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Bibliotheca Alexandrina is hosting an interesting site called Memory of Modern Egypt which contains scans of photographs, documents and maps from 1800 till now.
They have a low resolution scan of a map of Cairo from 1800 in French. The city is the darker area. Notice the flood basin surrounding the Nile and how the Zamalek island looks weird compared to the Roda island (Manial) which looks almost like what it really looks like.
Overlaying it on Google Earth was simple but not perfect. Perhaps the old map doesn't match the precision of a satellite photograph. The dark area is what was inhabited back then compared to what's enclosed by the faint yellow ring road surrounding the modern city. Cairo was 4.5 km by 2.5 km 210 years ago and now Cairo and Giza are 35 by 13 kilometers (if you consider 6th of October and all the new developments it's 65 kilometers across).
Click on the image to open a larger version in a new window.
Interestingly, you can see that some of the old streets overlap with the faint yellow streets of today. Especially Port Said street which ran in the middle of the old town. Does anyone know what was it's name back then?