Strange coordinates for San Francisco: 5979762, 2085921 from Shapefile

Question

I have a Shapefile (.shp, .dbf, .shx, .prj) that's supposed to represent street data for San Francisco, which I'm writing a parser for. The problem is, when I parse the bounding box according to the ESRI format for Shapefile here, I get back around (5979762, 2085921) for the min coordinate of the bounding box (note that I actually get floats, but I rounded them down here for clarity). These are not UTM coordinates for SF, as I originally thought, and they're definitely not lat/lons.

I'm not sure what units these are or how to interpret them. The content of the .prj file is as follows:

PROJCS[
    "NAD_1983_StatePlane_California_III_FIPS_0403_Feet",
    GEOGCS[
        "GCS_North_American_1983",
        DATUM[
             "D_North_American_1983",
             SPHEROID["GRS_1980",6378137.0,298.257 222101]
        ],
        PRIMEM["Greenwich",0.0],
        UNIT["Degree",0.0174532925199433]
    ],
    PROJECTION["Lambert_Conformal_Conic"],
    PARAMETER["False_Easting",6561666.666666666],
    PARAMETER["False_Northing",1640416.666666667],
    PARAMETER["Central_Meridian",-120.5],
    PARAMETER["Standard_Parallel_1",37.06666666666667],
    PARAMETER["Standard_Parallel_2",38.43333333333333],
    PARAMETER["Latitude_Of_Origin",36.5],
    UNIT["Foot_US",0.3048006096012192]
]

My problem is that I really don't have any experience with .prj files, so I don't know how to interpret this format. Can anyone explain what this format is and how I should interpret it, or where I can find any more resources on how to do so?

Answer 1

Welcome to GIS SE!

No worries here, you've got a shapefile whose coordinates subscribe to a State Plane Coordinate System, recorded in feet. The minimum bounding geometry coordinate you've retrieved can be thought of as the bottom-left bounding geometry corner relative to sort of an arbitrarily placed origin point on an XY plane. So, instead of 0,0, you start counting at 6561666, 1640416, in feet, as the map units. Usage of State Plane Coordinate Systems is common and these coordinate systems are generally most accurate for the region called out in their name. California appears to have 7 State Plane Zones available. For visualizations at the county or regional level, State Plane systems are often a solid choice (of course, this is not meant to start an argument about the "best" projection for any particular application).

Following @DLY's advice, you are free to project these coordinates into an alternative coordinate system should you decide to.

Best Luck.

Answer 2

These are not strange coordinates. San Francisco's coordinate reference system is EPSG 2227 which is what is described by your .prj file. EPSG 2227 is in feet and not meters. To get UTM coordinates, you need to reproject to from EPSG2227 to UTM using something like QGIS, ARCGIS, or GDAL.