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I used QGIS to find the extents/bounding boxes of a list of features from census data.

I used the following method: Vector - Research tools - Polygon From Layer Extent, which creates a new shapefile with a table of extent values for each feature.

However, the resulting extent values (minx, miny, etc.) are not LAT/LONG. I'm not sure what they are. The properties show CRS as "USER:100000 - Generated CRS (+proj=aea +lat_1=29.5 +lat_2=45.5 +lat_0=37.5 +lon_0=-96 +x_0=0" even though I set the project default to WGS 84 EPSG: 4326. If I change it back to WGS84, the data stays to same.

What CRS format are QGIS extents and how can I convert them to standard Lat/long so that I can plot them?

user2559519
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Extents are computed in the native projection of the data. If you want "standard lat/lon" I believe that the easiest way is to re-project your source data physically into EPSG:4326 and compute extents from the projected file.

user30184
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  • That's the problem, the source data is EPSG:4326. Everything, by defualt, is EPSG:4326, except the resulting extents, for some reason. – user2559519 Sep 19 '14 at 08:27
  • I used this for testing, could you repeat http://www.nacis.org/naturalearth/10m/cultural/ne_10m_admin_0_boundary_lines_land.zip – user30184 Sep 19 '14 at 08:30
  • Thanks, but the link appears to be broken. Is there a way to compute the lat/long using the current CRS? I'm using a Google Maps API. Perhaps I can add code that computes the Lat/Long using the generated CRS? – user2559519 Sep 19 '14 at 08:43
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    The link seems to be now http://www.naturalearthdata.com/http//www.naturalearthdata.com/download/10m/cultural/ne_10m_admin_0_countries.zip. But do not start fixing anything before knowing what is broken. QGIS believes your data is in user projection, you believe it is in EPSG:4326 and you must find the reason. What does ogrinfo tell about the projection? Have you played with custom projections http://docs.qgis.org/2.0/en/docs/user_manual/working_with_projections/working_with_projections.html#sec-custom-projections? – user30184 Sep 19 '14 at 09:11
  • I tried the data you provided and it looks like the bounding box is in EPSG: 4326. I am not sure what configurations are changing my data. I'm using: https://www.dropbox.com/s/6sv07u61vuxi1xj/nhgis0006_shapefile_tl2000_us_tract_1930.zip?dl=0 I simply use Vector - Research tools - Polygon From Layer Extent but the results are not in lat/long – – user2559519 Sep 20 '14 at 05:38
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    Have a look at the .prj parts of your shapefiles. They define the projection and there you can read PROJCS["USA_Contiguous_Albers_Equal_Area_Conic",GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Albers"],PARAMETER["False_Easting",0.0],PARAMETER["False_Northing",0.0],PARAMETER["Central_Meridian",-96.0],PARAMETER["Standard_Parallel_1",29.5],PARAMETER["Standard_Parallel_2",45.5],PARAMETER["Latitude_Of_Origin",37.5],UNIT["Meter",1.0]]. Reproject your data to 4326. – user30184 Sep 20 '14 at 08:08
  • Oh, Thank you. I now reprojected by saving as..and it looks like it's working. I just don't understand why changing the CRS in both the layer and project properties is so pointless. – user2559519 Sep 20 '14 at 16:13