How to read RAF wartime aerial photographs for a conservation area appraisal

Most conservation area appraisals lean on the same three sources: the tithe map, the first-edition OS, and whatever aerial cover happens to survive from the 1940s. That third one is the hardest to use well, because nobody teaches you how in planning school. You get handed a box of prints or a set of scans from the Historic England Archive and you're expected to just read them.

A few habits make the difference.

Finding and ordering the right sortie

RAF vertical cover from the war and the immediate postwar years comes in sorties, not individual photographs. Each sortie has a reference like RAF/106G or RAF/CPE/UK, followed by a run and frame number. The CPE (Central Photographic Establishment) survey of 1945-47 is usually the best starting point for England, because it's near-complete and the prints have held up reasonably well in storage. Earlier wartime sorties exist but coverage is patchier and quality varies a lot depending on who flew it and why.

The National Archives and the Historic England Archive both hold index maps showing which sorties cover which parish. Order by frame, not by sortie, since a single run might be forty or fifty frames long and you only need two or three for a given conservation area boundary. Ask for the frames either side of your target too. Overlap between consecutive frames is what lets you view the ground in stereo with a basic pocket stereoscope, and that's where ridge and furrow, old quarry faces, and collapsed field boundaries show up in relief, far clearer than they ever do on a flat single print.

Reading tone, shadow, and scale

A vertical sortie print has no color and no modern cartographic symbols, so you're reading tone and shadow the way a photo-interpreter would have in 1946. Low sun angle in winter sorties throws long shadows that reveal earthworks a summer pass would flatten out completely, which is why winter cover is often more useful for landscape archaeology than anything flown in July. Ridge and furrow shows as parallel corrugation. A filled-in pond reads as a dark, slightly sunken patch even decades after infilling. Demolished farm buildings leave a pale rectangular scar in the grass where rubble or hardcore was never properly cleared.

Scale varies between sorties, commonly somewhere around 1:10,000 to 1:25,000, and it isn't printed on the photograph itself. You work it out from known ground distances, usually the gap between two features also shown on a contemporary OS map, or you use the published sortie specification if the archive's catalogue entry includes one. Without that step you can't measure anything with confidence, and a conservation area appraisal that states a field boundary moved "significantly" without a measured basis is not going to survive scrutiny at examination.

Once you've got scale sorted, the next job is lining the wartime print up with the modern townscape so the two periods sit on the same base. Picking control points, church towers, road junctions, surviving boundary walls, and transforming the old print onto the current OS grid is the part that takes longest by hand. That's the step pairing the archive scan against a recent high-res pass can shortcut, giving you a proper before-and-after without warping each frame yourself.

What to flag in the appraisal text

The appraisal document doesn't need a photo-interpretation lecture. It needs specific, dateable observations: the orchard behind the vicarage shown on the 1946 sortie and gone by the first postwar OS revision; the former quarry edge that's now obscured by self-seeded woodland but still legible as a scarp; the three outbuildings on the farm range demolished sometime between the wartime sortie and the 1960s survey, narrowing a gap that documentary sources alone leave vague. These are the details that give a conservation area statement of significance some actual teeth, rather than the usual paragraph about "historic character" with nothing underneath it.

Keep a note of the sortie and frame reference next to every claim you draw from it. An inspector or a future officer revisiting the appraisal will want to check your reading, and a reference lets them order the same frame rather than take your word for it.

If the manual overlay work is the bottleneck, it's worth seeing what a decadal comparison against current imagery looks like for your own conservation area before you commit a day to it by hand.