you're at. The accepted answer to this question explains it well: Calculating longitude length in miles?
Using your test data, this:
> gDistance(sp_poly, sp_pts)
returns the distance from the polygon to the nearest point, but if you add byid=TRUE:
> gDistance(sp_poly, sp_pts, byid=TRUE)
you get a matrix of distances - each row is one of your points. If you have more than one polygon ...
If you raster's projection unit is meter, then the output unit of the proximity raster will be meters if you specify georeferenced units. In case your raster's projection unit is not meter (e.g. is degree because it is on WGS84), you'll want to reproject your raster to another coordinate system (such as UTM, for example).
Here is an example code which subclasses QgsMapToolEmitPoint to create a custom map tool which should do what you want. It also implements the QgsRubberBand class to draw a temporary line on the map canvas to track your start and endpoints. You can paste this script into a new editor in the python console and click 'Run script'. You will see the cursor is ...
I don't know about the tools you mention but you can try a query like this one :
select id, st_distance(poly.geometry, line.geometry) from poly, line
That should do the job ...
You input the query in QGIS DB Manager :
You choose Database / Database Manager / Database Manager then Virtual Layers / Qgis Layers.
Adapt the query to your ...
Create a scratch layer (line).
Apply a label with this expression:
'dy = ' || to_string( y(start_point( $geometry))-y(end_point( $geometry))) || ' , dx = ' || to_string( x(start_point( $geometry))-x(end_point( $geometry)))
It will look like: dy = -0.7004608294930876 , dx = -0.9370199692780339.
You can change the text as you want, eg if you want it to ...
The distance-distortion in the map projection will be independent of the software (ignoring blunders); it is instead dependent on the projection used, and is also spatially variable.
Ordnance Survey National Grid is a Transverse Mercator projection chosen that the distances on the ground are identical to distances in the grid along two meridians: two lines ...