I think of this suggestion different than just "add contour lines to a photo". For that suggestion, we need to render (or pre-render) a set of contour lines based on the elevation model in TNP. Then layer those lines onto the photo - similar to the way the Street layer works today.

For this suggestion, I envision the ability to make certain colors of the USGS map transparent, which would allow the remaining colors to "shine through" to the aerial photo. For example, if the green, white, and blue colors of a USGS map were removed to allow the photo to be revealed underneath, then you could create a hybrid map.

Incidentally, we are working towards both of these features, but I can't announce a timeline for availability, yet.

Should this (Bill Neve's) suggestion be combined with the closely-related suggestion that gives a method to "Add contours to aerial photos"? And combine Votes for an impressive total of 97 votes? (as of today)

I would dump all my votes on this suggestion if I could. To have an easy way to transpose USGS quad lines on an aerial would make my work with TNP so much easier. Have to use markers now to get section corners on aerial maps within TNP - a real pain.

Overlaying contour lines on aerial photos would be awesome!

Alternate approaches to overlaying semitransparent USGS quads can be done by creating TNP overlays for the contours and PLS TSR lines, which then may be nicely displayed on any of the TNP maps:

CONTOURS
1a. TNP could (Suggestion!) calculate vector contour lines using the DEM already in TNP (similar info as in the usual Map-GPS's), and apply as an overlay. These contours would differ somewhat from the USGS quad.
(1b. Contour lines might be available as SHP files from your local government GIS department, for import to TNP as overlays. Again, these contours as overlays would differ somewhat from the USGS quad.)
(1c. for the more advanced adventuresome TNP user. Contours almost identical to those shown on the USGS topos can be downloaded from the USGS as hypsography files in a DLG format, USGS quad-by-quad, which can be converted to SHP format and imported to TNP as overlays- details omitted here.)

PLS TSR
2a. TNP can now (current versions) display the PLS TSR lines as a Custom Grid. (Try it!)
2b. TNP users should be able to download PLS TSR data as SHP files from their local government GIS Department, and then import to TNP as overlays.

The vector overlays would have better versatility in display as overlays than would the semitransparent raster USGS quads.