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Comment by smcl | Hacker News Reader

Anyone know why Philadelphia has a kinda NSEW orientation like the other cities but rotated ever so slightly? Manhattan is similar and it makes sense (they are aligned with the island itself), but Philadelphia is so close to the actual compass points and not aligned with anything really else.

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As I understand it, the grid plan originated in Philadelphia in a stretch of land between the Delaware and Schyulkhill rivers. This portion, which is now generally called Center City, has a grid that aligns itself with the Delaware which runs at that point a little East of North. The rest of the city grew up extending that grid.

Also, that particular point was picked, I believe, because it was the shortest distance between the Delaware and the Schuylkill. (Well, except for a few miles downriver where the Schuylkill flows into the Delaware, but South Philly was basically swamps back then.) See for example https://hiddencityphila.org/2015/08/courtyard-compass-reveal...

The ~same offset appears in Penn's original plan.[0]

Maybe it was based on geomagnetic north? The magnetic declination in Philadelphia is now about -13 degrees, but it was maybe -8 degrees in 1682.[1]

0) http://www.ushistory.org/philadelphia/images/philadelphia1.g...

1) https://en.wikipedia.org/wiki/Magnetic_declination

River. The almost-north-south orientation runs parallel to a stretch of the Delaware; same way that Manhattan's uptown/downtown/crosstown are rectangular, but not aligned with the compass points.

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