I am trying to plot a proximity raster for secondary roads in a particular area. What I have is a set of polyline representing the roads. Now, I want to create a proximity raster based on the polyline. First of all I need to create a raster from the polyline which I am perfectly able to. Now, how can I insert burn in value of the pixel to be it's lat and long coordinate?
You can create a proximity raster using the Proximity (raster distance) processing tool directly from your rasterized polyline. No need to burn coordinates as new rasters...
However, if your coordinates are lon, lat then your data is in a geographic (lon, lat) CRS and the horizontal units are degrees which are not a good choice for distance measurements.
I suggest reprojecting your polyline to a projected coordinate reference system (CRS) with horizontal units in metres before rasterizing.